Population and Parity Levels of Aedes Aegypti Collected in Tucson

7/28/2019 Population and Parity Levels of Aedes Aegypti Collected in Tucson

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June, 2003 Journal of Vector Ecology 1

Population and parity levels ofAedes aegypti collected in Tucson

Paquita A. E. Hoeck, Frank B. Ramberg, Samuel A. Merrill, Carlos Moll and Henry H. Hagedorn*

Department of Entomology and Center for Insect Science

University of Arizona, Tucson, AZ 85721 U.S.A.

*Corresponding Author: [email protected]

Received 25 October 2002; Accepted 10 November 2002

ABSTRACT: Oviposition traps were used to follow changes in the population ofAedes aegypti (L.)

(Diptera:Culicidae) in a seven-block area in a midtown region of Tucson, Arizona. About 20,000 eggs were collected

over a period from 1 June to 14 October 2000. Peak mosquito populations were correlated with the late summer

rains. Mosquitoes seeking a blood meal were collected and dissected to determine if they had previously fed, i.e.

if they were parous. Of the 241 females examined, 44% were parous, with a range from 0% to 80%. Females that had

blood in their guts were collected and the source of blood was identified using an ELISA. Preliminary results

suggest that 80% of them had fed on humans. These data suggest that the reproductive history of Tucson

populations ofAe. aegypti could be conducive for transmission of dengue viruses. Journal of Vector Ecology 28

(1): 2003.

Keyword Index: mosquito, parity, oviposition, blood source, dengue.

INTRODUCTION

In late 1994, Aedes aegypti, the yellow fever

mosquito, reappeared in southern Arizona after a nearly

40-year absence. Since then, it has become widely

distributed in southeastern Arizona from Nogales north

to Tucson (Fink et al. 1998) and Phoenix (unpublishedobservations). Because it feeds preferentially on

humans and is a major vector of dengue viruses, and

because dengue cases have been reported from several

areas of Mexico and Texas, the potential for transmission

of the viruses by Ae. aegypti in Arizona is of critical

interest. These studies were done to evaluate and

understand factors of the biology and ecology of these

mosquitoes that are helpful in controlling their

populations and predicting their ability to transmit the

dengue fever virus in Tucson. We examined changes in

the mosquito population during the season, the parity

of the mosquitoes taking blood meals and the sources

of blood meals.

Initial efforts to determine the distribution ofAe.

aegypti in Tucson using CO2

traps showed that they

were present throughout the city (Fink et al. 1998). In

this study we used oviposition traps to follow changes

inAe. aegypti presence in a 7 block area in midtown

Tucson during the fall of 2000 as part of a larger project

to study the population genetics of this species in

Tucson. Oviposition traps provide an indirect estimate

of the feeding activity of the mosquito population.

The capacity of a vector to transmit a pathogen is a

complex interaction of several factors including the

density of the vector and its hosts, its feeding frequency,

its competence to transfer the pathogen, and the

likelihood that it will survive long enough to bite a host

again and actually transmit the pathogen (Black and

Moore 1996). These factors are often difficult to measure,

but it is not difficult to measure the proportion of thevector population that is parous, i.e. that have laid at

least one set of eggs (Corbet 1963). This is important

because when, for example, dengue virus is taken up by

the mosquito during blood feeding, a period of 10-14

days elapses before the virus can be transmitted to a

human during a subsequent blood meal (Watts et al.

1987).Ae. aegypti frequently take more than one blood

meal during a gonotrophic cycle and, once infected with

the virus, remain infective for life (Scott et al. 1993, 2000b,

Putnam and Scott 1995). An estimate of the percentage

of parous females in the population is, therefore, useful

because it measures the likelihood that a feeding

mosquito has fed before and may be infectious.

Identifying the source of the blood meal is an

important factor in pathogen transmission. If the vector

feeds on several animals other than humans, the

likelihood of transmission is reduced.Ae. aegypti has

long been considered to feed preferentially on humans

(Scott et al. 1993, 2000b, Christophers 1960, Tempelis

1975). However, it will feed on other hosts as well

(Tempelis 1975) and may be forced to feed on other

animals if humans are not available.


