Population Dynamics of Aedes aegypti and Dengue as Influenced by Weather and Human Behavior in San Juan, Puerto Rico

Abstract

Previous studies on the influence of weather on Aedes aegypti dynamics in Puerto Rico suggested that rainfall was a significant driver of immature mosquito populations and dengue incidence, but mostly in the drier areas of the island. We conducted a longitudinal study of Ae. aegypti in two neighborhoods of the metropolitan area of San Juan city, Puerto Rico where rainfall is more uniformly distributed throughout the year. We assessed the impacts of rainfall, temperature, and human activities on the temporal dynamics of adult Ae. aegypti and oviposition. Changes in adult mosquitoes were monitored with BG-Sentinel traps and oviposition activity with CDC enhanced ovitraps. Pupal surveys were conducted during the drier and wetter parts of the year in both neighborhoods to determine the contribution of humans and rains to mosquito production. Mosquito dynamics in each neighborhood was compared with dengue incidence in their respective municipalities during the study. Our results showed that: 1. Most pupae were produced in containers managed by people, which explains the prevalence of adult mosquitoes at times when rainfall was scant; 2. Water meters were documented for the first time as productive habitats for Ae. aegypti; 3. Even though Puerto Rico has a reliable supply of tap water and an active tire recycling program, water storage containers and discarded tires were important mosquito producers; 4. Peaks in mosquito density preceded maximum dengue incidence; and 5. Ae. aegypti dynamics were driven by weather and human activity and oviposition was significantly correlated with dengue incidence.

Full text

Population Dynamics of
Aedes aegypti
and Dengue as
Influenced by Weather and Human Behavior in San Juan,
Puerto Rico
Roberto Barrera*, Manuel Amador, Andrew J. MacKay
Entomology and Ecology Activity, Dengue Branch, Centers for Disease Control and Prevention, Calle Can
˜ada, San Juan, Puerto Rico
Abstract
Previous studies on the influence of weather on Aedes aegypti dynamics in Puerto Rico suggested that rainfall was a
significant driver of immature mosquito populations and dengue incidence, but mostly in the drier areas of the island. We
conducted a longitudinal study of Ae. aegypti in two neighborhoods of the metropolitan area of San Juan city, Puerto Rico
where rainfall is more uniformly distributed throughout the year. We assessed the impacts of rainfall, temperature, and
human activities on the temporal dynamics of adult Ae. aegypti and oviposition. Changes in adult mosquitoes were
monitored with BG-Sentinel traps and oviposition activity with CDC enhanced ovitraps. Pupal surveys were conducted
during the drier and wetter parts of the year in both neighborhoods to determine the contribution of humans and rains to
mosquito production. Mosquito dynamics in each neighborhood was compared with dengue incidence in their respective
municipalities during the study. Our results showed that: 1. Most pupae were produced in containers managed by people,
which explains the prevalence of adult mosquitoes at times when rainfall was scant; 2. Water meters were documented for
the first time as productive habitats for Ae. aegypti; 3. Even though Puerto Rico has a reliable supply of tap water and an
active tire recycling program, water storage containers and discarded tires were important mosquito producers; 4. Peaks in
mosquito density preceded maximum dengue incidence; and 5. Ae. aegypti dynamics were driven by weather and human
activity and oviposition was significantly correlated with dengue incidence.
Citation: Barrera R, Amador M, MacKay AJ (2011) Population Dynamics of Aedes aegypti and Dengue as Influenced by Weather and Human Behavior in San Juan,
Puerto Rico. PLoS Negl Trop Dis 5(12): e1378. doi:10.1371/journal.pntd.0001378
Editor: Michael J. Turell, USAMRIID, United States of America
Received April 27, 2011; Accepted September 13, 2011; Published December 20, 2011
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: The authors have no support or funding to report.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: rbarrera@cdc.gov
Introduction
There are three main patterns of dengue virus transmission that
merit a better understanding of the involvement of Aedes aegypti:1-
Major dengue epidemics occur every few years [1,2]; 2- Dengue
viruses have become endemic or hyperendemic (co-circulation of
two or more serotypes) in many countries [3,4], and 3- There is
intra-annual, seasonal dengue transmission with peak incidence
during the second half of the year in the northern hemisphere and
during the first half of the year in the southern hemisphere, each
one associated with elevated temperature and precipitation [5–7].
