Role of environmental poliovirus surveillance in global
polio eradication and beyond
T. HOVI1*, L. M. SHULMAN2, H. VAN DER AVOORT3, J. DESHPANDE4,
M. ROIVAINEN1AND E. M. DE GOURVILLE5
1National Institute for Health and Welfare (THL), Helsinki, Finland
2Central Virology Laboratory (CVL), Ministry of Health, Sheba Medical Center, Tel-Hashomer, Israel
3National Institute of Public Health and the Environment (RIVM), Bilthoven, The Netherlands
4Enterovirus Research Centre (ERC), Mumbai, India
5Global Poliomyelitis Eradication Initiative, WHO, Geneva, Switzerland
(Accepted 21 December 2010)
Environmental poliovirus surveillance (ENV) means monitoring of poliovirus (PV) transmission
in human populations by examining environmental specimens supposedly contaminated by
human faeces. The rationale is based on the fact that PV-infected individuals, whether presenting
with disease symptoms or not, shed large amounts of PV in the faeces for several weeks. As the
morbidity:infection ratio of PV infection is very low, this fact contributes to the sensitivity of
ENV which under optimal conditions can be better than that of the standard acute flaccid
paralysis (AFP) surveillance. The World Health Organization has included ENV in the new
Strategic Plan of the Global Polio Eradication Initiative for years 2010–2012 to be increasingly
used in PV surveillance, supplementing AFP surveillance. In this paper we review the feasibility
of using ENV to monitor wild PV and vaccine-derived PV circulation in human populations,
based on global experiences in defined epidemiological situations. This article may not be
reprinted or reused in any way in order to promote any commercial products or services.
Key words: Infectious disease epidemiology, polio, polio vaccine virus, public health, surveillance.
Success and remaining challenges of the
Global Polio Eradication Initiative (GPEI)
The GPEI, launched at the World Health Assembly
(WHA) in 1988, is the single largest, internationally
coordinated public health project the world has ever
known . It is spearheaded by national governments,
the World Health Organization (WHO), Rotary
International, the US Centers for Disease Control
and Prevention (CDC) and UNICEF. Administration
of poliovirus (PV) vaccine to every child had been
recommended by WHO since the 1970s in the context
of the Expanded Programme of Immunizations (EPI)
but the coverage never reached sufficiently high levels
to stop PV circulation globally. In contrast, exploiting
the key principles of GPEI [1, 2], especially together
with the designated supplementary immunization ac-
tivities (SIA, Table 1), rapidly changed the situation.
Although the initially planned deadline of eradi-
cation, the year 2000, is well in the past, the success of
the programme has been marked (Table 2). The num-
ber of new cases has been reduced by more than 99%
and no cases due to wild-type poliovirus 2 (WPV2)
have been identified since 1999 [5, 6]. Furthermore,
genetic diversity of the remaining WPV of serotypes 1
* Author for correspondence: Professor T. Hovi, National
Institute for Health and Welfare (THL), Mannerheimintie 166,
PO Box 30, Helsinki 00271, Finland.
Epidemiol. Infect., Page 1 of 13.
WHO has granted permission to Cambridge University Press to publish the contribution written by WHO.
This article may not be reprinted or reused in any way in order to promote any commercial products or services.
f Cambridge University Press and World Health Organization, 2011.
and 3 has strongly decreased, indicating limited cir-
culation. Since the start of the millennium, fewer than
2000 cases caused by WPVs have been reported an-
In spite of the success, obstacles still confront
the desired target. Re-emergence of WPV poliomyel-
itis in many previously polio-free countries demon-
strates that herd immunity in some polio-free
developing countries may be extremely fragile .
Even temporary deficits in vaccination coverage can
sometimes be sufficient to facilitate circulation of
imported WPV . At the dawn of the millennium,
yet another obstacle for global eradication of polio-
myelitis was recognized: vaccine-derived polioviruses
(VDPVs), genetically drifted strains that regain
neurovirulence, have caused several outbreaks of
paralytic poliomyelitis in different parts of the
The standard approach recommended by WHO for
polio surveillance is the detection and investigation of
cases of acute flaccid paralysis (AFP), which includes
standardized virological analysis of two faecal sam-
ples of the patient, and/or sometimes those from
contacts . Although AFP surveillance is in principle
applicable to any human population at any time,
there are situations in which there are good reasons to
suspect that negative results of AFP surveillance are
not reliable. Supplementary information is required in
such situations and one approach for that is environ-
mental surveillance (ENV), in which a search for PV is
made in environmental specimens contaminated by
human faeces .
Table 2. Selected indicators of the success of the Global Polio Eradication Initiative
No. of new cases per year
No. of endemic countries
No. of endemic WHO Regions
No. of circulating wild WPV serotypes
No. of remaining WPV genotypes#
WPV, Wild-type poliovirus.
Since 1988, >2 billion children around the world have been immunized against polio, thanks to the unprecedented
cooperation of more than 200 countries and 20 million volunteers, backed by an international investment of more than
US$ 5 billion.
# A genotype is a group of genetically closely related poliovirus strains (difference in capsid protein VP1 coding sequence
f15%) that are considered to have epidemiological linkage with each other .
$ O. Kew, personal communication.
