Transmission Assessment Surveys (TAS) to Define Endpoints for Lymphatic Filariasis Mass Drug Administration: A Multicenter Evaluation.
ABSTRACT Lymphatic filariasis (LF) is targeted for global elimination through treatment of entire at-risk populations with repeated annual mass drug administration (MDA). Essential for program success is defining and confirming the appropriate endpoint for MDA when transmission is presumed to have reached a level low enough that it cannot be sustained even in the absence of drug intervention. Guidelines advanced by WHO call for a transmission assessment survey (TAS) to determine if MDA can be stopped within an LF evaluation unit (EU) after at least five effective rounds of annual treatment. To test the value and practicality of these guidelines, a multicenter operational research trial was undertaken in 11 countries covering various geographic and epidemiological settings.
The TAS was conducted twice in each EU with TAS-1 and TAS-2 approximately 24 months apart. Lot quality assurance sampling (LQAS) formed the basis of the TAS survey design but specific EU characteristics defined the survey site (school or community), eligible population (6-7 year olds or 1(st)-2(nd) graders), survey type (systematic or cluster-sampling), target sample size, and critical cutoff (a statistically powered threshold below which transmission is expected to be no longer sustainable). The primary diagnostic tools were the immunochromatographic (ICT) test for W. bancrofti EUs and the BmR1 test (Brugia Rapid or PanLF) for Brugia spp. EUs.
In 10 of 11 EUs, the number of TAS-1 positive cases was below the critical cutoff, indicating that MDA could be stopped. The same results were found in the follow-up TAS-2, therefore, confirming the previous decision outcome. Sample sizes were highly sex and age-representative and closely matched the target value after factoring in estimates of non-participation. The TAS was determined to be a practical and effective evaluation tool for stopping MDA although its validity for longer-term post-MDA surveillance requires further investigation.
- SourceAvailable from: Asha Dilrukshi Wijegunawardana[Show abstract] [Hide abstract]
ABSTRACT: The Sri Lankan Anti-Filariasis Campaign conducted 5 rounds of mass drug administration (MDA) with diethycarbamazine plus albendazole between 2002 and 2006. We now report results of a comprehensive surveillance program that assessed the lymphatic filariasis (LF) situation in Sri Lanka 6 years after cessation of MDA.PLoS Neglected Tropical Diseases 11/2014; 8(11):e3281. · 4.49 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: As part of the Global Programme to Eliminate Lymphatic Filariasis (LF), American Samoa conducted mass drug administration (MDA) from 2000-2006, and passed transmission assessment surveys in 2011-2012. We examined the seroprevalence and spatial epidemiology of LF post-MDA to inform strategies for ongoing surveillance and to reduce resurgence risk.PLoS Neglected Tropical Diseases 11/2014; 8(11):e3297. · 4.49 Impact Factor
- PLoS neglected tropical diseases. 12/2014; 8(12):e3328.
Transmission Assessment Surveys (TAS) to Define
Endpoints for Lymphatic Filariasis Mass Drug
Administration: A Multicenter Evaluation
Brian K. Chu1*, Michael Deming2, Nana-Kwadwo Biritwum3, Windtare ´ R. Bougma4, Ame ´yo M. Dorkenoo5,
Maged El-Setouhy6, Peter U. Fischer7, Katherine Gass1, Manuel Gonzalez de Pen ˜a8, Leda Mercado-
Hernandez9, Dominique Kyelem1, Patrick J. Lammie2, Rebecca M. Flueckiger1, Upendo J. Mwingira10,
Rahmah Noordin11, Irene Offei Owusu12, Eric A. Ottesen1, Alexandre Pavluck1, Nils Pilotte13,
Ramakrishna U. Rao14, Dilhani Samarasekera15, Mark A. Schmaedick16, Sunil Settinayake15,
Paul E. Simonsen17, Taniawati Supali18, Fasihah Taleo19, Melissa Torres13, Gary J. Weil7, Kimberly Y. Won2
1Neglected Tropical Diseases Support Center, Task Force for Global Health, Decatur, Georgia, United States of America, 2Division of Parasitic Diseases and Malaria,
Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America, 3Department of Public Health, Ghana Health Service, Accra, Ghana, 4Programme
National d’E´limination de la Filariose Lymphatique, Ministe `re de la Sante ´, Ouagadougou, Burkina Faso, 5Programme National d’E´limination de la Filariose Lymphatique,
Ministe `re de la Sante ´, Lome ´, Togo, 6Department of Community, Environmental and Occupational Medicine, Ain Shams University, Cairo, Egypt, 7Infectious Diseases
Division, Washington University School of Medicine, St. Louis, Missouri, United States of America, 8Centro National de Control de Enfermedades Tropicales, Santo
Domingo, Dominican Republic, 9Infectious Disease Office, National Center for Disease Prevention & Control, Manila, Philippines, 10Neglected Tropical Diseases Control
Programme, National institute for Medical Research, Dar es Salaam, Tanzania, 11Institute for Research in Molecular Medicine, Universiti Sains Malaysia, Penang, Malaysia,
12Epidemiology Department, Noguchi Memorial Institute for Medical Research, Legon, Ghana, 13Department of Biological Sciences, Smith College, Northampton,
Massachusetts, United States of America, 14Department of Internal Medicine, Washington University School of Medicine, St. Louis, Missouri, United States of America,
15Anti Filariasis Campaign, Ministry of Health, Colombo, Sri Lanka, 16Division of Community and Natural Resources, American Samoa Community College, Pago Pago,
American Samoa, 17Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark, 18Department of Parasitology, University of Indonesia,
Jakarta, Indonesia, 19Neglected Tropical Diseases Unit, Public Health Directorate, Port Vila, Vanuatu
Background: Lymphatic filariasis (LF) is targeted for global elimination through treatment of entire at-risk populations with
repeated annual mass drug administration (MDA). Essential for program success is defining and confirming the appropriate
endpoint for MDA when transmission is presumed to have reached a level low enough that it cannot be sustained even in
the absence of drug intervention. Guidelines advanced by WHO call for a transmission assessment survey (TAS) to
determine if MDA can be stopped within an LF evaluation unit (EU) after at least five effective rounds of annual treatment.
To test the value and practicality of these guidelines, a multicenter operational research trial was undertaken in 11 countries
covering various geographic and epidemiological settings.
Methodology: The TAS was conducted twice in each EU with TAS-1 and TAS-2 approximately 24 months apart. Lot quality
assurance sampling (LQAS) formed the basis of the TAS survey design but specific EU characteristics defined the survey site
(school or community), eligible population (6–7 year olds or 1st–2ndgraders), survey type (systematic or cluster-sampling),
target sample size, and critical cutoff (a statistically powered threshold below which transmission is expected to be no
longer sustainable). The primary diagnostic tools were the immunochromatographic (ICT) test for W. bancrofti EUs and the
BmR1 test (Brugia Rapid or PanLF) for Brugia spp. EUs.
Principal Findings/Conclusions: In 10 of 11 EUs, the number of TAS-1 positive cases was below the critical cutoff, indicating
that MDA could be stopped. The same results were found in the follow-up TAS-2, therefore, confirming the previous
decision outcome. Sample sizes were highly sex and age-representative and closely matched the target value after factoring
in estimates of non-participation. The TAS was determined to be a practical and effective evaluation tool for stopping MDA
although its validity for longer-term post-MDA surveillance requires further investigation.