There is a lack of long-term, longitudinal studies on the
temporal dynamics of Ae. aegypti that would, otherwise, allow us to
understand whether mosquito outbreaks are responsible for inter-
annual dengue epidemics. It is common to observe relatively large
densities of Ae. aegypti that do not result in major outbreaks,
particularly after major epidemics [8], suggesting that other factors
such as temporal changes in population immunity or the
introduction of new serotypes can significantly influence inter-
annual epidemic patterns [9–11]. Climate variability, particularly
El Nin˜o Southern Oscillation (ENSO) teleconnections with local
weather, has been associated with inter-annual dengue epidemics
[2], although it would seem that the relationship is complex, non-
linear, and perhaps non-stationary [1,12,13]. Amarakoon et al. [6]
found significant effects of temperature on dengue epidemics in the
Caribbean, particularly one year after the onset of an ENSO
event.
Because dengue viruses are transmitted by the bite of infected
mosquitoes, dengue virus endemicity or hyperendemicity requires
the existence of sufficient vectors to produce uninterrupted
transmission in spite of adverse, seasonal weather conditions (e.g.,
lack of rain), such as that observed in urban areas with long dry
seasons [14]. Recurrent virus introductions facilitate dengue
endemicity. In dengue endemic/hyperendemic countries, dengue
viruses are disseminated among regions so that virus re-introductions
are frequent and do not depend solely on virus import [4,15]. There
is evidence showing that even in relatively small countries such as
Puerto Rico, some dengue genotypes can circulate uninterruptedly
for prolonged periods of time [16]. Vertical transmission of dengue
virus in Ae. aegypti could play a role in the maintenance of endemicity
but more evidence is required to understand its role in nature [17].
Perhaps, the single, most important factor determining dengue
endemicity is the habit of people of adding water to containers,
which can be for drinking, cooking or bathing (water-storage) and for
other purposes, such as ornamentation (fountains), watering plants,
etc. Production of Ae. aegypti in those containers can be so important
as to trigger dengue outbreaks during the dry season [18].
Additionally, the existence of cryptic containers with water
producing large numbers of Ae. aegypti has been more frequently
reported [19–22], and in at least one occasion, those recondite
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containers have been linked to local dengue virus transmission [23].
Some cryptic containers, such as septic tanks in Puerto Rico, can
produce Ae. aegypti throughout the year [24].
There is evidence showing that the intra-annual cycle of dengue
transmission is driven by weather and mosquitoes [7–9,25–29].
However, there seems to be spatial variability in the temporal
dynamics of Ae. aegypti. For example, detailed studies of the
temporal change in adult and immature stages of Ae. aegypti in a
temple compound in Bangkok, Thailand in 1966 failed to show an
association between weather, mosquitoes, and dengue incidence
[30,31]. The lack of seasonal fluctuation in the Ae. aegypti
population was explained by the prevalence of containers that
were manually filled with water by people, such as water-storage
jars and ant-traps. Assuming that there were enough mosquitoes
throughout the year, dengue incidence could have increased due
to temperature-induced rises in biting rates, and shortened
mosquito gonotrophic cycles and virus extrinsic incubations
periods that can significantly increase vectorial capacity [32,33].
On the other hand, entomological surveys conducted in five places
in Bangkok in 1962 showed a sharp increase in the number of Ae.
aegypti at the beginning of the rainy season, followed by a peak in
cases of dengue hemorrhagic fever two months later [34]. Similar
observations were reported for Koh Samui Island, Thailand [35].
Contrasting observations on the dynamics of Ae. aegypti have also
been reported in Puerto Rico. Moore et al. [36] found positive
correlations between rainfall, immature mosquito populations
(Breteau Index), and dengue, and that the relationship between
rainfall and mosquitoes was more marked in the drier, southern
parts of the island. The authors explained that most larval habitats
of Ae. aegypti occurred outdoors and were filled at least partly by rain;
a result that agrees with more recent surveys [37]. In contrast, Scott
et al. [38] did not find any significant associations between rainfall
or temperature and female adult Ae. aegypti in the wetter, northern
San Juan city area. Given that intra-annual dengue transmission is
sharply seasonal in Puerto Rico, with maximum transmission
during the hot and rainy seasons [39,40], we decided to investigate
the role of weather and human influence on the temporal dynamics
of Ae. aegypti and dengue in the San Juan metropolitan area. Efficient
Ae. aegypti adult trap devices (BG-Sentinel) and CDC enhanced
ovitraps were used to monitor the mosquito populations. Our
observations were structured to minimize temporal and spatial
dependence, which tend to undermine p-values in statistical
analyses [41]. Also, weather and dengue variables were designed
to represent direct mechanistic relationships with the number of
mosquitoes. Our results evidenced significant effects of rainfall,
temperature, and human behavior on the temporal dynamics of Ae.
aegypti and dengue in northern Puerto Rico.
