The influence of adult female body size of Anopheles gambiae s.l. on development of midgut and salivary gland infections by the parasite Plasmodium falciparum was investigated in a field study carried out in Tanzania. The proportion of mosquitoes infected during a blood meal was independent of size. However, the number of oocysts harboured by infected mosquitoes increased with size of the mosquito. The proportion of mosquitoes with sporozoites, and thus potentially infective to humans, was highest in intermediate-sized mosquitoes, whereas the largest and smallest mosquitoes were less likely to have sporozoites. This pattern is interpreted as a combination of high survival rate of large, uninfected mosquitoes and of low survival rate of mosquitoes infected with many oocyst

Koella, J. C.

Lyimo, E. O.

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233Relationship between body size of adult Anopheles gambiaes.l. and infection with the malaria parasite PlasmodiumfalciparumE. O. LYIMO1 and}. C. KOELLA2*1 Ifakara Centre, Box 53, Ifakara, Tanzania2 Swiss Tropical Institute, Socinstrasse 57, CH-4002 Basel, Switzerland(Received 2 July 1991; revised 22 August 1991; accepted 23 August 1991)SUMMARYThe influence of adult female body size of Anopheles gambiae s.l. on development of midgut and salivary gland infectionsby the parasite Plasmodium falciparum was investigated in a field study carried out in Tanzania. The proportion ofmosquitoes infected during a blood meal was independent of size. However, the number of oocysts harboured by infectedmosquitoes increased with size of the mosquito. The proportion of mosquitoes with sporozoites, and thus potentiallyinfective to humans, was highest in intermediate-sized mosquitoes, whereas the largest and smallest mosquitoes were lesslikely to have sporozoites. This pattern is interpreted as a combination of high survival rate of large, uninfected mosquitoesand of low survival rate of mosquitoes infected with many oocysts.Key words: Anopheles gambiae, body size, malaria, Plasmodium falciparum, oocyst, sporozoite, transmission.INTRODUCTIONVariability within mosquito populations of factorsaffecting malaria transmission has received only littleattention, although theoretical studies have shownthat it affects the pattern of transmission (Dye &Hasibeder, 1986; Kingsolver, 1987; Koella, 1991).One of the factors contributing to variability intransmission may be body size of the mosquitovector. Considerable variation in body size withinpopulations has been observed for many mosquitospecies (Fish, 1985), including Anopheles gambiae(Gillies & De Meillon, 1968), the main vector ofmalaria in Africa. In addition, body size has beenfound to affect transmission of several viral diseases.Small individuals of Culex tritaeniorhynchus are morelikely to be infected with Japanese encephalitis virus(Takahashi, 1976) and West Nile virus (Baqar et al.1980) during a blood meal than larger individuals.Grimstad & Haramis (1984) have shown thatnutritionally deprived larvae of Aedes triseriatusdeveloped into small adults that are infected with LaCrosse virus at higher rates than larger, well-nourished individuals.The effect of body size on malaria transmissionhas only rarely been studied. Kitthawee, Ehman &Sattabongkot (1990) artificially fed four size-classesof laboratory-reared Anopheles dims with Plasmo-dium falciparum and showed that the largest size-* Present address and address for reprint requests:Department of Biology, Imperial College, Prince ConsortRoad, London SW7 2BB, UK.class developed the highest number of oocysts,though the proportion of infected mosquitoes wasindependent of size. In contrast, two earlier studiesfound no relationship between weight of Aedesaegypti and probability of infection by Plasmodiumgallinaceum (Hovanitz, 1947) or between wing lengthof Anopheles stephensi and the number of oocysts ofPlasmodium yoelii nigeriensis (Ichimori, 1989).In the present study, we examine the relationshipbetween adult size of the vector An. gambiae andinfection with P. falciparum. We examine oocysts asa measure of infection from humans to mosquitoes aswell as sporozoites as a measure of infection frommosquitoes to humans.MATERIALS AND METHODSThe study was carried out in