Table 1. Original principles and current objectives of the Global Polio Eradication Initiative
Original key principles  Current key objectives 
(1) High coverage routine immunization of
newborns with the oral poliovirus vaccine (OPV)
(2) Active surveillance for and standardized
investigation of patients with acute flaccid
paralysis (AFP) including virological analysis
of faecal samples to detect polioviruses
(3) Supplementary immunization activities (SIA)
including National Immunization days and
house-to-house targeted mop-up rounds to
rapidly boost immunity to polioviruses in
children aged <5 years
(1) Interrupting wild poliovirus transmission in Asia
(2) Interrupting wild poliovirus transmission in Africa
(3) Enhancing global surveillance and outbreak response
(4) Strengthening immunization systems
2T. Hovi and others
ENV OF PV CIRCULATION
Rationale of ENV is based on the natural course
of PV infection
It is well known that the ‘typical paralytic case of
poliomyelitis’ is actually an exception in the natural
course of PV infection: more than 99% of PV infec-
tions proceed without paralysis. Irrespective of the
presence or absence of clinical symptoms, PV repli-
cation is considered to continue in the para-intestinal
submucosal lymphatic tissue from several weeks to a
few months. The virus is excreted into the faeces and
shed into the environment. This forms the rationale
be excreted in the nasopharyngeal mucosa during the
first 2 weeks of infection. This is considered to have a
role in virus transmission between close contacts but
is not likely to contribute strongly to the load of virus
in the environment. The amount of virus excreted into
stools is known to be variable with maximal amounts
reaching 107infectious doses/day per person . PVs
are relatively stable in aqueous environments at am-
bient temperatures and adsorption to various solid
materials in the environment may further extend the
time over which at least part of the infectivity can be
recovered. On the other hand, industrial waste, and in
tropical countries, increase of temperature and UV
irradiation from sunlight inactivate the virus in a
time-dependent manner, a fact that has to be taken in
account in designing ENV. In short, ENV offers an
anonymous, non-invasive approach for monitoring
PV circulation in populations at risk.
Selected strategic and technical aspects
as a supplementary approach in GPEI were published
in WHO Guidelines for Environmental Poliovirus
Surveillance . The Guidelines describe situations
when it might be necessary to consider ENV as a sup-
plementary approach, contain principles for selecting
sampling sites, propose methods for sample proces-
sing, and suggest possible programme responses to
PV detection in sewage. Further, the Guidelines state
the need for identification of resources for sample col-
lection and transport to a dedicated laboratory as well
as availability of trained staff, necessary equipment
and supplies in the laboratory. Two main factors limit
the wider application of this approach; first, the lack
of sewer networks in most of the world poses a chal-
lenge for identifying representative sampling sites,
and second, the analysis of the samples with current
of us (E.dG.) has calculated for GPEI that a ‘start-up’
cost of equipment would be about US$ 50000 for
a laboratory already involved in PV isolation from
clinical specimens, and the cost of supplies for ana-
lysing 100 specimens about US$ 33000. Sample col-
lection is not expensive but training and staff salaries
to an almost 10-year experience in Egypt, processing
and analysis of 100 sewage samples in the laboratory
requires trained staff at about the same level as for
processing and analysis of stools from 200 AFP cases
with two specimens from each case (L. El-Bassioni,
Selecting sampling sites
For systematic environmental PV surveillance it is
optimal but not obligatory that most, if not all,
households are equipped with water closets connected
to a converging sewer network (Fig. 1) allowing
collection of downstream samples that represent a
large number of people living in the catchment area.
The number of people living in the catchment area
affects the sensitivity of PV detection in a population
in two ways: by increasing the area it is possible to
monitormore people withfewersamples.Onthe other
hand, this increase is likely to diminish the sample
sensitivity, i.e. capacity to detect small numbers of PV
excretors in the population as the increasing number
of non-excretors may dilute the virus to below the
limits of detection. In practice, the size of the source
population in established systems where WPV and
VDPV have been detected has varied from tens of
thousands to a few millions.
Modelling of factors affecting sensitivity
Ranta and co-workers  have presented a math-
ematical model visualizing how different factors in-
fluence the proportion of excreted PV which can be
recovered at a downstream sampling site, and thus
sample sensitivity or population sensitivity, respect-
ively, in ENV (Fig. 2). The model also demonstrated
how different factors affecting the sensitivity of the
approach are inter-related, e.g. how less than optimal
virus detection sensitivity could be compensated
for by collecting larger volumes for analysis, and re-
vealed, unsurprisingly, that for detection of emerging
Environmental poliovirus surveillance3
outbreaks of virus circulation, frequently repeated
sampling is critical. Applied at the standard bi-weekly
analysis of 1 litre of sewage for PV in the Greater
Helsinki Region representing about 700000 in-
habitants, the model predicted that an emerging PV
circulation would have been detected within a few
months or at least as quickly as by using optimal AFP
surveillance. This prediction was in reasonable agree-
the fate of a known amount of attenuated PV flushed
into the sewerage system. The authors of the report
of that trial  calculated that by analysing a single
400-ml specimen, PV circulation could have been de-
tected if about 100 individuals were infected with PV.
Optimization of sample collection schedules
When monitoring large populations served by a
complex sewer network, simple daily grab samples are
likely to be sufficient as transport of any input virus
bolus is partially delayed, and virus concentration at
the sampling site is not affected by toilet-use fre-
quency of the source population . The closer the
sampling site is to the source, the higher the prob-
ability of detection. However, if the sampling site is
very close to a suspect population, samples may not
be sufficiently dispersed and a composite sampling
system, i.e. daily pooled samples composed of hourly
collected aliquots, may be necessary in order not to
miss the peak virus concentrations in the sewage.