Materials and Methods
Study sites
The study was carried out in two separate neighborhoods, each
consisting of two contiguous US census tracts of the Metro Area of
San Juan, Puerto Rico (Figure 1). One pair of bordering census
tracts was located in urbanization ‘‘Villa Carolina’’, Carolina
municipality (VC; 9240 persons and 1996 buildings; 18u239520N
65u579260W; US Census 2000).We studied a second pair of
adjacent census tracts approximately 3 km from VC: urbanization
‘‘Extension El Comandante’’ in Carolina municipality and
urbanization ‘‘El Comandante’’ in San Juan municipality (EC;
6951 persons and 1979 buildings; 18u249020N65u599300W; US
Census 2000). We will be referring to these latter two census tracts
as ‘‘El Comandante’’ in most of the report.
Carolina Municipality has a spatial, insecticide spraying
program (truck-mounted Ultra Low Volume equipment) that is
active throughout the year, whereas the San Juan Municipality
uses a similar insecticide spraying technique but mostly around
notified cases of dengue. Thus, both census tracts in VC were
subjected to frequent ULV insecticide treatments, whereas in EC
only the census tract that belongs to the Carolina Municipality
may have been regularly sprayed. Establishing the frequency and
coverage of insecticide spraying was attempted but unsuccessful.
Weather variables
Total annual rainfall and mean daily temperature were
1,388 mm and 27.0uC, respectively at the nearby (4–7 km)
Mun˜oz-Marin International Airport in 2008. Rainfall in the San
Juan area occurs year round, with a short, relatively dry season
(,100 mm/month) between January and March and two rain
maxima around April-May and September-October (Figure 2).
Air temperature reached minimum and maximum values in
January and August, respectively (Figure 2).
We computed two derived weather variables: adjusted rainfall
(ADJRAIN) and temperature (ADJTEMP). Adjusted rainfall was
set to be the accumulated rainfall during the third and second
weeks before each mosquito sampling date (every three weeks) and
adjusted temperature was the average of mean daily temperature
for the 21 days before each mosquito sampling date. We did not
include rainfall during the week preceding mosquito sampling
because the immature cycle of Ae. aegypti takes at least seven days.
Adjusted temperature was the average of the previous three weeks
because this variable could have directly influenced the number of
adults present at the time of sampling. It can be observed that
adjusted temperature was similar to the average monthly
temperature, but adjusted rainfall markedly differed from monthly
rainfall (Figure 2).
Adult Ae. aegypti mosquitoes
Aedes aegypti adults were concurrently captured in each
neighborhood using 40 lured BG-Sentinel mosquito traps
Author Summary
Previous studies on the influence of weather on Aedes
aegypti in Puerto Rico suggested that rainfall influenced
mosquito populations and dengue incidence in the drier
areas of the island. We studied temporal changes in Ae.
aegypti in areas where rainfall is more uniformly distribut-
ed throughout the year. Changes in adult mosquitoes
were monitored with BG-Sentinel traps and oviposition
activity with CDC enhanced ovitraps. We also counted the
number of mosquito pupae in containers with water
during the drier and wetter parts of the year to determine
the contribution of humans and rainfall to mosquito
production. Mosquito dynamics was compared with
dengue incidence in the municipalities investigated in
the study (November 2007–December 2008). We found
that the population of Ae. aegypti was driven by weather
and human activities, and peaks in mosquito density
preceded maximum dengue incidence during the rainy
season. Even though Puerto Rico has a reliable supply of
tap water and an active tire recycling program, water
storage containers (e.g., 5-gal pails, drums) and discarded
tires were important mosquito producers. We also
documented for the first time that water meters are
important producers of Ae. aegypti. This longitudinal study
contributes to a better understanding of the complex
dynamics of weather, human behavior, mosquito vectors,
and dengue virus transmission in an endemic country.
Population Dynamics of Aedes aegypti
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(Biogents, Regensburg, Germany) from November 2007 to
December 2008 (20 sampling dates each; total 6,107 samples).