Citation: Chu BK, Deming M, Biritwum N-K, Bougma WR, Dorkenoo AM, et al. (2013) Transmission Assessment Surveys (TAS) to Define Endpoints for Lymphatic
Filariasis Mass Drug Administration: A Multicenter Evaluation. PLoS Negl Trop Dis 7(12): e2584. doi:10.1371/journal.pntd.0002584
Editor: Charles D. Mackenzie, Michigan State University, United States of America
Received July 16, 2013; Accepted October 29, 2013; Published December 5, 2013
This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for
any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.
Funding: Funding for this study was provided by the Bill and Melinda Gates Foundation, Grant #OPP43922. The funders had no role in study design, data
collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: email@example.com, firstname.lastname@example.org.
PLOS Neglected Tropical Diseases | www.plosntds.org1December 2013 | Volume 7 | Issue 12 | e2584
Lymphatic filariasis (LF) is a mosquito-borne parasitic disease
endemic to 73 countries worldwide. An estimated 1.4 billion
people are said to be at-risk of LF disease with approximately 120
million infected and 40 million suffering from the crippling and
stigmatizing clinical manifestations of the disease, especially
lymphoedema and hydrocele . As such, LF is one of the
leading causes of chronic disability worldwide.
The primary focus for control and elimination of LF is the
interruption of disease transmission through treatment of the
entire at-risk population with repeated annual mass drug
administration (MDA) using a single-dose combination of
albendazole with either diethylcarbamazine (DEC) or ivermectin
. Since 2000, these efforts have been coordinated through the
World Health Organization’s (WHO) Global Programme to
Eliminate Lymphatic Filariasis (GPELF), a collaborative public
health program that has delivered to date nearly 4 billion drug
treatments to over 950 million individuals in 53 countries . This
extraordinary achievement, made possible through the drug
donations of manufacturers Merck (ivermectin) and GlaxoSmith-
Kline (albendazole) has resulted in a marked reduction of infection
prevalence in endemic areas, along with sizeable health and
economic benefits to the affected populations [3,4].
Essential to the Global Programme’s success in combating LF is
the important challenge of defining and confirming endpoints for
MDA when disease transmission is presumed to have reached a
level low enough that it cannot be sustained even in the absence of
drug intervention. Given the biology and parasitic life cycle of LF,
this threshold of infection is most likely to be reached following 4–
6 annual MDA rounds with effective population coverage and a
resulting microfilaria (mf) prevalence rate ,1% (or Circulating
Filarial Antigen [CFA] prevalence ,2%) [2,5–7]. The priority
need, therefore, has been a standardized, robust evaluation tool to
determine statistically whether this critical threshold has been met
and recrudescence not likely to ensue if MDA is stopped. Earlier
WHO guidelines for stopping MDA were notably cumbersome,
[8–10]. Because of these shortcomings, new guidelines were
drafted  with the introduction of Transmission Assessment
Surveys (TAS) that propose to be more logistically feasible and
adaptable to varying demographic and epidemiologic conditions
The present study was a multi-country operational research
assessment of the TAS protocol, specifically aimed at evaluating
the assumptions and accuracy of the TAS sampling strategy, as
well as identifying best practices for TAS implementation. Results
are informative for refining TAS methodologies and improving
standard procedures going forward. Indeed, several of the
preliminary results captured in this study were used by WHO to
inform its development of the updated GPELF monitoring and
evaluation guidelines .
Ethical approval was obtained for each participating country
through the following review boards and institutions: Centers for
Disease Control and Prevention Institutional Review Board
(American Samoa), Comite ´ d’ethique pour la Recherche en Sante ´
du Ministe `re de la Sante ´ (Burkina Faso), Consejo Nacional de
Bioe ´tica en Salud (Dominican Republic), Noguchi Memorial
Institute for Medical Research Institutional Review Board
(Ghana), University of Indonesia Committee of the Medical
Research Ethics (Indonesia), Ministry of Health Research and
Ethics Committee (Malaysia), National Center for Disease
Prevention and Control (Philippines), Ministry of Health (Sri
Lanka), National Institute For Medical Research Clearance
Committee (Tanzania), Comite ´ Bioe ´thique de Recherche en
Sante ´ du Ministe `re de la Sante ´ (Togo), Government of the
Republic of Vanuatu Public Health Services (Vanuatu).
Informed assent was required from all sampled children in
addition to written or oral consent from their parent or guardian.
Oral consent was marked as part of the electronic record,
witnessed by a teacher or other family member, and only used
where allowed by the local ethical review board and not as a
replacement for those countries requiring written consent (Amer-
ican Samoa, Malaysia, and Togo). Only data remaining in the
country was identifiable; those electronically sent to the Task
Force for Global Health were de-identified at the point of
transmission. Positive children by any diagnostic test were treated
with the appropriate medicines either during or immediately
following the study. In Malaysia, however, only mf positive
children were treated after the first TAS while all Brugia Rapid
and mf positives were treated after the second TAS.
For this study, eleven countries – American Samoa, Burkina
Faso, Dominican Republic, Ghana, Indonesia, Malaysia, Philip-
pines, Sri Lanka, Tanzania, Togo, and Vanuatu – took part in
implementing and evaluating the TAS according to standard
operating procedures (SOP) and guidelines [11,12]. With the TAS
applicable for both stop-MDA and post-MDA surveillance decision-
making, countries were specifically chosen and sub-divided into
these two categories based on their current MDA status. An initial
TAS (referred to in this paper as TAS-1) was conducted over the
period of September 2009 to February 2011. As prescribed by the
TAS protocol, a second TAS (TAS-2) was done approximately
two years later to re-evaluate the initial outcomes and decisions of
TAS-1. All countries completed TAS-2 from September 2011 to
April 2012 with the exception of American Samoa whose
Lymphatic filariasis (LF) is targeted for global elimination
through a strategy of repeated annual mass drug
administration (MDA) to entire at-risk populations. A
transmission assessment survey (TAS) is designed to
evaluate whether transmission of LF is presumed to have
reached a level low enough that it cannot be sustained in
the absence of drug intervention and, therefore, MDA can
be stopped. This multicenter operational research trial
examines the value and practicality of the TAS guidelines
through its implementation in 11 countries of diverse
geographical and epidemiologic profiles. The field expe-
riences support the TAS survey design methodology with
particular respect to school and cluster-based sampling
strategies. We found that sample sizes were age and sex
representative and met the target values after factoring in
estimates of non-participation rates. In 10 of 11 countries,
the TAS found the number of positive cases in the
evaluation unit to be no more than the statistically
powered critical threshold. These results were corroborat-
ed in a follow-up TAS approximately 24 months later. We
conclude the TAS is a valuable and effective tool for
stopping MDA but its utility for longer-term post-MDA
surveillance needs further empirical evidence and may be
best supported with complementary tools and methods.