Detection of rare viruses from a complex mixture
Various techniques have been used for sewage sample
collection and processing. In most cases, the proces-
sing includes a concentration step, up to 100-fold. It is
obvious that the exploited approaches differ in sensi-
tivity but no systematic comparative studies have
been published, so the relative specificities and effi-
cacies of different methods will not be commented
upon here. Thus far, attempts to directly use mol-
ecular detection methods for analysing sewage
site in the
x, y, z, q, r, s
Fig. 1. Use of converging sewage systems for monitoring poliovirus circulation. A schematic picture of a sewer network with
small branches starting from a single house or group of houses, joining stepwise to larger sewer branches, and finally ending
in a sewage treatment plant. The inlet to the plant is the primary sampling site for monitoring the population served by this
network. Two specific features are visualized. (1) Small circles with lower-case letters represent different viruses circulating in
different locations of excretors throughout the catchment area of main sewer line A. At the main sampling site these viruses,
plus those putatively derived from main sewers B and C, may appear as a complex mixture. (2) The dashed circle labelled PV
represents a situation where a single or small group of poliovirus (PV) excretor(s) live in a geographically restricted subregion
of the catchment area. Decreasing numbers at successive sites of network junctions ( ) along the trunk lines leading towards
the processing plant indicate a decreasing likelihood of PV detection as the concentration of virus is diluted by wastewater
from converging lines. In case wild type or vaccine-derived poliovirus is detected at the primary sampling site, the same
numbers reflect the order and positioning for secondary (and stepwise forward) sampling sites that are necessary in order to
reveal the PV excretor(s).
4 T. Hovi and others
concentrates have not been very successful presum-
ably because, among other things, one or more com-
ponents of the sample may inhibit the reverse
transcriptase or DNA polymerase used in the meth-
ods. It is hoped that these obstacles can be eliminated
in the near future because using cell culture for PV
detection is slow, labour-intensive, and expensive.
In the cell culture approach, a PV-selective cell line
(L20B) is used to avoid masking by other cytopathic
ent in the sample. Regular immunizations or cam-
paigns with oral poliovirus vaccine (OPV) in the
target population may result in abundance of Sabin-
like (SL) PVs in the sewage, which have the potential
to interfere with the detection of lower concentrations
of WPV or VDPV potentially present in the same
samples. Use of multiple parallel cell vials with stan-
dard monolayer culture has coped with this problem
surprisingly well, as described below for Egypt and
India (Table 3). In Egypt, increasing the number of
parallel vials inoculated with a given sample usually
increased the number of different virus strains iso-
lated, i.e. improved the sample sensitivity. At the same
time, it limited the number of samples that could be
bly decreased the population sensitivity of detecting
virus circulation . While the laboratory workload
per sewage sample is several times that caused by a
faecal sample, it should be borne in mind that with
sewage samples it is possible to monitor a large group
of people rather than an individual.
In Israel, another cell culture-based approach has
been used. When OPV was used alone, or in combi-
nation with inactivated poliovirus vaccine (IPV), the
sewage surveillance protocol employed plaque iso-
lation of all PVs in a sewage sample on L20B cells
PVs plaques in each sewage sample by a 5-day tem-
perature selection at 40 xC of each plaque on HEp2C
cells, and subsequent molecular analysis of the heat-
resistant strains [14, 15]. This 5-day temperature selec-
tion step was discontinued when the detection of
Fig. 2. Factors influencing recovery of poliovirus (PV) from the sewage system. Circles with a cross represent steps where
uncontrollable factors (steps 1 and 2) or controllable factors (steps 3 and 4) may affect the end results. PV aggregates are
dispersed and fluctuations of concentrations smoothed by the complexity of the sewage systems that include turbulence from
high flow rates and converging streams from multiple trunk lines at branch points and in some cases from pumping stations
that lift sewage upwards from low-lying areas to allow continuation of gravitational flow to the processing plant. All PVs
excreted by the source population will not be readily detectable at the sampling site as some will be inactivated during transit,
faecal matter from disposable baby diapers is disposed elsewhere among solid waste, and the complexity of the sewerage may
result in retention of some of the input virus preventing it from ever reaching the sampling site or delaying its passage so that
it arrives after the sample is taken.
Environmental poliovirus surveillance5
vaccine viruses in the samples stopped after Israel
switched to exclusive use of IPV in 2006.
Quality assurance of a non-standardized approach
While the virus detection from ENV specimens by cell
culture isolation and characterization of possible PV
isolates follows the standard principles and is carried
out by the laboratory network designed for AFP sur-
veillance, the sample processing methods have not
been standardized. A suggested quality assurance
standard for sewage surveillance is that at least 30%
of all sewage samples at any sampling site should be
positive for non-polio enteroviruses ). The figure
may depend on various factors but experience from
routine use indicates that it is easy to reach in different
countries. In both Finland and Israel the percentage
has been about 80% in recent years (routine surveil-
lance data of THL and Central Virology Laboratory,
In the following sections we will briefly discuss ex-
periences from the use of ENV in attempts to answer
selected specific epidemiological questions and review
some relevant observations from routine use of ENV
in PV surveillance.
Use of ENV as a tool to solve specific epidemiological
Studies aimed at assessing the extent of a polio
outbreak in a population
PV infections occur at least 100-fold more frequently
than paralytic cases caused by the infection. In a
routine AFP reporting system as well as under passive
notification of paralytic cases of poliomyelitis, infor-
mation about the circulation of the causative PV
may underestimate the geographical range, the sub-
populations affected and the length of circulation. As
ENV is not case-driven but is capable of revealing any
type of infections, it has been used to complement the
picture of outbreaks based on reported clinical cases.