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2 Journal of Vector Ecology June, 2003

MATERIALS AND METHODS

Oviposition trapping

Oviposition traps were modeled on those described

by Reiter et al. (1991, 1997). They consisted of 1-quart

canning jars painted black outside, containing plantinfusion water and a circular sheet of paper (#76 seed

germination paper, extra heavy weight, Anchor Paper

Co., www.anchorpaper.com). The infusion was made from

a 1:1 mixture of high protein rabbit chow and soy meal,

finely ground in a blender. Approximately 1 pound of

the mixture was incubated in a 30-gallon garbage can

filled with water for 2-3 weeks. Two jars were used at

each site, one containing full strength infusion water

and the second containing a 1/10 dilution. As the jar

containing the full strength infusion was attractive to

lizards, it was covered by a fiberglass screen held in

place by the lid ring closure.

The oviposition sites were in a 7-block region ofthe Sam Hughes region of Tucson that is just east of

the University of Arizona campus. Sam Hughes is an

older residential neighborhood with mainly single family

residences on lots of variable size. Vegetation varies

considerably in density and type. Oleander hedges are

common along property boundaries, and yards often

have plants that require irrigation. Evaporative coolers

or air conditioning are used to cool the houses. Property

maintenance varied considerably, and potential

mosquito habitat was not scarce. Homes were separated

by double east-west alleys. The area between the alleys

generally contained no inhabited buildings. Each block-

long alley had three traps, one at each end and one in

the center (Figure 1). Traps were placed along the edges

of these alleys adjacent to yards. The position of traps

is indicated in the figure. Very few traps were disturbed

during the study. We tried to place the traps in a wide

range of sites according to sun exposure and type of

adjacent property. Trapping started on 1 June with 24

traps, and expanded to 36 traps on 17 July (traps 25-36).

Trapping continued until 14 November. The papers were

replaced twice weekly, and infusion water was replaced

in the full-strength jars as needed. In the laboratory,

egg papers were removed, and eggs were counted.

Climate data were obtained from the daily temperatureand precipitation data summaries recorded by the

University of Arizona Atmospheric Science Department

(http://www.atmo.arizona.edu/cgi-bin/uawxstn/

wxtail5.pl). The department weather station is on the

university campus, about 1 mile from the study site.

Collection of mosquitoes for parity determination

266 female Ae. aegypti were collected during 22

collections in backyards and alleys adjacent to residential

areas in a region 3 to 4 km from downtown Tucson (Figure

1) between 13 September and 2 November, 2000. The

collections were done during approximately two hours

preceding sunset, corresponding to part of the known

daily activity period ofAe. aegypti (Corbet 1974).

Two collectors working together aspiratedmosquitoes attracted to themselves. To prevent actual

biting, clothing covered arms, legs, and feet. The

aspirators were constructed from 16-20 inch lengths of

flexible Tygon tubing (0.63 cm) with mouth tips made

from short lengths of 5 mm plastic pipette tips and with

screens of cotton organdy. The collected females were

held in pint-sized paper carton cages with wetted paper

wipes attached to maintain a high humidity. In order to

slow blood meal digestion and ovarial development, each

evenings collection was stored overnight in an

incubator at 16oC.

On the morning after collection, each female was

anaesthetized with CO2 and transferred to a drop of0.675% saline solution (Rosay 1969). The last abdominal

segments were torn and the gut and ovaries were pulled

gently out of the abdominal cavity. Guts containing blood

were recorded and stored in fixative (Frings and Frings

1971) at 4oC. The ovaries were either analyzed

immediately, or they were stored in a saline-glycerol

solution (0.675% saline solution, 10% glycerol) at 4o C

for later analysis. Dissections were done under a

dissecting microscope.

Analysis of ovaries

Although several methods have been developed

for determining parity in mosquitoes, for this study the

primary methods used were those of Polovodova (1941,

1949) and Detinova (1945, 1949) because they have been

used forAe. aegypti (Corbet and Smith 1974). These

methods depend upon changes in ovarial structure

during egg maturation and oviposition. As the ovary

expands during the initial gonotrophic cycle, the

tracheoles supplying it irreversibly uncoil and are thus

good indicators of parity when dried out on slides

(Detinova 1945). During ovulation, the mature oocyte

passes from the ovariole into the calyx lumen through

the basal body (Hoc and Schaub 1995). Subsequently,

granulation occurs in the basal body cells, which canbe seen in neutral-red stained preparations. The

expanded ovariolar sheaths contract after oviposition

and form dilatations containing the follicular remnants

(Rosay 1969, Polovodova 1949, Corbet and Smith 1974).