Transmission Assessment Surveys for LF Endpoints
PLOS Neglected Tropical Diseases | www.plosntds.org2December 2013 | Volume 7 | Issue 12 | e2584
projected TAS-2 date extended beyond the timeline of this
Within each country, TAS was carried out in a newly defined
evaluation unit (EU) that was based on LF IUs used for MDA and
follow-up assessments. An EU may consist of part of an IU, a
whole IU, or a combination of multiple IUs that typically have
contiguous borders and share similar epidemiologic profiles .
All EUs in this study met the TAS eligibility requirements of
having completed at least five effective rounds of MDA in all IUs
with coverage $65% of the total population, mf rates ,1% (or
CFA,2%) in all sentinel and spot-check sites post-5thMDA, and a
total population less than two million. In addition, at least six
months had passed since the last MDA. Table 1 outlines each of
the participating countries and corresponding EUs at the time of
The TAS survey design depends upon factors such as the net
primary school enrolment rate in each EU, the target population
size, number of schools, vector type and parasite species to
determine the required survey site, target population, survey type,
sample size, critical cutoff, and diagnostic tool– all of which are
described below and summarized in Table 2. A Microsoft Excel
computer tool entitled Survey Sample Builder (SSB) was used to assist
principal investigators (PI) in navigating the TAS protocol and
inputting the required data . From these inputs, the SSB
produced random number lists and automated survey design
calculations, including sample size and sampling intervals to
facilitate rigorous sampling. In accordance with the TAS protocol,
survey methodologies were identical for both stop-MDA and
periodic post-MDA assessments.
School surveys were conducted in American
Samoa, Ghana, Indonesia, Malaysia, Philippines, Sri Lanka,
Togo, and Vanuatu, where net primary enrolment rates in the EU
were $75%. In Burkina Faso and Tanzania where EU primary
enrolment were ,75%, community-based surveys were imple-
mented using census enumeration areas (EA) and hamlets as the
primary sampling units (i.e. clusters). Although school enrolment
was $75% in the Dominican Republic, community-based surveys
have commonly been used for LF evaluation here and were
therefore preferred for the TAS with villages as the primary
sampling unit. In this publication, the term EA will be used
universally to designate primary sampling units for a TAS using
Children 6–7 years old represent the
target age group for TAS because they have lived most or all their
lives during MDA and, therefore, positive filarial serology would
be more indicative of recent LF transmission than it would be in
older children or adults who may have been previously exposed.
For school surveys, 1stand 2ndgrade children were chosen as a
proxy for 6–7 year olds; all children in these grades were eligible
including those outside this age range. In Togo, however, only 6–7
year old children in 1stand 2ndgrade were surveyed. Another
exception was in Vanuatu where only 1stgrade children were
enrolled due to the expected large proportion of children aged
nine and above in Grade 2. Community surveys specifically
targeted and sampled 6–7 year old children only.
For each country, it was determined whether a
cluster or systematic survey was most appropriate. Cluster-sample
surveys were used in Burkina Faso, Dominican Republic, Ghana,
Indonesia, Malaysia, Philippines, Tanzania, and Togo where EUs
had a sampling frame of at least forty primary sampling units and
a large number of target eligible children. In these countries, only
a subset ($30) of total schools or EAs was randomly selected for
Table 1. Evaluation Unit key characteristics at time of TAS-1.
# of IUs
Brugia timori, W. bancrofti
Transmission Assessment Surveys for LF Endpoints
PLOS Neglected Tropical Diseases | www.plosntds.org3December 2013 | Volume 7 | Issue 12 | e2584
sampling. For EUs in American Samoa and Sri Lanka with
smaller target populations and fewer primary sampling units, the
TAS required systematic sampling rather than cluster sampling. In
Vanuatu where the target population was very small, the selection
method was a census where all primary schools and eligible children
in the EU were surveyed.
Sample size and selection.
lation size, and LF vector in the EU determined the target sample
size for each country. All else being equal, TAS sample sizes were
larger in EUs where cluster surveys were required. The TAS
assumes a cluster survey design effect of 1.5 if the target population
is ,2400 (,5000 for Aedes EUs) and 2.0 if $2400 ($5000 for Aedes
EUs); these initial estimates were based on low expected
prevalence of filarial antigenaemia following several MDAs and,
therefore, lower probability of prominent clustering. TAS sample
sizes are also larger where Aedes is the primary LF vector because it
is known to be more efficient at transmitting the infection. Target
sample sizes in this study were provided by SSB and ranged from
684 in Sri Lanka (systematic sampling) to 1556 in Burkina Faso
and Ghana (cluster sampling).
The selection of clusters was done by systematic sampling
without regard to school or EA size. Schools or EAs were listed in
order of proximity from a central point to ensure a geographically
representative sample across the entire EU. A random starting site
was selected by SSB, based on a random number chosen between
one and the sampling interval, and the calculated sampling
interval then applied to determine the remaining survey sites. If
the target sample size was not met after all clusters were surveyed,
an additional set of at least 5 clusters was randomly selected from
the remaining pool without replacement.
The same systematic sampling approach was used in cluster-
sample surveys to select children within each school or EA in
Burkina Faso, Ghana, Indonesia, Malaysia, and Togo, where
average cluster size was large enough that only a fraction of
eligible children were needed at each site to reach the required
The TAS critical cutoff value represents the
threshold of infection prevalence below which transmission is
expected to be no longer sustainable, even in the absence of MDA.
TAS estimates the EU’s relationship to this threshold by the
The survey type, target popu-
number of serologic antigen- or antibody-positive cases. If the total
number of positive cases is at or below the critical cutoff, the EU
‘passes’ the survey and the decision to stop MDA can be made. If
the total number of positive cases is above the critical cutoff, MDA
should continue in the EU for at least two more rounds .
TAS sample sizes and critical cutoff values are powered so that
the EU has at least a 75% chance of passing if the true antigen or
antibody prevalence is half the threshold level (2% for Culex,
Anopheles, and Mansonia vector areas, and 1% for Aedes vector
areas). In addition, there is no more than a 5% chance of passing if
the true prevalence is greater than or equal to the threshold level.
Following a comprehensive multicenter
study evaluating potential diagnostic tools , the immunochro-
matographic (ICT) test for filarial antigen and the BmR1 antibody
test (PanLF or Brugia Rapid) were recommended for TAS in W.
bancrofti and Brugia spp. endemic areas, respectively. These were,
therefore, the principal diagnostic tools used for evaluation in this
study, in accord with test-specific SOPs and the following
i.For TAS-1, positive ICT tests were immediately followed up
with a second confirmatory ICT test. If the second test was
also positive, the child was considered positive; however, if
the second test was negative, the child was considered
negative. Because of satisfactory reproducibility results in
TAS-1, the repeat of positive ICT tests was dropped in TAS-
For TAS-1, the PanLF test was used in Brugia spp. countries
(i.e. Indonesia, Malaysia). For TAS-2, the Brugia Rapid test
was used due to wider availability; however, both tests
incorporate the same BmR1 recombinant filarial antigen to
measure antibodies and proved to have similar sensitivity in
multicenter evaluations .
With the EU in Indonesia endemic for both W. bancrofti and
Brugia spp., ICT and PanLF tests were individually conducted
for all sampled children in TAS-1. For TAS-2, however, only
Brugia Rapid tests were used; ICT tests have yet to be
completed because of logistic complications importing the
cards into the country.