Three major series of observations in this category
will be briefly described, corresponding to outbreaks
in Finland (1984), Israel (1988) and The Netherlands
Finland had been free of WPV for 20 years when an
outbreak of WPV3 took the country by surprise in
late 1984 [16, 17]. As several contacts of the index case
also excreted a similar WPV3, ENV was utilized to
assess the extent of virus circulation, first in the
Greater Helsinki region and later throughout the
country . The epidemic WPV3 strain was found in
18/26 samples collected in December 1984–January
1985 in 13/15 different locations in the Helsinki region
and in 14/21 sites elsewhere in the country. Most of
the latter locations were sampled only once and most
of the respective districts had shown no PV infection-
associated cases of AFP at all . In other words,
under these conditions ENV provided crucial sup-
plementary information indicating a wide geographi-
cal extent of WPV3 circulation in the country.
In Israel, the anti-polio immunization programme
towards the end of 1980s was mainly based on the use
of OPV but included a small region of the country
with exclusive enhanced-potency IPV administration.
An outbreak discovered in 1987 was caused by WPV1
with cases in different parts of the country without a
Table 3. Examples of variation of isolation results from five parallel L20B cell vials (modified from a table
originally published in )
Isolation results for individual vials
Vial no. 1 Vial no. 2 Vial no. 3Vial no. 4Vial no. 5
–, No cytopathic effect; WPV, wild type poliovirus=non-Sabin-like; SL, Sabin (OPV)-like poliovirus; DR, double reactive
poliovirus (inconclusive result in the intratypic-differentiation test).
Numbers after WPV, SL, and DR indicate the serotype of poliovirus.
6T. Hovi and others
simple correlation to the previous immunization his-
tory of the respective population . Here also, the
response after the detection of the outbreak included
establishment of monthly ENV extending into wider
regions of the country. WPV1 was detected in 6/197
sewage samples collected during the outbreak in
1987–1988. As in Finland, ENV detected the epidemic
virus in areas of Israel, where no paralysed cases were
reported [19, 20].
In The Netherlands, in 1992–1993 an outbreak of
poliomyelitis was detected in the unvaccinated min-
ority group of the Dutch population that refuses
vaccination for religious reasons . Neither cases
nor virus were detected in the majority of the popu-
lation immunized with IPV only. Overall, WPV3 was
detected in 23/269 sewage samples collected at 120
virus was found only in sewage derived from villages
mainly inhabited by the unvaccinated people, but not
in regions where the rest of the regularly immunized
Dutch population lived. In some locations, the virus
was detected in sewage a few weeks before the first
cases of poliomyelitis were reported in the same re-
gion, demonstrating the potential power of ENV to
herald epidemics and in outbreak investigations .
Search for putative WPV reservoirs and monitoring
the decline of WPV transmission
Molecular epidemiological analysis of PV strains iso-
lated from successive cases in a region occasionally
shows ‘jumps’ of evolution in a given lineage. This
is interpreted as indicating either suboptimal AFP
surveillance resulting in missed cases, or, importation
of the virus from an unknown reservoir of virus
transmission. Targeted ENV could be used as a sup-
plementary tool to attempt to discover which of the
two alternatives is likely to be correct.
At the end of 1990s, Egypt was faced with sus-
pected gaps in PV surveillance. ENV was initiated in
2000 with sewage samples collected from inlets of
sewage plants in two provinces and later extended to
cover most provinces of the country. Rather than
uncovering a presumed distinct reservoir region, ENV
rapidly showed that WPV1 was circulating in many
regions of the country, not only in those provinces
which had shown paralytic cases in recent years, but
in several others as well including some distant ones
. Interestingly, in 2002 ENV in the Gaza district
detected importation of PV related the Egypt ENV
lineages . These observations resulted in inten-
sified immunization campaigns and improved AFP
surveillance throughout Egypt. The still continuing,
intensive ENV in Egypt later revealed the last in-
digenous WPV1 case in January 2005, 1 year after
the last paralytic case, and, in 2008, two introductions
of WPV belonging to separate non-indigenous geno-
types (Table 4). No associated paralytic cases were
found in Egypt during follow-up investigations, but
responsive precautionary SIAs were conducted .
stages of the eradication programme. Like the situ-
ation in Egypt, one of two genetic lineages of WPV
isolated from samples collected in open canals in
healthy contacts investigated using AFP surveillance
In India, following the nationallaunch of the GPEI,
WPV transmission significantly declined and by the
early 2000s was mainly restricted to the northern
states of Uttar Pradesh and Bihar while, for instance,
in the western coast island city of Mumbai, only
sporadic cases occurred, mainly in the slum areas. In
the absence of sewage networks, wastewater samples
were collected from the open canals containing waste
from populous areas  WPV strains were frequently
isolated from samples even in the absence of concur-
rent paralytic cases in the corresponding population
(Table 5). Phylogenetic analysis later showed that
virus strains detected in these samples were rather di-
verse as regards their origin with representatives from
the still endemic northern parts of India. Rather than
Table 4. Wild poliovirus (WPV) transmission in Egypt
monitored with acute flaccid paralysis (AFP)
AFP cases Sewage samples
* Two separate importations of WPV1 strains genetically
unrelated to indigenous strains.