Each trap was operated for four consecutive days every three
weeks. We assumed that the maximum longevity of Ae. aegypti was
three weeks, so adult mosquitoes captured in consecutive
samplings dates should have been born between collection dates,
thus minimizing generational overlap and temporal auto-correla-
tions. Collection bags were replaced every day and batteries were
replaced after two days of operation. Traps were uniformly
distributed across each neighborhood, resulting in inter-trap
average distances of 132 m in EC and 137 m in VC to minimize
spatial dependence and trap interactions. We calculated the
average number of female Ae. aegypti captured per trap per day as a
measure of relative abundance. We used a Geographical
Information System (GIS; ArcView 9.2, Esri, Redlands, CA) to
facilitate trap placement. The GIS had the following geo-spatial
layers: polygons showing house boundaries (Municipal Tax
Revenue Agency, Puerto Rico) and census tracts, digitized lines
representing streets, and derived house centroids representing trap
locations.
Ovitrap sampling
In order to compare the number of eggs collected in standard
CDC ovitraps [42] with the number of female adults of Ae. aegypti
in BG traps, and to evaluate relationships with weather variables
and dengue, we placed a pair of ovitraps (100% hay infusion or
10% dilution) across the street from the location of each BG trap
in both neighborhoods (40 pairs of ovitraps per community). The
number of eggs per ovitrap per day for four consecutive days was
monitored every three weeks from May to December, 2008 on the
same dates as the BGtrap sampling scheme. Eggs were counted
under a stereoscope microscope. Given the low prevalence of Ae.
mediovittatus in the study areas (0.2–0.6% of all container Aedes spp.;
Table 1), most of the collected eggs should have been Ae. aegypti.
We averaged the number of eggs in both ovitraps over the four
days of collection as a measure of oviposition activity (eggs/
ovitrap/day) [43].
Aedes aegypti pupal surveys. To determine the types of
containers producing Ae. aegypti pupae during the relatively dry
season, we conducted simultaneous pupal surveys in VC and EC
(15 January–2 February, 2008). The locations of all houses in each
study area were incorporated as a point layer in the GIS and used
to randomly select houses to be sampled (156 and 152 houses in
EC and VC, respectively).We repeated the pupal surveys in both
neighborhoods during the beginning of the rainy season (16–25
June, 2008) and increased the sample size to 225 and 235 houses
in EC and VC, respectively. The number of houses sampled
resulted from a compromise between our capacity: 1- to conduct
Figure 1. Map of the study areas. The map shows the municipalities of San Juan city, Puerto Rico and the location of the airport in relation to the
two neighborhoods investigated. Each neighborhood is composed of two adjacent census tracts.
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simultaneous pupal assessments in both communities in a short
period of time and 2- to obtain a number of samples that could
reliably inform on the main types of containers producing Ae.
aegypti [44]. Each container with water was located and recorded
from each sampled house and mosquito pupae were taken to the
laboratory in 80% ethanol for species identification. The pupal
survey methodology used in this study has been described in detail
[37].
Ethics statement
Although the database from which the information was
obtained contains identifiable information, only non-identifiable
data was provided to the investigators for this work by an
individual not otherwise involved in the conduct of the study.
Therefore it was determined that this activity did not involve
human subjects as described under the human subjects protection
regulations at 45 CFR 46.102 (f) and an IRB review was not
required.
Dengue data
Suspected and confirmed cases of dengue by onset of symptoms
during the period of study were obtained from the dengue
database of the passive surveillance system that is in place for
Puerto Rico at the Dengue Branch, Centers for Disease Control
and Prevention. Dengue cases reported for the study areas were
aggregated and extracted using the boundaries of the census tracts
within the GIS. A variable named adjusted dengue cases
(ADJDENGUE) was constructed from the accumulated number
of dengue cases reported during the second, third and fourth
weeks after mosquito sampling. The reason for creating this
variable was that recently emerged mosquitoes cannot transmit
dengue virus until after a period that includes: biting an infected
person and acquiring the virus (2–3 days), incubation of the virus
in the vector (7–10 days) [32], then biting a susceptible person.
Transmission is followed by the intrinsic incubation of the virus (5
days), and seeing a doctor after symptoms’ onset (1–5 days).
The number of dengue cases reported to the passive surveillance
system from November 2007 to December 2008 in the study areas
in EC and VC were two and six, respectively. This small number
of cases did not allow us exploring the associations between
mosquitoes, weather variables, and dengue within the investigated
areas. One plausible reason for such small number of cases is that
the passive surveillance system usually captures a small number of
symptomatic cases and misses most asymptomatic ones. There-
fore, we decided to explore the relationship between female Ae.
aegypti abundance in each neighborhood and dengue cases per
100000 inhabitants (inh.) in the San Juan and Carolina
municipalities where mosquito surveys were conducted.