Table 2. Survey design summary for TAS-1 and TAS-2.
CountryEU name Survey site
American Samoa TutuilaSchool1st–2ndgraders Systematic1,0426 ICT
Burkina Faso Dafra-KV-LenaCommunity 6–7 year olds Cluster1,55618ICT
Dominican RepublicSouthwest FocusCommunity6–7 year olds Cluster1,53218ICT
GhanaAES-Agona School1st–2ndgradersCluster 1,55618 ICT
IndonesiaAlor School1st–2ndgraders Cluster1,548 18 PanLF, ICT (TAS-1) Brugia
Rapid, ICT (TAS-2)
Malaysia SabahSchool1st–2ndgraders Cluster1,368 16PanLF (TAS-1) Brugia
PhilippinesSorsogonSchool1st–2ndgradersCluster 1,55218 ICT
Sri Lanka Dehiwala School1st–2ndgraders Systematic6848 ICT
TanzaniaTandahimba Community6–7 year oldsCluster 1,54018ICT
TogoKozahSchool1st–2ndgradersCluster1,548 18 ICT
1The critical cutoff in Vanuatu can be calculated exactly as .02N because the TAS was a census without random sampling error.
Transmission Assessment Surveys for LF Endpoints
PLOS Neglected Tropical Diseases | www.plosntds.org4 December 2013 | Volume 7 | Issue 12 | e2584
iv. Positive ICT or PanLF or Brugia Rapid children were
followed up with mf testing. The three-line blood smear
technique was conducted in TAS-1 and TAS-2 in local in-
country laboratories . In Tanzania, the counting
chamber technique was used for the detection of mf in place
of the three-line blood smear. Additional mf testing using the
real-time polymerase chain reaction (PCR) procedure 
was performed at Smith College (Northampton, USA) but
only for TAS-1 in all countries.
For TAS-1, survey data were collected on personal digital
assistants (PDA) (Hewlett Packard iPAQ 211) using the Electronic
Data Gathering and Evaluation (EDGE) system designed at the
Task Force for Global Health (Decatur, USA) . Each child was
assigned a unique identification number that was printed on a
barcode label, scanned into the PDA with a Bluetooth scanner
(Socket Mobile CX2821-656), and affixed to blood vials and
diagnostic tests to facilitate specimen management. External global
positioning system (GPS) cards (GlobalSat BC-337) were attached
to the PDA to track GPS coordinates of each school or household
surveyed. The survey itself consisted of questions capturing basic
location, demographic, and test result information. Each night, the
PDA was synchronized to a central laptop and the data were sent
to a secure server at the Task Force for Global Health whenever
the Internet was next accessible.
For TAS-2, identical survey data were collected but on mobile
smartphones (Motorola Milestone XT720) through a modified
version of the OpenDataKit (ODK) application developed at the
University of Washington (Seattle, USA). The same barcode
identification and labeling procedure as TAS-1 was implemented
using the Barcode Scanner application (Zxing Team). All phones
were equipped with a built-in GPS. Collected data were
automatically sent through cellular service or wireless Internet to
a secure server at the Task Force for Global Health.
Each country used 3–5 field teams consisting of at least three
persons each: a data collector, phlebotomist, and supply manager.
All team members and supporting staff were trained on the study
SOPs and electronic data collection tools by external consultants.
For school surveys, field teams were assisted by teachers who
provided official class registers and identified all survey-eligible
children for enumeration and selection according to the SSB
randomized number lists. For each selected child, demographic
data (i.e. name, sex, age) were recorded and blood collected (at
least 100 ml for ICT, 60 ml for PanLF, and 35 ml Brugia Rapid
tests) via finger prick into an EDTA-coated tube . The single
exception was in Vanuatu TAS-1 where blood was collected into a
calibrated capillary tube and directly applied onto the ICT card
for reading at the school due to logistic challenges.
For community-based surveys, a household selection process
was required in the selected EAs. With the assistance of local
officials and sketch maps, field teams identified a route through the
EA passing and enumerating each house. All houses correspond-
ing to the SSB randomized number lists were selected and all 6–7
year olds residing at the chosen households were surveyed with the
requisite blood collected in anticoagulant tubes. One exception to
this procedure occurred in Tanzania where instead of going house
to house, hamlet leaders organized all 6–7 year olds at a central
location in advance of the survey team’s arrival for selection and
sampling. The small population and area of the hamlets, in
addition to accurate updated registries and strong working
relations between health staff and hamlet leaders, permitted this
strategy and mitigated concerns about potential selection bias. In
contrast, this approach was not operationally feasible in Burkina
Faso because of larger EA sizes including peri-urban areas, as well
as sampling intervals .1 so household enumeration was necessary.
Following specimen collection, all blood-filled tubes were stored
and transported via cold-chain to a nearby laboratory base to
process and record ICT (or PanLF, Brugia Rapid) test results.
Children testing positive were identified using the EDGE/ODK
systems and individually followed up at night during peak mf
hours to collect an additional blood sample for mf testing (10pm–
2am except in American Samoa where W. bancrofti shows diurnal
The number of children absent on the survey date was recorded
for all surveys. For community surveys, field teams made at least
one revisit to the absent child’s house before recording an official
absence. The number of selected children without consent or
refusing to participate was also captured in addition to invalid and
incomplete tests due to malfunction or insufficient blood.
Together, these absentees, refusals, and individuals with test
errors were designated as TAS non-participators.
All transmitted data were compiled into a central database at
the Task Force for Global Health and exported into Microsoft
Excel spreadsheets for final cleaning and approval by the
collaborating principal investigators. Statistical analysis was done
by importing the clean datasets into SAS v9.3 (SAS Institute).
Summary statistics of test results and univariate analyses with
regard to age, sex, and location were performed using the PROC
UNIVARIATE function. Design effect calculations were conduct-
ed using the PROC SURVEYFREQ function.
For W. bancrofti countries, TAS-1 and TAS-2 results are presented
in Table 3. All EUs passed TAS-1, meaning that the number of
ICT positive children was no greater than the critical cutoff value.
As recommended by the TAS, MDA was then stopped (or periodic
post-MDA surveillance continued) in those specific EUs for
approximately 24 months before conducting TAS-2. All W.
bancrofti EUs (with the exception of American Samoa and
Indonesia where follow-up assessments were not yet completed)
also passed TAS-2, thereby corroborating the TAS-1 stop-MDA
or post-MDA surveillance decision. Microfilaraemia (mf) tests
were conducted on ICT positive children using the three-line
blood smear (TAS-1 and TAS-2) and PCR (TAS-1) procedures.
The proportion of mf-positive children among antigen-positive
children identified in the TAS was low in the W. bancrofti countries.
The positive blood smear to positive ICT proportion was 12.9%
(4/31) for TAS-1 and 5.2% (1/19) for TAS-2, while the
proportion of positive PCR to positive ICT was 22.6% (7/31).