# Data up to 7 December.
Environmental poliovirus surveillance7
representing a local reservoir of PV circulation, re-
peated isolation of WPV in the slums thus appeared
to be caused by repeated introductions of the strains
along with people moving from the endemic regions
to Mumbai. Along with the success of control of
poliomyelitis in India, the number and the genetic
diversity of virus strains isolated from the sewage
in Mumbai has also decreased (routinely reported
data provided to the GPEI). WPV detection in the
Mumbai sewage is taken into account by local auth-
orities in planning SIAs. WPV has also been detected
in sewage in Lucknow, a city situated in a high-risk
area in northern India, during a research project be-
tween 2004 and 2006, and demonstrated silent circu-
lation of WPV in the absence of detected poliomyelitis
cases in that city . Since 2010, ENV is also being
used as a supplementary approach in Delhi.
Evaluation of emerging transmission of OPV-derived
viruses after immunization campaigns or a switch to
It is generally thought that possible transmission
chains after OPV administration are very short and
this is indeed likely to be true in OPV immunized
populations with good herd immunity. Recent circu-
lating VDPV (cVDPV) outbreaks have, however,
raised concern that, under certain circumstances, es-
pecially following decreased OPV coverage , the
OPV strains have the potential to cause sustained
transmission and paralytic disease [29–39]. A widely
held view is that IPV immunization does not effec-
tively protect against PV replication in gut mucosa,
and hence, a switch from OPV to IPV alone might
result in a situation where OPV-derived viruses from
the last period of use, from chronically infected per-
sons, or imported from other locations, might be able
to be transmitted. Several observational studies
provide evidence against this suspicion and, in some
situations, ENV has been utilized to gather this evi-
dence (Table 6). Many industrialized countries have
successfully switched from routine OPV immuniza-
tion, or from combined OPV-IPV schedules to IPV-
only programmes. Such countries generally rely on
routine AFP surveillance or on passive case notifi-
cation only. No signs of emerging transmission of
OPV-related viruses have been reported.
Thus, so far all available data support the view that
it is safe to switch from OPV to IPV in industrialized
countries with high polio immunization coverage.
However, there is a question about whether the same
will be true in developing countries with high popu-
lation density and poor sanitation. To evaluate this
question, a study was designed and has been im-
plemented on the island of Java, Indonesia, to moni-
tor the range of genetic drift of PV strains detected in
the environment before and after a switch from OPV
to IPV in Yogjakarta province. The switch took place
in 2007, and to date there has been no evidence of
circulating OPV-derived strains (R. Sutter et al., un-
Detection of importations of OPV strains, WPV
and VDPV by routine ENV
Both Finland and Israel have used ENV as the main
approach to PV surveillance for decades. In addition,
several countries are currently using regular ENV as a
Table 5. Wild poliovirus (WPV) and vaccine-derived poliovirus (VDPV) detection in sewage in Mumbai
compared to results of acute flaccid paralysis (AFP) surveillance
Greater Mumbai regionAll India
Environmental AFP surveillanceAFP surveillance
No. with WPVNo. of
No. with WPVNo. of
No. with WPV
Type 1Type 3 Type 1Type 3 Type 1Type 3
* The laboratory processing environmental surveillance specimens was closed from mid-April 2006 to the end of April 2007
because of a fire.
# One VDPV strain of each serotype.
8 T. Hovi and others
supplementary approach in their national PV sur-
In countries using OPV in regular immunizations,
PV SL strains are frequently isolated from sewage.
IPV is exclusively used for polio immunizations in
Finland, but annually millions of tourists travel to or
from neighbouring Estonia and/or Russia where OPV
has been used until very recently. About 60 samples
are collected annually in Finland and the sampling
sites cover about 20% of the population. While not a
single PV was isolated from the Finnish sewage sam-
ples between autumn 1985 and 2005, at least one SL
PV has been isolated from the samples annually since
2006 (routine data of THL). Similarly, ENV has de-
tected OPV strains in sewage samples in another
exclusive IPV country, The Netherlands (routine data
of RIVM), suggesting that ENV might also readily
detect WPV importations. Indeed, WPV importations
have been detected in the absence of clinical cases in
Israel and the Gaza district in spite of vaccine cover-
age above 95% and use of OPV in regular immu-
nizations [14, 15].
The catchment areas in Israel currently serve
30–40% of the population and since 1989, >25 sites
were sampled monthly between 1989 and 2001, and
>11 sites from 2002 to the present. Silent infections,
i.e. in the absence of AFP cases, of WPV1, primarily
in the Gaza district, were documented by ENV be-
tween May and September 1991, between October
1994 and June 1995, and in December 1996 .
Phylogenetic analyses of the WPV1 isolated from
sewage in the Gaza district in 1994–1995 were con-
sistent with local circulation of one of three WPV1
lineages introduced into the Gaza district from
Egypt . Routine monthly ENV during 1994–1995
also documented the interruption of this silent
transmission by a subregional vaccination campaign
conducted in response to the finding [14, 15].