Statistical analyses
Average abundance of female Ae. aegypti captured per BG trap
per day was compared between neighborhoods using a linear
mixed model with ADJRAIN and ADJTEMP as covariates over
the 20 sampling dates. A linear mixed model was also used to
compare the average number of eggs per ovitrap per sampling
period between neighborhoods. A mixed model accounts for the
possible temporal correlation in this type of longitudinal data and
includes the calculation of random factors for each site.
Covariance structure for repeated measures was modeled as being
autoregressive of order one. Analyses were performed using SPSS
12.0.
Results
Main types of containers producing Ae. Aegypti
During the drier season (January/February 2008), 5-gal buckets,
barrels, plant pots, and water meters collectively produced 74% of
the Ae. aegypti pupae in EC and 72% of the Ae. aegypti pupae in VC.
Water meters are located in a 1–1.5 gal cavity below ground level
in front of every house. During the rainy season, 5-gal buckets,
barrels, plant pots, and water meters were producing 60% and
57% of all pupae in EC and VC, respectively. The contribution of
barrels and water meters decreased whereas the contribution of
tires increased in the rainy season (Figure 3).
Immature Ae. aegypti indicators revealed similarities between
sites in June 2008: House Index (30% in EC; 32% in VC) and total
pupae collected (705 in EC; 746 in VC). The number of pupae per
house was not statistically different (log10+1 transformed, t = 0.12;
df = 458; P.0.05) between EC (3.1360.90) and VC (3.1760.77).
By contrast, the mean number of adult Ae. aegypti captured per BG
trap per day by the end of June 2008 was larger in EC
(10.3861.10) than in VC (6.3960.44), though the difference was
not statistically significant at a= 0.05 (log10+1 transformed,
unequal variances assumed, t = 1.79; df = 302; P.0.05). The
pupal sex ratios (male: female) were 0.9:1 in EC and 1:1 in VC.
Adult Aedes aegypti dynamics in BG-Sentinel traps. A
total of 20 mosquito species was captured in the study sites from
November 2007 to December 2008 in BG traps (Table 1). The
Figure 2. Weather variables at Mun
˜oz-Marin International
Airport in 2008, San Juan, Puerto Rico. Panel A shows mean
monthly temperature and adjusted temperature. Adjusted temperature
is the average of daily mean temperature for 21 days before mosquito
sampling. Panel B shows monthly rainfall and adjusted rainfall. Adjusted
rainfall is the accumulated rainfall during the third and second weeks
before each mosquito sampling, which was conducted every three
weeks.
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most abundant mosquito species were Culex quinquefasciatus and
Aedes aegypti. The number of adult Cx. quinquefasciatus collected in
EC was several times larger than in VC (Table 1). We observed
that several houses in the eastern part of EC had septic tanks, but
we could not sample them. The number of adult males of Ae.
aegypti captured in VC was half of that captured in EC (Table 1).
The overall mean numbers of Ae. aegypti females and males per
trap per day were 4.7660.22 (695% CI) and 4.0660.29,
respectively in EC (n = 3059 trap days). There were 3.8060.14
females and 2.1360.11 males per trap per day in VC (n = 3048).
The results of a linear mixed model comparing the average
number of females per trap per day (log10-trasformed) between
the two study sites for the 20 temporal observations every three
weeks, with ADJRAIN and ADJTEMP as covariates, indicated
significant differences between sites (F = 15.9; P,0.01) and
significant effects of rainfall (F = 33.2; P,0.01) and temperature
(F = 19.3; P,0.01).
The number of female Ae. aegypti captured in BG traps
decreased from November 2007 through April 2008 in both study
areas, concurrently with a sustained decrease in rainfall (Figure 4),
with the exception of an intense precipitation event that occurred
on January 19–20 that was associated with slight increases in
mosquito densities in both neighborhoods (Figure 4). The density
of Ae. aegypti in both study sites was similar from November 2007
through April 2008, but after that date (rainy season) the density
was higher in EC than in VC (Figure 4). Additionally, although
rainfall reached minimum values in March - April, mosquito
densities did not proportionally decrease in the neighborhoods.
Maximum mosquito densities in BG traps during 2008 were
registered in June and September in EC and in June and August-
September in VC. The number of females Ae. aegypti captured in
BG traps was positively and significantly correlated with adjusted
rainfall (Figure 5A) in both neighborhoods. Adjusted temperature
was positively and significantly correlated with the number of Ae.
aegypti females in BG traps in EC (r = 0.50; P,0.05) but not in VC
(r = 0.10; P.0.05).