Michenga, KilomberoDistrict, southeastern Tanzania. The area is holo- tohyperendemic for malaria with peak transmission inJune and July. The main malaria species is P.falciparum, occurring in about 95 % of all infections,and its main vectors are An. gambiae and An.funestus. The study area has been described in detailby Tanner et al. (1987) and Biro (1987).An. gambiae s.l. were sampled from 5 houses inJuly and the beginning of August, 1990. Femalemosquitoes were caught as they were resting on thewalls inside houses in the early morning. Weassumed that blood-fed mosquitoes had fed only onhumans because no animals were kept in the houses.For oocyst detection freshly fed female mosquitoeswere selected. They were fed on a 10% glucoseParasitology (1992), 104, 233-237 Printed in Great Britainhttps:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0031182000061667Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 13:18:10, subject to the Cambridge Core terms of use, available atE. O. Lyimo and J. C. Koella 234160140120> 100 Ho§ 80 H£ 60 -4020 -2-45 2-65 2-85 305 3-25 3-452-55 2-75 2-95 3-15 3-35 3-55Wing length (mm)coeoD.£0-50-40-3 -0-2 -0-1 -2-64 2-76 2-84 2-91 2-99 304 3-12 3-22Wing length (mm)103 4 5 6 7 8 9Number of oocystsFig. 1. Frequency distributions of (A) wing lengths and(B) oocyst numbers in infected mosquitoes. In (A) thewing lengths are grouped into 01 mm intervals and eachinterval is labelled with its midpoint.solution for 5 days, when their abdomens weredissected and examined for oocysts. For sporozoitedetection mosquitoes were killed immediately aftercapture. Their heads and thoraxes were investigatedwith an ELISA specific for the (NANP)40 repeatregion of the circumsporozoite protein (Campbellet al. 1987). The cut-off value for the optical densityseparating infected from uninfected mosquitoes wasset to the optical density of a control well on the sameplate containing 500 sporozoites, which allows acompromise between the number of false positivesand false negatives (N. Weiss, unpublished data).The size of a mosquito was measured as the lengthof its wing, which correlated with dry weight(E. Lyimo, unpublished data). Wings were measuredto the nearest 0-01 mm with an ocular micrometerfrom the distal end of the alula to the tip, excludingthe fringe scales.For the analysis of the proportions of mosquitoeswith oocysts and with sporozoites the wing lengthswere grouped into 8 size classes. The cut-off valuesbetween classes were determined so that the numberof mosquitoes in each class was similar. A logisticregression, with wing length and the square of thecotoQ.o0-8 -0-6 -0-4 -0-2 -n -r2-68 3-112-85 2-97Wing length (mm)Fig. 2. Association between infection with oocysts andwing length. The x axis is labelled with the mean winglength of the mosquitoes in each size-class. (A)Proportions of mosquitoes infected in 8 size-classes. Thevertical lines denote the 95 % confidence intervals. Thehorizontal, dotted line denotes the overall proportion ofinfected mosquitoes. (B) Proportions of the infectedmosquitoes with 2 or more oocysts in 4 size-classes. Themeasured proportions are shown as bars, the 95 %confidence intervals as vertical lines. The horizontal,dotted line denotes the overall proportion of infectedmosquitoes with 2 or more oocysts. The proportionspredicted by a logistic regression (see text and Table 1)are shown as connected diamonds.wing length as independent variables, tested for anassociation between wing length and proportionspositive. The relationship between wing length andproportion infected, / , was thus estimated as theparameters of the modelIn = A + B (wing length) + C (wing length)2.Because the distribution of oocysts was over-dispersed, with many mosquitoes harbouring only 1oocyst, the relationship between number of oocystsand wing length could not be found with aproduct—moment correlation analysis. Therefore therelationship between wing length and the proportionof those infected mosquitoes harbouring 2 or morehttps:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0031182000061667Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 13:18:10, subject to the Cambridge Core terms of use, available atMosquito size and malaria infection 235Table 1. Analysis of the relationship