Highly diverged neurovirulent VDPVs have been
isolated by ENV from many geographical locations
such as Egypt, Greece, Haiti, India, South Africa and
Switzerland [33, 42–46]. Two independent epidemio-
logical sources of type 2 VDPV have been identified in
central Israel. Based on the rate of accumulation of
single nucleotide substitutions, one of the sources was
exposed to OPV around 1988, and 47 isolates from
32 sewage sample isolates from this source have been
recovered from sewage between 1998 and 2010 [47, 48]
(L. M. Shulman, unpublished data). It is noteworthy
that the ENV protocol used in Israel was capable
of detecting ambiguous VDPVs (aVDPVs) at a time
when sewage also contained OPV from national
vaccination . Similarly, mutually related, highly
diverged type 3 aVDPVs have been isolated from
sewage in Estonia between 2002 and 2008, as well as
two similar type 2 aVDPVs in 2008 and 2009, in
the presence of OPV strains  (M. Roivainen et al.,
Several aVDPVs were found in sewage in Slovakia
in 2003–2005 . This country has been using sup-
plementary ENV with samples collected at 47 sites
dispersed throughout the country, usually at 2-month
intervals. Starting from the capital Bratislava in April
2003, more than 100 related type 2 aVDPV strains
with vast sequence divergence from Sabin 2 strain
have been characterized (T. Hovi et al., unpublished
data). All but two of the strains were found in sewage
from a small town of Skalica, located y60 km north
of the capital. With a retrospective ‘walk’ of sampling
sites along the bifurcations of the sewage network
(Fig. 2) the virus excretor(s) was located among y500
people living in a few blocks of flats, but despite much
Table 6. Environmental surveillance in documentation of short-lived oral poliovirus vaccine (OPV) circulation
after cessation of OPV administration
Country, year of
OPV cessation Mode of OPV use
Last SL isolate,
Finland, 19855-week campaign; coverage
95% of all age groups
Annual National Immunization
Regular childhood immunization
Cuba, 1997–199824 
New Zealand, 20023 12
SL, Sabin-like poliovirus strain.
Environmental poliovirus surveillance9
effort the source of the strain could not be identified
before the occurrence of the virus in the sewage
samples completely stopped. Throughout the 2 years,
the virus was neither found in clinical specimens from
infectious disease patients or from immune-deficient
individuals nor in sewage in locations other than
Skalica, and initially in 2003 in Bratislava. It was
therefore concluded, that virus excretion was highly
local perhaps due to one single individual with poss-
ibly a few occasional contacts.
Since December 2008, several highly divergent
VDPVs have been isolated intermittently from sewage
specimens collected in Tampere, a city in the southern
part of Finland. Interestingly, four out of five positive
specimens contained VDPVs of more than one sero-
type and one specimen VDPV of all three serotypes.
All VDPVs were highly divergent from the corre-
sponding vaccine strains (similarities <88%) sug-
gesting that they might be originally derived from
a chronically infected, still unidentified, immune-
deficient individual(s) .
On top of the threat of emerging cVDPV outbreaks
under suboptimal OPV vaccination coverage [33–42],
existence of immune-deficient long-term PV (iVDPV)
excretors present a challenge to GPEI. Frequent
isolation by ENV of vastly drifted aVDPV strains
genetically resembling iVDPV [47, 49–51] suggests
that long-term PV excretion is not limited to immune-
deficient individuals known to the health system, or
if known to the health system not recognized as
chronically excreting PV.
CONCLUDING REMARKS AND
Experiences from several countries confirm that ENV
can detect introduction and silent circulation of WPV
and VDPV (Table 7), sometimes before any AFP
cases occur. Likewise, ceasing PV transmission may
be monitored with ENV with greater sensitivity than
with AFP surveillance, as described above for Egypt
and India. Further, ENV can be used to monitor the
efficacy of immunization interventions when it is re-
quired. The list of countries routinely employing ENV
currently includes Czech Republic, Egypt, Estonia,
New Zealand, Pakistan, Russia, Slovakia, and
Switzerland. In Finland and Israel, it is considered to
be the main approach of PV surveillance. ENV also
has a role in the new WHO strategy for intensified
efforts to complete poliomyelitis eradication .
Proper programme responses to detection of WPV or
VDPV by ENV depend on the actual observation and
epidemiological situation but always include an alert
to improve surveillance in all possible aspects .
With conventional laboratory techniques, the ap-
proach is rather labour and resource intensive.
Simpler laboratory techniques and high-capacity
equipment allowing for handling large workloads are
perhaps needed in order to increase the use of this
approach. Another significant factor limiting more
wide-scale use of ENV is the fact that dwellings of
most people living in developing countries are not
connected to converging sewage networks. Under
these conditions, although suitable environmental
material is usually abundantly available, individual
samples do not represent large numbers of people.
Experience in India, however, has shown that careful
selection of sampling sites representing open sewers in
highly populated or high-risk areas can overcome this
limitation. Significant resources are currently avail-
able to implement AFP surveillance in developing
countries but such surveillance and investment may
not be sustainable in the long term. In certain situ-
ations, ENV can provide an alternative approach for
monitoring PV circulation, independent of the local
healthcare infrastructure, with design of sample col-
lection based on knowledge of the sanitary organiz-
ation of the municipalities. Yet, it is clear that in order
to provide useful information, ENV also requires
dedicated resources, trained laboratory staff and close
cooperation and coordination with municipal, re-
gional and national authorities which should be
guaranteed before the decision to start ENV is made.
After eradication of WPV transmission, the risk of
re-emergence of polio remains extremely high as long
OPV continues to be used routinely or in campaigns
organized for any reason, and thus provides a poten-
tial source of circulating neurovirulent VDPV strains
. Environmental contamination with faecal ef-
fluents, increasing use of wastewater in agriculture,
and declining PV population immunity are all likely
future risk factors for ingestion and transmission of
PVs . ENV can be an important tool for shorten-
ing the response time between awareness of a PV re-
emergence event and response. ENV should be avail-
able and in place during the critical period between
interruption of WPV transmission and certification of
polio eradication, and, should continue into the post-
eradication and OPV cessation periods to monitor for
the emergence of VDPVs, re-emergence of WPVs, or
disappearance of all OPV-related strains. Research
10 T. Hovi and others
and development to improve and simplify the tech-
nology required to demonstrate PV in environmental
specimens should be encouraged and allocated with
sufficient resources in order to make this approach
more widely available. An added benefit is that the
methodology and infrastructure of ENV can also be
adapted to monitor for non-polio enterovirus and
other environmental pathogens.