Aedes aegypti dynamics in ovitraps. Overall average
number of eggs/ovitrap/day was similar in EC (31.9461.07;
n = 1745; Total eggs = 55743) and VC (29.7661.02; n = 1705;
Total eggs = 50737). A linear mixed model did not find significant
differences in the average number of eggs per ovitrap between
neighborhoods (F = 0.19, P.0.05). The number of eggs in the
ovitraps was correlated with the number of females captured per
BG trap in EC (r = 0.61; P,0.05) and VC (r = 0.69; ,0.05).
The number of mosquito eggs per ovitrap reached peak values
in June and September in EC and in June, September and
November in VC (Figure 4). The numbers of eggs per ovitrap
show a linear tendency with adjusted rainfall for most samples,
however the correlation was not significant due to an outlier
sample in each neighborhood (Figure 5B). Both outliers corre-
sponded to samples taken in June, right after a large rainfall event
that was associated with a peak in the number of females in both
neighborhoods (Figure 4). Adjusted temperature was positively
correlated with the number of eggs per ovitrap in VC (r = 0.66;
P,0.05) but not in EC (r = 0.39; P.0.05).
Dengue dynamics
Dengue prevalence during the period of study was similar in
San Juan (71 cases per 100000 inh.) and Carolina (77 cases per
100000 inh.) municipalities. Dengue incidence reached maximum
values in San Juan municipality during the beginning of 2008 and
in September 2008; in both cases, peak dengue incidence followed
Table 1. Composition and abundance of adult mosquito species captured in BG traps.
Mosquito species
Villa Carolina
(n = 3,048 trap 6days)
El Comandante
(n = 3,059 trap 6days)
Aedes (Stegomyia) aegypti (L.) (females) 11,690 14,589
Ae. (Stg.) aegypti (L.) (males) 6,534 12,505
Ae. (Gymnometopa) mediovittatus (Coquillett) 32 162
Ae. (Ochlerotatus) taeniorhynchus (Wiedemann) 25 4
Ae. (Och.) tortilis (Theobald) 14 30
Anopheles (Anopheles) grabhamii Theobald 28 7
Anopheles (Ano.) vestitipennis Dyar & Knab 1 3
Culex (Culex) bahamensis Dyar & Knab 7 7
Cx. (Cux.) chidesteri Dyar 5 9
Cx. (Cux.) habilitator Dyar & Knab 81 214
Cx. (Cux.) janitor Theobald 28 37
Cx. (Cux) nigripalpus Theobald 52 103
Cx. (Cux.) quinquefasciatus Say 11,275 76,425
Cx. (Melanoconion) atratus Theobald 149 19
Cx. (Mel.) iolambdis Dyar 21 48
Cx. (Mel.) taeniopus Dyar & Knab 92 230
Cx. (Micraedes) antillummagnorum Dyar 7 1
Mansonia (Mansonia) flaveola (Coquillett) 1 0
Psorophora (Grabhamia) jamaicensis (Theobald) 0 2
Uranotaenia (Uranotaenia) cooki Root 6 0
Ur. (Ura.) lowii Theobald 5 2
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peaks in female mosquito density in EC (Figure 4A). The peaks of
mosquito density observed in June 2008 were not associated with a
large increase in dengue incidence (Figure 4). There were also two
maxima of dengue incidence in Villa Carolina municipality; one at
the end of December 2007 and the other in November 2008
(Figure 4B). Dengue incidence reached the lowest levels by the end
of the ‘‘drier season’’ (San Juan) or beginning of the rainy season
(Carolina; Figure 2, 4). Dengue incidence was positively and
significantly correlated with the number of female Ae. aegypti in
both neighborhoods (Figure 6A). Similarly, dengue incidence was
positively correlated with the number of eggs per ovitrap in EC but
did not reach statistical significance in VC (Figure 6B). It can be
noted that mosquito density never reached values below two in BG
traps or below ten in ovitraps during the study (Figure 6). With the
exception of one sample in VC (May 2008), all vector density
values observed in this study were associated with dengue
incidence.