For Brugia spp. countries, Indonesia passed TAS-1 and TAS-2 and
only one mf positive was found across both surveys (Table 4). The
number of PanLF positive children in Malaysia (Sabah), however,
exceeded the critical cutoff value in TAS-1. MDA was, therefore,
continued before re-testing in TAS-2, but for only one round in 8
IUs due to DEC supply problems. Results for TAS-2 using the
Brugia Rapid test were still greater than the critical cutoff value so
consequently, MDA has been recommended to continue in the
EU for two more rounds before conducting another TAS
evaluation. Mf results in Malaysia (Sabah, not peninsular
Malaysia) confirmed a high likelihood of active transmission with
Transmission Assessment Surveys for LF Endpoints
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a TAS-1 positive blood smear to positive PanLF proportion of
35.6% (32/90) and positive PCR to positive PanLF proportion of
52.2% (47/90). The TAS-2 positive blood smear to positive Brugia
Rapid proportion decreased to 20.5% (15/73) following the
additional rounds of MDA.
Population and Sampling Characteristics
The proportions of male and female children sampled were very
even across all school and community-based surveys in both TAS-
1 and TAS-2 (Table 5). In addition, no one country in either
survey had more than 54% male or female children in the sample.
The target age group for TAS is 6 and 7 year old children,
approximated by 1stand 2ndgraders in school surveys. In W.
bancrofti EUs, 84% of the total sample in school surveys was aged 6
and 7 and 95% between 6 and 10 years old (Table 6). The Brugia
spp. EUs in Indonesia and Malaysia found a higher proportion of 8
year olds in the TAS sample due to 1stand 2ndgrade in both
countries primarily consisting of 7 and 8 year old children. No
positive cases were detected outside the 6–10 year old range
although one positive ICT test was associated with a child of
Table 7 is informative because it displays the target and actual
sample sizes for TAS-1 and TAS-2 along with the number of
clusters (schools or EAs) surveyed to achieve the total. The target
sample size was mostly met in both surveys with a few notable
exceptions. In American Samoa TAS-1, there was insufficient
blood to perform the ICT test in a number of collected samples.
Likewise in Indonesia TAS-1, ICT and PanLF tests were
unavailable at the time of sampling; therefore, they were
conducted retroactively using preserved serum and several samples
did not have enough quantity to complete the test. For TAS-2 in
Malaysia, the actual sample size greatly exceeded the target due to
the random selection of several large schools in addition to a lower
non-participation rate than initially estimated.
Table 7 also presents the number of original clusters selected
and the number of extra clusters needed to meet the sampling
requirements. In TAS-1, a total of 63 extra clusters were required,
most prominently in Ghana, Indonesia, Philippines, and Tanza-
nia. In contrast, only 10 total extra clusters were required in TAS-
2, primarily as a result of factoring the non- participation rates into
the SSB survey design calculation. The non- participation rate
includes children – enrolled in first and second grade (for school
surveys) or residing in the selected house (for community-based
surveys) – absent on the survey date and those refusing to
participate or without consent. The rate was a combined 14.0%
for TAS-1 and 10.2% for TAS-2 but varied by country and survey
Table 3. ICT, blood smear, and PCR results for W. bancrofti countries.
ICT (Ag) Blood smear (mf) PCR (mf)
value TAS-1 positive1
6 2/949n/a0/2 (0.0%)n/a 0/2 (0.0%)
Burkina Faso18 13/15715/15912/13 (15.4%) 0/5 (0.0%)5/13 (38.5%)
Dom. Rep. 180/1609 3/1558- 1/3 (33.3%)-
Ghana 18 2/1557 0/15140/2 (0.0%)- 0/2 (0.0%)
18 6/1312 n/a4
Philippines182/1599 1/16560/2 (0.0%)0/1 (0.0%) 0/2 (0.0%)
80/679 1/698- 0/1 (0.0%)-
Togo 182/1571 0/1550 1/2 (50.0%)-1/2 (50.0%)
Tanzania 18 10/15619/1588 1/10 (10.0%) 0/9 (0.0%)1/9 (11.1%)
Vanuatu 18, 195
0/933 2/954-0/2 (0.0%)-
1% of total survey population.
2% of ICT+ individuals; some individuals could not be retraced for mf testing.
3Systematic sampling was used in American Samoa and Sri Lanka.
4Indonesia EU of Alor+Pantar islands is endemic for both W. bancrofti and Brugia timori. TAS-2 ICT tests were not available due to logistic problems importing diagnostic
tests into the country.
5Census critical cutoff value is equal to .02N for EUs with Culex, Anopheles, or Mansonia as primary LF vector.
Table 4. PanLF, Brugia Rapid, blood smear, and PCR results for Brugia spp. countries.
PanLF or Brugia Rapid (Ab)Blood smear (mf)PCR (mf)
1812/1353 14/1622 0/12 (0.0%)1/14 (7.1%) 0/12 (0.0%)
Malaysia1690/1429 73/168431/87 (35.6%) 15/73 (20.5%)46/86 (53.4%)
1% of total survey population.
2% of PanLF(+) or Brugia Rapid(+) individuals; some individuals could not be retraced for mf testing.
3Indonesia EU of Alor and Pantar islands is endemic for both W. bancrofti and Brugia timori.
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(Table 8). Non-participators also include invalid (i.e. malfunction-
ing) diagnostic tests or samples that were collected but had
insufficient quantity or other barriers preventing completion of the
test (e.g. blood clotting). These specific non- participation factors
accounted for approximately 4% of total TAS-1 and 2% of total
TAS-2 samples but were also dependent on country and survey.
Some non- participation rates were not tracked or estimated in
American Samoa (TAS-1), Burkina Faso (TAS-1), and Sri Lanka
Design effects for TAS-1 and TAS-2 cluster surveys are listed in
Table 9. All W. bancrofti countries had design effects less than the
TAS estimated value of 2 (for target populations .2400),
indicating the required sample size was not underestimated.
Conversely, Indonesia and Malaysia, both Brugia spp. EUs, had
design effects larger than 2 that may be associated with the more
sensitive detection of antibody versus antigenemia, and with the
subsequently larger number of positive cases found, particularly in
Time and Costs of These Studies
The overall average number of field days required for TAS was
26 in TAS-1 (range: 9–60) and 27 for TAS-2 (range: 12–50), using
an average number of 4 field teams (range: 3–6) with 3–4 persons
per team (Table 10). School surveys took 24–27 days on average
versus 26–33 for community surveys but the overall survey length
was highly dependent on country-specific factors including
weather, distance, and other logistic delays, particularly in the
Philippines, Dominican Republic, Indonesia, and Vanuatu.
The mean and median TAS costs in this operational research
study were $25,500 and $24,900 with the largest proportion of
costs allocated to personnel (33%) and transportation (24%)
(Tables 11 and 12). Community surveys (mean $26,800, median
$26,000) required slightly more resources than school surveys
(average $24,900, median $23,800). Project cost was moderately
correlated to the area of the EU (R2=.39). It should be noted,
however, that all costs referenced here reflect research budgets and
objectives including training, foreign consultants, and extra
specimen shipment and analysis; carried out for programmatic
purposes, costs would be expected to be less.