The preserved ovaries were analyzed for parity and

for presence of yolk. They were transferred from the

saline-glycerol solution into a drop of 0.675% saline

solution on a microscope slide. Two drops of neutral-

red (1 mg/ml) were added to stain the ovarioles. The


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Figure 1. Map of oviposition sites in the Sam Hughes region of midtown Tucson. Dotted lines indicate alleys.

Dots indicate placement of oviposition traps; round symbols indicate traps that were present for the

entire collection period, square symbols indicate traps that were added on July 17. Numbers above the

symbols indicate the total number of eggs collected from each site during the entire collection period.


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ovaries were teased apart with fine needles so that the

single ovarioles could be seen. Each ovariole was

analyzed under a Nikon compound microscope

(magnification 100 x) for the presence of the following

structures and conditions that indicate that the female

is parous:1) Expansion of the ovariolar sheath: Immediately

after a mature egg leaves the ovariole, the ovariole

sheath and the pedicel remain as a loose sac stretched

by the developed egg as it passes into the calyx (Rosay

1969).

2) Gonotrophic dilatations: Within 48 hours after

oviposition the pedicel shrinks, and the ovariole sheath

contracts to form a dilation that contains the follicular

relic (Rosay 1969). As these dilations may be resorbed,

their presence is only evidence of recent gonotrophic

cycles.

3) Granular basal bodies: During ovulation, the

mature oocyte passes from the ovariole into the calyxlumen through the basal body. Subsequently,

granulation occurs in the basal body (Hoc and Schaub

1995). These granular basal bodies can be seen in intact

ovaries that have been stained with neutral red.

4) Tracheole skeins: In each of 52 females, one

ovary was examined as described above and the second

was allowed to dry on a slide for analysis of the

tracheoles. The tips of ovarial tracheoles of nulliparous

females are coiled, whereas the tracheoles of parous

females are extended as a result of the expansion of the

ovary during an earlier gonotrophic cycle (Detinova

1962, Nasci 1986).

Analysis of yolk

Ovaries that contained yolk or fully developed eggs

were analyzed by measuring the lengths of the follicle

and of the opaque yolk of the 7 largest follicles at 100x

magnification. The average of these measurements was

taken in order to calculate the ratio of the amount of

yolk to the size of the follicle. If the ratio was 0.3 or

higher, the follicle was considered to contain yolk,

indicating a previous blood meal.

The ratio was set conservatively high at 0.3 because

at very low densities yolk presence can be difficult to

judge. The choice of this ratio will possibly result in an

underestimation of the number of ovaries that contained

yolk. Relic fully developed eggs were considered as

indicators of parity, but they were not measured for yolkanalysis.

Blood meal identification

The ELISA was performed as described by Chow et

al. (1993). Blocking buffer (BB): 0.5% casein was

suspended in 0.1N NaOH and boiled; 0.02% phenol red

and 0.1% thimerosal were then added and pH was

adjusted to 7.4 with HCl at room temperature. PBS (pH

7.4) was used to bring the blocking buffer solution to

the final volume. Wash: PBS with 0.05% Tween 20 and

0.1% thimerosal. Grinding solution: PBS with 0.1%

thimerosal. TMB substrate: TMB ELISA substrate.

Capture antibodies: Anticat IgG heavy chain (H),antidog IgG (H), antihuman IgG (H), and antichicken

IgG (heavy and light chain) were obtained from Jackson

ImmonoResearch (http://www.jacksonimmuno.com/

home/). Conjugate antibodies: Horseradish peroxidase

labeled anticat IgG (H), antidog IgG (H), antihuman IgG

(H), and antichicken IgG (H & L) were also obtained

from Jackson.

Field-caught Ae. aegypti were collected using

carbon dioxide traps and resting specimens were

collected with an aspirator made from a handheld

vacuum. Centrifuge tubes were incubated with blocking

buffer for 3 h at room temperature, rinsed with PBS, and

allowed to dry. Each blood-fed mosquito was placed

into a blocked 1 ml centrifuge tube with 20 l of grinding

solution and homogenized with a micro pestle. The

pestle was then washed with 130 l of grinding solution

giving a total volume of approximately 160 l. All tubes

were then centrifuged at 14000 rpm for two minutes and

the supernatant was used for the assay.

Cross reactivity was evaluated by checkerboard

titration of species capture and conjugate antibodies

with non-species blood samples and non-fed femaleAe.