We are grateful to members of the environmental
virology teams at our home laboratories, including
Soile Blomqvist and Mirja Stenvik at the National
Institute for Health and Welfare (THL), Finland;
Yossi Manor, Ella Mendelson and Danit Sofer at the
Central Virological Laboratory (CVL), Israel; Ron
Altena and Edin Jusic at the National Institute of
Public Health and the Environment (RIVM), The
Netherlands; and Sushmitha Shetty at the Entero-
virus Research Centre (ERC), India. Roland Sutter
and Olen Kew are acknowledged for providing
unpublished data for our use.
DECLARATION OF INTEREST
One of the authors (E.M.dG.) is employed by the
WHO and all other authors are working in member
Table 7. Global detection of wild and vaccine-derived polioviruses in sewage waters in 1984–2010
Category of virusCountry City or regionYear Serotype Genotype*Source of virusRef.
NEAF Israel and GazaMultiple
Tallinn 2002, 2008
* Relevant to the wild viruses only. Four first digits in the genotype name refer to the geographical origin of the genotype;
NE, north-eastern; WE, western; SO, southern; AF, Africa; AS, Asia.
# Routine notification to WHO.
$ National Institute for Health and Welfare (THL), Finland (unpublished observations).
· Environmental Research Centre (ERC), India (unpublished observations).
k Central Virological Laboratory (CVL), Ministry of Health, Israel (unpublished observations).
Environmental poliovirus surveillance11
laboratories of the WHO Polio Laboratory Network,
which have received partial support from the WHO.
1. World Health Assembly. Global eradication of polio-
myelitis by the year 2000. Geneva, Switzerland: WHA
resolution no. WHA41.28, 1988.
2. WHO. Field guide for supplementary activities aimed
at achieving polio eradication, publication no. WHO/
EPI/GEN/95.1. Geneva: World Health Organization,
3. The GlobalPolio Eradication
2010.pdf). Accessed 6 October 2010.
4. Rico-Hesse R, et al. Geographic distribution of
wild poliovirus type 1 genotypes. Virology 1987; 160:
5. Anon. Progress towards interrupting wild poliovirus
transmission worldwide, 2008. Weekly Epidemiological
Records 2009: 84; 110–116.
6. Roberts L. Polio eradication. Looking for a little luck.
Science 2009; 323: 702–705.
7. Anon. Resurgence of wild poliovirus types 1 and 3 in
15 African countries, January 2008–March 2009.
Weekly Epidemiological Records 2009; 84: 133–140.
8. Anon. CDC Update on vaccine-derived polioviruses –
worldwide, January 2008–June 2009. Mobridity and
Mortality Weekly Report 2009; 58: 1002–1006.
9. WHO. Guidelines for environmental surveillance of
poliovirus circulation. World Health Organization,
Department of Vaccines and Biologicals, 2003. (http://
htm). Accessed 6 October 2010.
10. Dowdle W, et al. Containment of polioviruses after
eradication and OPV cessation: characterizing risks
to improve management. Risk Analysis 2006; 26:
11. Ranta J, Hovi T, Arjas E. Poliovirus surveillance by
examining sewage water specimens: studies on detec-
tion probability using simulation models. Risk Analysis
2001; 21: 1087–1096.
12. Hovi T, et al. Poliovirus surveillance by examining
sewage specimens. Quantitative recovery of virus after
introduction into sewerage
13. Hovi T, et al. Environmental surveillance of wild
poliovirus circulation in Egypt – balancing between de-
tection sensitivity and workload. Journal of Virological
Methods 2005; 126: 127–134.
14. Manor Y, et al. Detection of poliovirus circulation by
environmental surveillance in the absence of clinical
cases in Israel and the Palestinian authority. Journal of
Clinical Microbiology 1999; 37: 1670–1675.
15. Manor Y, et al. Advanced environmental surveillance
and molecular analyses indicate separate importations
rather than endemic circulation of wild type 1
poliovirus in Gaza district in 2002. Applied and
Environmental Microbiology 2007; 73: 5954–5958.
16. Hovi T, et al. Outbreak of paralytic poliomyelitis in
Finland: widespread circulation of antigenically altered
poliovirus type 3 in a vaccinated population. Lancet
1986; 1: 1427–1432.
17. Kinnunen E, Hovi T, Stenvik M. Outbreak of polio-
myelitis in Finland in 1984. Description of nine cases
with persisting paralysis. Scandinavian Journal of In-
fectious Diseases 1986; 18: 15–18.
18. Po ¨ yry T, Stenvik M, Hovi T. Viruses in sewage waters
during and after a poliomyelitis outbreak and sub-
campaign in Finland. Applied and Enviromental Micro-
biology 1988; 54: 371–374.
19. Slater PE, et al. Poliomyelitis outbreak in Israel in
1988: a report with two commentaries. Lancet 1990;
335: 1192–1195; discussion 1196–1198.
20. Shulman LM, et al. Resolution of the pathways of
poliovirus type 1 transmission during an outbreak.
Journal of Clinical Microbiology 2000; 38: 945–952.
21. Oostvogel PM, et al. Poliomyelitis outbreak in an un-
vaccinated community in The Netherlands, 1992–93.