LF elimination programs require a standardized methodology
that is statistically robust and programmatically feasible in order to
assure confidence in making stop-MDA and post-MDA surveil-
lance decisions. In this regard, Transmission Assessment Surveys
offer a more pragmatic approach than previous WHO guidelines
and with 22 implementations of the TAS in 11 countries, this
operational research study provides the first report of a large-scale
rollout of the TAS at a programmatic level. Indeed, these field
experiences in multiple geographic and epidemiological settings
have offered a prime opportunity to evaluate the TAS protocol
critically and identify both best practices for future implementation
and important remaining research gaps.
TAS Results and Sampling Strategy
Consistent results were seen across TAS-1 and TAS-2. In the 10
EUs that passed TAS-1, the recommended decision to stop MDA
was validated in TAS-2, as no resurgence of infection was
observed above the critical cutoff value where active transmission
is anticipated as likely to occur. This finding is extremely
important from a programmatic perspective because if the TAS-
2 result had differed from TAS-1, MDA might have needed to be
restarted in the EU, which is not only a resource intensive process
but one that could be politically and socially undesirable. A final
TAS evaluation is recommended in these EUs after another 2–3
years to confirm the absence of reemerging transmission
detectable by the TAS.
The results were in-line with anticipated outcomes of the TAS
survey design and sampling strategy. Design effects for W. bancrofti
Table 5. TAS sample size by sex for school and community-based surveys.
Sex School TAS (16 surveys) Community-based TAS (6 surveys)Total (22 surveys)
Male9,894 (50.2%) 4,752 (50.1%) 14,646 (50.2%)
Female 9,798 (49.8%) 4,725 (49.9%)14,523 (49.8%)
19,692 (100.0%)9,477 (100.0%)29,169 (100.0%)
157 records were missing sex identification data.
Table 6. TAS results by age for school surveys in W. bancrofti and Brugia. spp. countries.
W. bancrofti countries1
Brugia spp. countries
Age (years)n (% of total) ICT+ + (% of age) n (% of total)
PanLF or Brugia Rapid+ + (% of
,6 694 (4.7%)0 (0.0%) 160 (2.6%)2 (1.3%)
6–712,479 (83.7%) 11 (0.1%)2,713 (44.5%)79 (2.9%)
8–101,689 (11.3%)6 (0.4%) 3,213 (52.8%) 108 (3.4%)
.1037 (0.3%) 0 (0.0%)2 (0.1%)0 (0.0%)
14,899 (100.0%)17 (0.1%) 6,088 (100.0%) 189 (3.1%)
1Includes TAS-1 ICT tests for Indonesia.
273 records were missing age data (including 1 ICT+).
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EUs fell within expected limits, and participant age and sex
reflected distributions in the target population. One notable
advantage of the TAS protocol is its inclusion of cluster surveys to
reduce the number of survey sites and overall sample size. In this
study, 8 of 11 countries used a cluster survey design although
sampling efficiency differed from TAS-1 to TAS-2. For TAS-1, a
Table 7. Comparison of target and actual sample sizes and number of clusters.
CountrySurvey Target sampleActual sample1
Am. SamoaTAS-11,042 949
Burkina Faso TAS-11,5561,5711.0% 301
TAS-2 1,556 1,5912.2% 308
Dom. Rep. TAS-11,532 1,609 5.0% 308
TAS-2 1,532 1,5581.7%400
Ghana TAS-11,556 1,5570.1% 3010
TAS-2 1,556 1,514
TAS-21,548 1,622 4.8%300
Malaysia TAS-1 1,368 1,4294.5%302
Philippines TAS-11,552 1,599 3.0%3510
TAS-2 1,5521,6566.7% 350
Sri Lanka TAS-1684679
TAS-2684 6982.0% 322
Togo TAS-11,5481,5711.5% 301
Tanzania TAS-11,540 1,5611.4%51 18
TAS-2 1,5401,588 3.1%700
Vanuatu TAS-1933 9330.0% 630
TAS-2 954 954 0.0%630
TotalTAS-1 14,859 14,811
2 20.3%390 63
TAS-2 13,830 14,4154.2% 402 10
1Excluding invalid tests and specimens unable to be tested.
2Systematic sampling; all eligible primary sampling units surveyed.
Table 8. Non-participation rates observed in TAS-1 and TAS-2.
Absent, refused, or no consent Invalid test or Unable to be tested
Country Survey siteTAS-1 TAS-2TAS-1TAS-2
Am. Samoa School--16.0%-
Burkina Faso Community-7.5% 0.9%0.3%
Dom. Rep. Community 12.6%7.2%0.6%0.1%
Ghana School 15.0%15.0%0.1% 2.9%
Indonesia School20.0%10.0% 18.3%9.5%
Malaysia School 22.9%20.4%0.3% 0.5%
PhilippinesSchool 4.0%3.0% 4.0%1.3%
Sri LankaSchool- 9.3% 0.0%1.4%
TogoSchool 12.0%8.0% 0.0% 0.0%
TanzaniaCommunity 14.7% 5.7% 0.6%1.1%
VanuatuSchool 10.7% 15.7%0.0%0.0%
Total-14.0%10.2% 3.8% 1.9%
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total of 63 extra clusters had to be selected and surveyed in
addition to the originally planned sample in order to fulfill the
target sample size. Such a process proved burdensome to survey
planning and resource allotment. In contrast, only 10 extra clusters
were needed in TAS-2 to achieve the target objective. This vast
improvement in TAS-2 is largely because of factoring in ‘non-
participators’ (i.e. absent children and those refusing to participate
or without consent) into the initial survey design calculation.
Estimates of the non- participation rate, however, might be
difficult to obtain or measure during TAS planning, as was the
experience in several of the countries in TAS-2. In such cases, a
10–15% estimated non- participation rate can be recommended
based on the results from this study (Table 9), although this rate
may vary greatly by EU and survey location. Community-based
surveys, in particular, may experience a larger non- participation
rate than school surveys because of the unreliable availability of
eligible children at specific times of the day. The amount of TAS
pre-planning and school or community sensitization is also likely
to influence non- participation rates considerably. Because the
TAS uses a fixed sampling fraction within each cluster, the
inclusion of an accurate non- participation rate into the survey
design calculation is also necessary to achieve a more accurate
sample size. More specifically, underestimating the non- partici-
pation rate would result in larger sampling intervals and, therefore,
fewer children sampled per cluster than required given the
number of clusters selected. Since the TAS presumes an equal
probability sample, extra clusters would be needed to make up the
sample size difference, as seen most notably in TAS-1.
Despite best efforts to reach sample size targets efficiently using
non- participation rates and extra clusters, our study found that
discrepancies may persist because of outdated population or
enrollment estimates, school closures, inclement weather, and
other factors including the selection by chance of several large or
small outlier schools. Non-participation is also not unprecedented
in such types of surveys and because absentees were randomly
spread out across clusters, sampling bias was likely not introduced.
Furthermore, the inclusion of extra clusters improved sample
robustness and reduced intraclass correlation between clusters.
Probability proportional to estimated size (PPES) sampling has
been investigated but preliminary assessment suggests the uncer-
tainties of actual school size and number of smaller schools with
target children below the fixed number needed would increase the
average clusters required and likely offset benefits to standardizing
the sample size . Strategic approaches to harmonize the target
and actual sample size will likely evolve as the TAS is further field
tested and evaluated. Several improvements have already been
made to the SSB tool including the input of an estimated non-
participation rate and the automatic random selection of ‘backup
Table 9. Design effects calculated for TAS-1 and TAS-2 cluster
Dom. Rep.- 1.6
Philippines 1.0 1.0
Tanzania 1.1 1.1
Table 10. Number of field days required to complete TAS-1 and TAS-2.