Table 1. Final capture antibody and conjugate antibody concentrations used.

IgG to be identified Capture antibody Conjugate antibody Negative cut-off

(g/ml) (g/ml) absorbance

Cat 0.5 0.08 0.09

Human 4 0.16 0.09

Dog 0.5 0.08 0.09

Bird 2 0.16 0.09


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Figure 2. The percent of oviposition traps containingAe. aegypti eggs per collecting day in Tucson Arizona from

1 June to 14 November 2000. From 1 June to 17 July, 24 traps were used. Thereafter, 36 traps were used. The data

are uncorrected for the number of traps.

Figure 3. Rainfall (bars) and the maximum (solid line) and minimum (dotted line) temperatures from 1 June through

7 November 2000, on the campus of the University of Arizona in Tucson, about 1 mile from the

collection site.


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aegypti. Final capture antibody and conjugate antibody

concentrations are shown in Table 1.

For human IgG positive controls, femaleAe. aegypti

from a lab colony were fed on a human source until fully

engorged and prepared as described above. For cat,

dog and bird positive IgG controls, 4 l of cat, dog orchicken blood was placed in each centrifuge tube, then

a non-blood-fed mosquito was added and prepared as

listed above. Whole cat and dog blood were obtained

from University Pet Clinic, Tucson, AZ. Chicken blood

was obtained from the Diagnostic Facility, University

of Arizona.

Each mosquito was tested twice per ELISA plate

for reaction to each antibody type. A positive

consisted of both wells for an antibody having an

absorbence of 0.09 or greater. Control experiments

showed that the ELISA assay was capable of detecting

human IgG 52 hours after feeding.

RESULTS

Oviposition trapping

Oviposition traps were set out in a 7-block region

of midtown Tucson on 1 June (Figure 1). The first eggs

appeared in traps on 22 June (Figure 2). The rise in eggs

trapped was correlated with the beginning of the summer

rains in late June (Figure 3). Most of the eggs were laid

during this period, which is characterized by violent

thunderstorms that move through the area sometimes

producing large amounts of water during short periods.

Temperatures are moderated and humidity increases.

These rains last into September, and most of the areas

yearly precipitation occurs during this period. Higher

than usual precipitation also occurred during Octoberin 2000 (Figure 3). However, the numbers of eggs

collected fell steadily through October as the

temperature declined.

Eggs collected through the season totaled 20,591,

averaging 447.6 eggs/ trap collection day. A log

transformation of the collecting data (log n+1) yielded a

geometric mean of 99.75 (SE = 0.16) eggs/ collection

day. Individual trap totals varied considerably, even

within adjacent traps in a single alley (Figure 1), with a

range from 44 to1,764 eggs for the season. The geometric

mean compensates for the skewing of the arithmetic

mean caused by large variability within traps. The data

in Figure 2 show the percent traps positive per collecting

day, which also tends to compensate for extreme variation

in individual trap counts.

Parity

Complete data were available for 241 of 266

dissected females. Data for 25 females were rejected

because of uncertainty of one or more of the criteria for

judging parity. Figure 4 shows the number of females

Figure 4. Parity levels inAe. aegypti collected in Tucson, Arizona in the fall of 2000. The number of mosquitoes

sampled is indicated above the bars.


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collected and the parity rate for each collecting day from

13 September to 2 November. The percentage of parous

females varied from 0% to 80% with an overall average

of 44%. An alternating high/low trend is evident. To

look at parity more carefully, the relationship between

parity levels and the presence of yolk and blood was

examined.

Of the 241 dissected females, the greatest number,

165, had no yolk or blood, 58 had yolk in their oocytes,and 40 had blood in their guts. Most interesting is that

of the 107 females that had previously laid eggs, 34

were developing a new set of eggs, and 21 still had

blood in their gut from an earlier blood meal, indicating

a high level of repetitive blood feeding by these

mosquitoes.

Blood Source

Analysis of blood in 37 wild-caught Ae. aegypti

showed that 80% had fed on humans (Table 2). Dog

blood represented 11% of the sample.

DISCUSSION

Oviposition trap data are seen primarily to be useful

for indicating presence of reproductively active (i.e.

blood-feeding) mosquitoes and for following the course

of a population through the season (Focks et al. 1993).

Our data do this, showing that the activity season

coincided with the late summer rainy season in the

Southwest. The mosquito activity dropped, however,

as the temperatures declined during October despite

the continuation of rain.