Discussion
Ae. aegypti dynamics and dengue endemism
The pupal surveys conducted during the drier and wetter parts
of the year in San Juan city during the end of 2007 and 2008
revealed that most pupae were produced in containers whose
water content was managed by humans: water storage vessels (5-
gal pails, barrels), leaking-water meters, and plant pots. This is the
first report on water meters as productive aquatic habitats for Ae.
aegypti. Containers that were mainly filled with water by rains
increased during the rainy season, most notably used tires
(Figure 3). Therefore, it would seem that the reason why the Ae.
aegypti population did not reach very low levels at a time when
rainfall was scant (Figure 4) was due to the production of
mosquitoes in containers managed by people. This phenomenon
was remarkably similar in both neighborhoods. A longitudinal
study that investigated the temporal changes in aquatic habitats
and oviposition of Ae. aegypti in northern Venezuela showed similar
dynamics, although in such study the main habitats producing
mosquitoes during the prolonged dry seasons were 55-gal barrels
used for water storage [14]. Lambdin et al. [45] found that
buckets, barrels, and tires were the most productive containers
during the dry and wet seasons in American Samoa. It is
perplexing that Ae. aegypti mosquitoes were being produced in
water storage containers and tires in Puerto Rico because the
country has a reliable supply of tap water and a tire-recycling
program. Our results seem to add support to the hypothesis that
Figure 3.
Aedes aegypti
pupae per type of container. Percentage
of all pupae found in pupal surveys conducted during the ‘‘drier’’ (Panel
A) and rainy (Panel B) periods, respectively, in neighborhoods ‘‘El
Comandante’’ (EC) and ‘‘Villa Carolina’’ (VC), San Juan city, Puerto Rico.
Numbers on top of bars indicate how many containers of each type
were found with pupae.
doi:10.1371/journal.pntd.0001378.g003
Figure 4. Temporal changes in rainfall, mosquitoes, and
dengue. Panel A shows changes in adjusted rainfall (mm), number
of Aedes aegypti females per BG-Sentinel trap per day, number of eggs
per CDC ovitrap per day, and adjusted dengue incidence (cases per
100000 inhabitants) in ‘‘El Comandante’’ (EC) and Panel B shows these
parameters in ‘‘Villa Carolina’’ (VC), San Juan city, Puerto Rico from
October 2007 to December 2008.
doi:10.1371/journal.pntd.0001378.g004
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dengue endemicity (uninterrupted transmission) is favored by the
persistent production of mosquitoes in containers whose water is
managed by people.
Weather effects
The present longitudinal study of Aedes aegypti in neighborhoods
of San Juan City revealed significant changes in the number of
adult mosquitoes in BG traps that were positively associated with
rainfall and temperature (Figures 4, 5). The typical bimodal
rainfall pattern of northern Puerto Rico [46] was associated with
two main peaks in mosquito abundance, particularly in EC
(Figure 4). The first peak in mosquito density was associated with a
small hump in the figure of dengue cases in June and the second
peak in mosquito density preceded maximum dengue incidence
later in the year (Figure 4). It could be observed that dengue
incidence decreased after reaching its peak, in association with
concomitant reductions in rainfall and Ae. aegypti density (Figure 4).
Thus, the present study is in agreement with a previous
longitudinal study of Ae. aegypti in Puerto Rico that showed
significant effects of rainfall on larval indices and dengue incidence
[36]. These authors suggested that such a relationship was patent
in the drier cities of the island because rainfall is more seasonal.
The lack of significant effects of weather on adult Ae. aegypti in a
previous study in San Juan city in northern Puerto Rico, where
rainfall is more uniformly distributed throughout the year [38],
seemed to confirm such hypothesis. However, in the present study
Ae. aegypti dynamics was strongly driven by weather and human
activity. The only previous study of Ae. aegypti dynamics using BG
traps in relation to weather variables was done in tropical Cairns,
Australia [47]. The authors used weekly values of temperature,
rainfall, and relative humidity and explored varying time lag
effects using multiple regression analyses. They did not find
significant effects of rainfall at any time lags, but significant effects
of relative humidity with a lag of two weeks and mean daytime
temperature at lag 0.
Oviposition (eggs/ovitrap/day) was also influenced by rainfall
and this variable was correlated with the abundance of Ae. aegypti
females in BG traps. Both female adults and oviposition were
correlated with dengue incidence. Such an association could be
more easily seen in EC (Figures 4, 6). Mogi et al. [48] reported
marked seasonal changes in Ae. aegypti oviposition associated with
the rainy season, with maximum numbers occurring one month
Figure 5. Relationships between mosquitoes and rainfall. Panel
A presents the number of female Ae. aegypti per BG-Sentinel trap per
day versus adjusted rainfall (mm) for each sampling date in ‘‘El
Comandante’’ (EC) and ‘‘Villa Carolina’’ (VC), San Juan city, Puerto Rico,
and Panel B shows the number of eggs per CDC ovitrap per day versus
adjusted rainfall in each neighborhood. The corresponding correlation
coefficients and Type I error probabilities are presented next to the
location.