Survey siteCountry Field days TAS-1Field days TAS-2
Field teams TAS-1 and TAS-
Ghana 20 184
Sri Lanka 26 323
Vanuatu 25 254
Community Burkina Faso19 183
Dom. Rep.57 423
All sites Average26 274
Table 11. Total TAS operational research costs for school and
Survey siteLow HighMean Median
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clusters’ to survey in case the target sample size is not initially met.
This study also validated the overall utility and convenience of the
SSB tool with regards to simply determining the proper survey
design, calculating sample sizes and sampling intervals, and
randomizing cluster and child selection lists. Future TAS should
continue using the SSB tool for survey planning.
The TAS protocol identifies 6–7 year old children as the target
age group. While no positive cases were found outside the 6–10
year age range, a narrower sampling frame of 6–7 year olds is
believed to be more epidemiologically accurate and programmat-
ically feasible to avoid larger sample sizes . In school surveys,
6–7 year olds are approximated by 1st–2ndgrade children. This
approximation, however, proved ambiguous in countries where
the target ages and grades did not effectively align. For example, in
Ghana, children 8–10 years were frequent in 1st–2ndgrade. In
Malaysia and Indonesia, 1st–2ndgrade typically corresponds to 7–
8 year old children. Furthermore, some countries including Togo
interpreted the guidelines as only including 6–7 year olds within
1st–2ndgrade as the target population. Therefore, although the
results show that 6–7 year old children still comprised the majority
of all school surveys, the clarification of the age requirement in the
TAS protocol is extremely important for planning and calculating
an accurate survey design. To this end, the general guideline in the
SSB tool has been revised for programs to specifically select the
grade(s) in which 6–7 year old children are most likely to be found
and then to use those grade(s) as the eligible target group for school
surveys. This refined terminology was implemented successfully in
the Vanuatu study and is likely to benefit and simplify future TAS
implementations as well.
Specimen Collection and Diagnostic Tests
Specimen collection procedures were closely examined within
the context of an operational research protocol that involved
collecting blood into an EDTA-coated tube that would be
transported and analyzed in a central laboratory, as opposed to
directly conducting the ICT (or PanLF, Brugia Rapid) tests in the
field. The perceived advantage of this method was to streamline
blood collection in the field while being able to perform the
diagnostic tests in a more controlled environment. This strategy
proved adequate under operational research conditions to evaluate
quality and consistency; however, it introduced logistic challenges
in terms of transportation, time, and supplies. In addition, it was
observed that field staff may be unfamiliar with drawing blood into
EDTA tubes and basic pipetting techniques. This method was also
more challenging for follow-up testing or where there was
insufficient blood quantity or clotting. As a result, it may be more
efficient programmatically for teams to conduct diagnostic tests in
the field, directly transferring blood from the finger prick to the
ICT or Brugia Rapid card with a calibrated capillary tube. This
process was carried out successfully in Vanuatu, Indonesia, and
Malaysia because of logistic restrictions that are likely to be
duplicated in other TAS-eligible EUs. However, because the rapid
diagnostic tests are extremely time sensitive and require good
lighting, it is highly recommended that one team member be
specifically assigned to timing and reading the tests in an area with
sufficient lighting. However, in community surveys where house-
to-house visits are more time consuming and on-the-spot
diagnostic testing is likely to exacerbate this constraint, especially
when surveys are conducted in the afternoon or evening, lighting
becomes more restricted and it might be preferable to collect
blood in EDTA tubes for later analysis.
The performance and reliability of the diagnostic tests used for
the TAS are undoubtedly critical to the success of the survey. In
TAS-1, all positive ICT tests were immediately followed-up with a
repeat test to confirm the initial finding. In all 33 positive cases, the
original and repeat ICT tests were both positive, indicating 100%
positive concordance. Despite this limited sample size, repeat ICT
tests are deemed unnecessary under current TAS programmatic
guidelines. More importantly, however, the field experiences here
showed that the quality and consistency of ICT results can be
strongly improved with robust training and strict adherence to
reading the cards after exactly ten minutes. A newer filariasis test
strip with potential greater sensitivity and reduced susceptibility to
heat will only improve the accuracy of TAS results although it may
require the adjustment of critical cutoff values and sample sizes
Mf tests using blood smear (TAS-1 and TAS-2) and PCR
methods (TAS-1 only) were examined in this study and showed
that positive concordance to antigen (W. bancrofti) and antibody
(Brugia spp.) results were comparable to previous studies, albeit with
much smaller sample sizes . Programmatically, however, the
ICT and Brugia Rapid tests remain more suitable as the primary
TAS diagnostic tool given their convenience advantages. Mf tests
may best be utilized as a positive-case follow-up tool to test for
potential hotspots, focal transmission, or spatial clustering.
School versus Community-Based TAS in Targeted EUs
The community-based TAS studies in Burkina Faso and
Tanzania highlighted several specific challenges; in particular,
both had trouble finding children in the daytime and poor census
and map accuracy led to difficulties estimating the target age
group, enumerating houses, and defining EA boundaries. While
not especially pronounced in these studies, non-participation rates,
cost, and time can all be reasonably assumed to be higher in
community TAS than in school TAS. Of note, the number of field
days for school surveys was heavily skewed by the considerable
Table 12. Allocation of TAS costs by spending category.
Description% of total costs
Personnel (per diems)33%
Transportation (fuel, vehicle hire) 24%
Diagnostic tests (procurement, shipment, customs)15%
Consumable supplies (e.g. lancets, EDTA tubes) 14%
Communication (e.g. printing, mobile phone data)3%
Other (e.g. training, consultants, sensitization, specimen shipment)11%
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time taken in the Philippines due to severe weather and poor
accessibility to insecure areas in the EU. Moreover, the level of
planning, training, sensitization, and field effort required for the
community-based surveys in Burkina Faso and Tanzania were
qualitatively much higher as reported by field staff and supervisors.
Perhaps if more community-based TAS were conducted in this
study and if time included the planning stage and was measured in
person-hours rather than days, differences between school and
community-based surveys would have been more evident. Com-
munity-based TAS are also limited by having to often sample
eligible children on evenings or weekends outside of regular school
hours. A more critical assessment of the 75% enrolment rate
requirement for TAS school surveys could, therefore, have
important implications if this threshold could be justifiably lowered.
A comparison of school and community-based TAS is also
important to disprove any selection bias that may occur by only
sampling school children, namely that those not attending school
may also not be attending MDAs and are at a higher risk for
infection. Preliminary results from separate TAS studies appear to
suggest there is no statistically significant difference or change in the
TAS-recommended outcome for EUs with school primary enrol-
ment rates as low as 59% . Although the majority of TAS EUs
are still likely to qualify for school surveys, validation of such results
would greatly streamline the overall efficiency of the TAS sampling
strategy if school surveys could be used on a wider or exclusivebasis.