We collected 20,591 eggs from a seven-square block

area over 131 days during the months of July to October,2000. About 100 eggs were collected per collection day.

Given that eggs were collected twice a week, the rate of

egg development is roughly three days (in Thailand,

Focks et al. 1993), and the maximum number of eggs

produced byAe. aegypti is near 100 (Christophers 1960),

the number of eggs trapped per collection day might

represent the eggs produced by one or two females.

However, the number of eggs per individual trap ranged

from 44-1,764. The higher numbers were collected during

the month of September. Clearly, the number of egg-

laying females varied considerably during the season.

SinceAe. aegypti is known to feed repeatedly during a

single gonotrophic cycle (Scott et al. 1993, 2000a, 2000b),

production of a single batch of eggs does not imply a

single blood meal. Without information on biting rates

or mosquitoes present per person, prediction of dengue

transmission rates is difficult. Also, estimating

population size from ovitrap data is complicated byirregularities in oviposition behavior, variation of habitat

and trap placement, competition from other available

oviposition sites, and weather fluctuations that affect

oviposition site availability (Focks et al. 1993). The

extreme variability in numbers of eggs laid in traps that

were closely situated was remarkable (Figure 1), and

would require a detailed knowledge of breeding and

resting sites within the neighborhood to understand.

Of the 241 females caught while seeking a blood

meal in Tucson, 44% were parous, with a range from 0%

to 80%. This indicates the high likelihood that many

females will survive the 10-14 days necessary for

transmission of a dengue virus after the female has taken

an infective blood meal. The gonotrophic cycle ofAe.

aegypti has been estimated at 3 days (Sheppard et al.

1969) or 4 days (Conway et al. 1974), and is, of course,

affected by ambient temperature. Thus, a female

surviving for 20 days could undergo up to 4 or 5

gonotrophic cycles with considerable potential for virus

transmission. Furthermore, Ae. aegypti takes several

blood meals during a single gonotrophic cycle (Scott et

al. 1993, 2000b, Canyon et al. 1995).

Three previous studies of parity in Ae. aegypti

populations showed levels of parity similar to those

found in Tucson. On the Koh Samui Island of Thailand26-53% were found to be parous (Gould et al. 1970),

while in a village in Thailand only 19% were parous

(Scott et al. 2000a). In Dar es Salaam, Tanzania, 28%

were parous (Corbet and Smith 1974) and in San Juan,

Puerto Rico, the rate of parity was 32% (Scott 2000b).

The study by Gould et al. (1970) is interesting because

the collections were broken down by month from August

through March, with parous levels varying from 26%

Table 2. Blood source in wild-caughtAe. aegypti in Tucson.

Blood Source # females % of total

Human 30 80

Dog 4 11

Cat 1 0.02Human and bird 1 0.02

Unidentified 1 0.02


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(November) to 53% (August); the average for the 8

months was 37%.

Because these studies vary considerably in

duration, locality, sampling techniques and sampling

frequency, the results are difficult to compare with those

of the Tucson study. For instance, the studies of Scottet al. (Scott et al. 2000a, 2000b) extended over three years,

whereas Corbet and Smith (1974) collected on only 3

days. However the similarity in parity levels is evident

and surprising given the very different ecological

conditions in the four areas studied. Tucson is in the

Sonoran desert, with an average of 10-12 inches of rain

per year. The long hot summer when temperatures can

rise above 40o C is interspersed with cool rainy weather

from June through February. The increase in mosquito

populations in the late summer and fall is correlated

with the late summer rains. The environment of the city

provides mosquito habitat that mitigate these extremes

and might explain why the parity levels are so similar tothose found in Thailand and Puerto Rico.

Only preliminary data were available from our ELISA

assays of the blood sources used by Ae. aegypt i.

However, these data do confirm the previous evidence

that this species feeds preferentially on humans (Scott

et al. 2000a).

The current study shows that the reproductive

history of Tucson populations ofAe. aegypt i is

favorable for transmission of dengue viruses.

Acknowledgments

We thank Dr. Richard Collins for advice on parity

determination and Cyrus Jones, Institute of Atmospheric

Physics, for assistance with the weather data. Support

for this project was supplied by CSREES-IPM funds

through the College of Agriculture at the University of

Arizona.

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Population and Parity Levels of Aedes Aegypti Collected in Tucson

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