doi:10.1371/journal.pntd.0001378.g005
Figure 6. Relationships between dengue incidence and mos-
quitoes. Panel A presents dengue incidence (cases per 100000
inhabitants) as a function of the number of female Ae. aegypti per
BG-Sentinel trap per day for each sampling date in ‘‘El Comandante’’
(EC) and ‘‘Villa Carolina’’ (VC), San Juan city, Puerto Rico, and Panel B
shows dengue incidence as a function of the number of eggs per CDC
ovitrap per day in each neighborhood. The corresponding correlation
coefficients and Type I error probabilities are presented next to the
location.
doi:10.1371/journal.pntd.0001378.g006
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before the peak of dengue cases in northern Thailand. It would
appear that ovitraps could be used as inexpensive indicators of the
risk of dengue transmission, although perhaps these traps may not
consistently reflect overall mosquito population abundance. BG
traps capture Ae. aegypti in various physiological states but
underestimate some groups, such as nulliparous females [49],
whereas ovitraps reflect the number of ovipositing or gravid
females [50,51]. Ovitraps have been used to monitor temporal
changes in Ae. aegypti populations and to assess the impact of
insecticide treatments [52]. The use of ovitraps for surveillance
purposes can be made easier because the number of eggs per
ovitrap can be significantly related to the percentage of positive
traps using an empirical model, so once the model is developed
there is no need to count the eggs collected in the traps [53]. We
propose a closer examination of the value of standard CDC
ovitraps or similar devices that monitor gravid females as
indicators of dengue transmission. The results obtained in this
investigation suggest that the levels of Ae. aegypti females per BG
trap or the number of eggs per ovitrap should be reduced well
below two and ten, respectively to prevent dengue transmission
(Figure 6). Mogi et al. [48] did not observe dengue hemorrhagic
fever cases in Chiang Mai, Thailand when the number of eggs in
ovitraps baited with water was two or less.
Variation between neighborhoods
Pupal surveys indicated rather similar entomological indices and
numbers of pupae of Ae. aegypti in both study areas in June, 2008.
However, the number of adult Ae. aegypti captured in BG traps was
significantly greater in EC than in VC. Among the main
differences observed between the two areas was the fact that there
was a deficit of male Ae. aegypti mosquitoes captured in BG traps in
VC (Table 1). This unbalance in the proportion of males was not
observed in the pupal surveys, suggesting that it resulted later on
during the adult life of the mosquitoes. Additionally, an
outstanding difference in dynamics between the two sites was
observed in August and September, when it appeared like the
second peak in mosquito abundance in VC was trimmed in
comparison with EC (Figure 4). A hypothesis explaining this
difference between neighborhoods is that there was more frequent
application of truck-mounted, Ultra Low Volume (ULV) spraying
of insecticide in VC, particularly during the dengue season.
Unfortunately, we could not gather enough data to document
when and where insecticides were applied in each neighborhood.
Yet, of the 78 municipalities of Puerto Rico, Carolina municipality
was the one that spent the most in the chemical control of
mosquitoes [54]. Focks et al. [55] reported that truck-mounted
ULV spraying killed an average of 88% of the males and 30% of
the females in New Orleans, Louisiana. This greater impact of
insecticide spraying on Ae. aegypti males is consistent with our
observations in VC. However, if insecticide spraying actually
reduced the number of mosquitoes captured in BG traps, it did not
significantly affect the number of eggs in ovitraps since there were
no differences between neighborhoods. The Carolina municipality
also carries out insecticide spraying to eliminate other biting
mosquito species that come from nearby marshes and mangroves
(Table 1). An additional difference between neighborhoods was
the comparatively larger density of Culex quinquefasciatus observed
in EC (Table 1), which was probably due to the presence of septic
tanks that were observed in some of the less urbanized sectors of
EC. It would appear that current levels of mosquito population
control were insufficient to make a difference in terms of dengue
transmission between municipalities because dengue prevalence
during the study was similar in both administrative areas.
Acknowledgments
We thank Belkis Caban, Veronica Acevedo, Gilberto Felix, Juan Medina,
Angel Berrios, Jesus Flores, and Orlando Gonzalez for their outstanding
field and laboratory work. We also thank the residents of Villa Carolina,
Extension El Comandante and El Comandante, for their support
throughout the study. We acknowledge the positive interaction with
officials from Carolina and San Juan municipalities, and the Department of
Health of Puerto Rico.
Author Contributions
Conceived and designed the experiments: RB. Performed the experiments:
RB MA AJM. Analyzed the data: RB. Contributed reagents/materials/
analysis tools: RB MA AJM. Wrote the paper: RB.
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