The composition of the TAS EU requires careful consideration
to ensure that uniform epidemiological conditions persist across
the EU. Despite the TAS being designed to provide an accurate
EU-wide assessment, an EU that is smaller in area would
presumably be more likely to include a self-sustaining subpopu-
lation in its cluster sample (if such a ‘hotspot’ existed), but it might
also be more cost prohibitive at a regional or national scale. In
contrast, combining multiple IUs into one larger EU is more cost-
effective, but clusters are spread more thinly across the EU and
may miss potential hotspots where infection may persist in a focal
area despite the overall EU successfully passing the TAS. A simple
linear regression analysis of the EUs in this study showed moderate
correlation between the cost of the TAS and EU area size,
although cost is dependent on the geographical setting (e.g.
transportation costs in Vanuatu were understandably greater than
in Togo and Ghana despite relatively similar EU area sizes). The
maximum limit of 2 million people for an EU also requires
evidence; however, as the average EU population here was
approximately 250,000 with a maximum of 682,000, no informa-
tion about the validity of extremely large EU populations can be
ascertained from this study.
Identification of cost and epidemiological appropriateness of
EUs may also be aided by spatial modeling or related research to
determine additional criteria that is pertinent to defining an ideal
EU size or cost for TAS. Although there was no evidence of major
differences between rural and non-rural clusters in our study,
MDA coverage and compliance might differ considerably in both
areas. Likewise, cross-border infection with high-endemic neigh-
boring IUs or other countries may increase the risk of transmission
into the TAS EU. In the Dominican Republic study, some
evidence of cross-border infection from Haitian immigrants was
described in bordering EAs. Other high-risk factors could persist in
specific parts of an EU but not others. In the Philippines, a census
evaluation of 533 TAS-eligible children was conducted in a sub-
area of the EU where there is a high concentration of certain
axillary plants known to support breeding of LF vectors and
increase inhabitants’ risk of exposure and infection. Though no
positive cases or significant difference from the rest of the EU was
detected in the high-risk area (unpublished data), such factors
should be carefully examined and accounted for when classifying
TAS EUs in order to maintain a fairly homogeneous EU so far as
risk of LF infection can be assessed.
TAS is currently recommended for EUs in post-MDA
surveillance mode using an identical methodology to EUs
evaluating the decision to stop or continue MDA. The results in
this study support the reliability of this strategy but because TAS is
not powered to detect change or designed to identify hotspots,
post-MDA surveillance would best be complemented in the short
and long term with other, complementary diagnostic tests and
surveillance methods. In particular, antibody testing using Bm14,
Bm33, or Wb123 assays may be highly suitable for post-MDA
surveillance because it is more sensitive than antigen testing and
may be superior to TAS for early detection of residual or resurgent
LF infection. Initial findings from American Samoa and Haiti
comparing filarial antigen and antibody responses seem to indicate
that the antibody responses may be early markers of infection and
not just exposure [19,20]. The development of multiplex tools for
NTD surveillance further facilitates the ability to conveniently
examine several parameters at once [21,22]. Xenomonitoring may
also be a useful complementary post-MDA surveillance strategy
because advances in molecular technology give it the potential to
identify low-level LF infection in vector mosquitoes while being
‘non-invasive’ to the human population. Particularly in the
majority of countries where filariasis is transmitted by Culex
mosquitoes, efficient collection techniques exist and early results
have been promising [23–25]. Furthermore, preliminary analysis
of mosquitoes collected in American Samoa and Sri Lanka, in
conjunction with these TAS studies, shows that xenomonitoring
may provide comparable transmission markers and offer a cost-
effective addition to the periodic post-MDA surveys where
appropriately trained entomology teams are available (unpub-
lished data). Longer term, post-TAS surveillance may also best be
met through passive surveillance strategies using appropriate
sentinel groups for routine blood monitoring or through malaria-
or other disease-surveillance efforts [12,22,26].
Utilizing the antibody-based critical cutoff values for Brugia spp.
EUs remains a concern for the current TAS protocol. While
successfully passing the TAS based on more conservative
thresholds increases the confidence of the results, the antibody-
based thresholds may be overly restrictive, compared to the
antigen-based thresholds for W. bancrofti. Additionally, the design
effects calculated in the two Brugia spp. TAS (Indonesia and
Malaysia) were notably higher than those assumed for calculating
TAS sample sizes. In Malaysia, the large design effect can be
partially attributed to a greater number of positive cases found in
the EU than normally presumed by TAS. In Indonesia, however,
the sample size and number of positive cases were similar to
Burkina Faso yet the design effect was 2–3 times greater. Such
findings may be indicative of inherent epidemiological differences
of the respective EUs, but also warrant further investigation of the
implications of evaluating filarial antigen and antibody using the
same decision criteria.
Interruption of ongoing LF transmission and cessation of MDA
in an LF endemic area are milestone achievements but ones that
require careful determination and accurate assessment. TAS
guidelines are currently in place for stopping MDA and post-MDA
surveillance and can be carried out effectively and efficiently with
recommendations and best practices identified through the
operational research experiences here. While the general sampling
strategy has proven to be robust and pragmatic, thresholds and
sample sizes may need to be modified as new diagnostic tools
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become available and validated. The ability of the TAS, however,
to detect recent or ongoing LF transmission in hotspots within an
EU that passes the critical threshold is still untested and requires
longer-term empirical evidence. Additional research into the
composition of EUs and mechanisms for hotspot detection and
post-MDA surveillance will only help evolve and strengthen the
current guidelines. From a broader perspective, the survey design
principle of the TAS can be realistically applied and adapted to
other NTDs as they reach similar points in their programs. The
TAS may also provide a very opportune platform and sampling
strategy to integrate assessments for co-endemic NTDs such as
onchocerciasis and STH. Continued deployment and refinement
of the TAS, therefore, is essential not only for LF elimination
programs but potentially to the wider NTD community as well.
The authors of this paper would like to express their deepest thanks for the
invaluable assistance of all local country field staff and the teachers,
community leaders, volunteers, and children participating in this study.
Sincere gratitude is also given to the many individuals who helped in the
planning, implementation, and reporting of this multicenter evaluation,
including Salissou Adamou Bathiri, Mark Bradley, Yaya Couilibaly,
Massitan Dembele, Johnny Gyapong, Rafe Henderson, PJ Hooper, Julie
Jacobson, Kazuyo Ichimori, Ramaiah Kapa, Sandra Laney, Gabriel
Matwale, Khalfan Mohammed, Chamila Nagodavithana, Catherine
Plichard, and Steve Williams.
Conceived and designed the experiments: BKC MD KG DK PJL EAO
KYW. Performed the experiments: BKC NKB WRB AMD MES PUF
KG MGdP LMH DK RMF UJM RN IOO AP RUR DS MAS SS PES
TS FT KYW. Analyzed the data: BKC MD NKB WRB AMD MGdP
LMH UJM RN IOO DS SS TS FT GJW KYW. Contributed reagents/
materials/analysis tools: AP NP MT. Wrote the paper: BKC MD EAO.
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Transmission Assessment Surveys for LF Endpoints
PLOS Neglected Tropical Diseases | www.plosntds.org12 December 2013 | Volume 7 | Issue 12 | e2584