Lancet 1994; 344: 665–670.
22. van der Avoort HG, et al. Isolation of epidemic polio-
virus from sewage during the 1992–3 type 3 outbreak in
The Netherlands. Epidemiology and Infection 1995; 114:
23. El Bassioni L, et al. Prolonged detection of indigenous
wild polioviruses in sewage from communities in Egypt.
American Journal of Epidemiology 2003; 158: 807–815.
24. WorldHealthOrganization. Globaldetectionofwidand
vaccine-derived polioviruses, January 2008–June 2009.
Weekly Epidemiological Records 2009; 36: 366–371.
25. Tambini G, et al. Direct detection of wild poliovirus
circulation by stool surveys of healthy children and
analysis of community wastewater. Journal of Infectious
Diseases 1993; 168: 1510–1514.
26. Deshpande JM, Shetty SJ, Siddiqui ZA. Environmental
surveillance system to track wild poliovirus trans-
mission. Applied and Environmental Microbiology 2003;
27. Chowdhary R, Dhole TN. Interrupting wild poliovirus
transmission using oral poliovirus vaccine: environ-
mental surveillance in high-risks area of India. Journal
of Medical Virology 2008; 80: 1477–1488.
28. Korotkova EA, et al. Retrospective analysis of a local
cessation of vaccination against poliomyelitis: a poss-
ible scenario for the future. Journal of Virology 2003;
29. Kew O, et al. Outbreak of poliomyelitis in Hispaniola
associated with circulating type 1 vaccine-derived
poliovirus. Science 2002; 296: 356–359.
30. Anon. Paralytic poliomyelitis in Madagascar, 2002.
Weekly Epidemiological Records 2002; 77: 241–242.
31. Dowdle WR, et al. Polio eradication: the OPV paradox.
Reviews in Medical Virology 2003; 13: 277–291.
32. Shimizu H, et al. Circulation of type 1 vaccine-derived
poliovirus in the Philippines in 2001. Journal of
Virolology 2004; 78: 13512–13521.
12T. Hovi and others
33. Kew OM, et al. Circulating vaccine-derived polio- Download full-text
viruses: current state of knowledge. Bulletin of World
Health Organization 2004; 82: 16–23.
34. Liang X, et al. An outbreak of poliomyelitis caused by
type 1 vaccine-derived poliovirus in China. Journal of
Infectious Diseases 2006; 194: 545–551.
35. Tebbens RJ, et al. Risks of paralytic disease due to wild
or vaccine-derived poliovirus after eradication. Risk
Analysis 2006; 26: 1471–1505.
36. Rakoto-Andrianarivelo M, et al. Reemergence of re-
combinant vaccine-derived poliovirus outbreak in
Madagascar. Journal of Infectious Diseases 2008; 197:
37. Estı´variz CF, et al. A large vaccine-derived poliovirus
outbreak on Madura Island – Indonesia, 2005. Journal
of Infectious Diseases 2008; 197: 347–354.
38. Jenkins HE, et al. Implications of a circulating vaccine-
derived poliovirus in Nigeria. New England Journal of
Medicine 2010; 362: 2360–2369.
39. Gumede N, et al. Identification of vaccine-derived polio-
viruses (VDPVS) in the DRC from 2005 to 2010. Com-
municable Diseases Surveillance Bulletin 2010; 8: 43–45.
40. Ma ´ s Lago P, et al. Poliovirus detection in wastewater
and stools following an immunization campaign in
Havana, Cuba. International Journal of Epidemiology
2003; 32: 772–777.
41. Huang QS, et al. Persistence of oral polio vaccine virus
after its removal from the immunization schedule in
New Zealand. Lancet 2005; 366: 394–396.
42. Kew OM, et al. Vaccine-derived polioviruses and the
endgame strategy for global polio eradication. Annual
Reviews of Microbiology 2005; 59: 587–635.
43. Dedepsidis E, et al. Retrospective characterization of a
vaccine-derived poliovirus type 1 isolate from sewage
in Greece. Applied and Environmental Microbiology
2007; 73: 6697–6704.
44. Vinje ´ J, et al. Isolation and characterization of cir-
culating type 1 vaccine-derived poliovirus from sewage
and stream waters in Hispaniola. Journal of Infectious
Diseases 2004; 189: 1168–1175.
45. Pavlov DN, et al. Prevalence of vaccine-derived polio-
viruses in sewage and river water in South Africa. Water
Research 2005; 39: 3309–3319.
46. Bundesamt fur Gesundheit. Communicable diseases.
European Region of World Health Organization free
of wild virus-induced poliomyelitis for five years
[in German]. Bulletin BAG 2007; 46: 824–826.
47. Shulman LM, et al. Neurovirulent vaccine-derived
polioviruses in sewage from highly immune popu-
lations. PLoS ONE 2006; 1: e69.
48. Shulman LM, et al. Type 2 polio still in our midst.
Science 2009; 324: 334.
49. Blomqvist S, et al. Characterization of a highly
evolved vaccine-derived poliovirus type 3 isolated
from sewage in Estonia. Journal of Virology 2004; 78:
50. Cerna ´ kova ´ B, et al. Isolation of vaccine-derived polio-
viruses in the Slovak Republic. European Journal of
Clinical Microbiology & Infectious Diseases 2005; 24:
51. Roivainen M, et al. Highly divergent neurovirulent
vaccine-derived polioviruses of all three serotypes are
recurrently detected in Finnish sewage. Eurosurveillance
Environmental poliovirus surveillance 13