between wing length andproportions of mosquitoes infected (a) with oocysts, (b) with 2 or moreoocysts, or (c) with sporozoites(The estimates are the parameters of a logistic regression. Thus the proportion ofinfected mosquitoes, / , was modelled as In (//I —f) = A + B (wing length) + C(wing length)2 for (a) and (c), where 8 classes were available, and asIn (//I —f) = A + B (wing length) for (6), where only 4 size-classes were available.)SourcerEstimate(a) Oocyst(b) > 1 Oocyst(c) SporozoiteInterceptWing length(Wing length)2Likelihood ratioInterceptWing lengthLikelihood ratioInterceptWing length(Wing length)2Likelihood ratio111511211150-300-300-344-885-996-340-3211-2511-5512003-470-5840-5830-5580-4310-0140-0120-853< 0001< 0001< 00010-62823-23-16-122-96514-34-51661090-76-1213-36oocysts was analysed. Only infected mosquitoeswere used for this analysis. So that size-classes hada sufficient number of mosquitoes, we formed 4 newsize-classes by merging adjacent classes mentionedabove. The relationship was found with a logisticregression with wing length as the independentvariable. No quadratic term was included becauseonly 4 size classes were involved. The logisticregressions were performed with the procedureCATMOD of the SAS package (SAS, 1988).Because of the procedures described above, oocystinfection was measured as an incidence, i.e. as theproportion of mosquitoes that acquired a newinfection during a blood meal, whereas sporozoiteinfection was measured as a prevalence, i.e. theproportion of mosquitoes that harboured sporozoitesat a given time. Thus the two proportions are notdirectly comparable.RESULTSA total of 324 mosquitoes was dissected for oocystsand 425 investigated for sporozoites.The wing lengths varied from 2-41 to 352 mm(Fig. 1 A) with a mean of 2-92 mm and a standarddeviation of 0-175 mm. It was found that 18-2% ofthe mosquitoes were infected with oocysts and in theinfected mosquitoes the number of oocysts rangedfrom 1 to 49 (Fig. 1 B) with a mean of 5-3 and amedian of 2-5. Of the infected mosquitoes 38-9% hadonly 1 oocyst. The proportion of mosquitoes withsporozoites was 28-1 %.The proportion of mosquitoes infected withoocysts (Fig. 2A) was independent of size (Tablela) . In contrast, the proportion of mosquitoesinfected with 2 or more oocysts (Fig. 2B) increasedwith wing length (Table \b). In the smallest size-class 37-5 % of the mosquitoes had more than•D0)CotoQ.O0 - 6 -0-5 -0-4 -0-3 - .0-2 -0-1 -n - -/I*— | jr -~N-] \12-62 2-76 2-84 2-91 2-99 304 3-12 3-25Wing length (mm)Fig. 3. Proportion of mosquitoes infected withsporozoites in 8 size-classes. The x axis is labelled withthe mean wing length of the mosquitoes in each sizeclass category. The vertical lines denote the 95 %confidence intervals. The horizontal, dotted line denotesthe overall proportion of infected mosquitoes. Theproportions predicted by a logistic regression (see textand Table 1) are shown as connected diamonds.1 oocyst, whereas 88-9 % of the mosquitoes in thelargest size-class had more than 1.The proportion of mosquitoes with sporozoitesincreased from 19-5 % in the smallest mosquitoes to41-1 % in intermediate size-classes and droppedagain to 7-1 % in the largest mosquitoes (Fig. 3). Thenon-linear relationship of wing length with pro-portion infected could be closely approximated witha quadratic function (Table 1 c). The parameters ofthe logistic regression (Table 1 c) estimated thehighest proportion at —B/2C = 2-85 mm, whichwas slightly below the mean wing length (Fig. 1 A).https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0031182000061667Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 13:18:10, subject to the Cambridge Core terms of use, available atE. O. Lyimo and J. C. Koella 236DISCUSSIONAdult size of An. gambiae affected key factors oftransmission of the malaria parasite P. falciparum.Although the risk of becoming infected during abloodmeal was independent of size, mosquitoestended to develop many oocysts only if they werelarge. This pattern was similar to that observed inlaboratory-reared mosquitoes (Kitthawee et al. 1990)and might have been due to differences in size of thebloodmeal. Size of the bloodmeal is positivelycorrelated with body size in several mosquito species(Reisen, 1975; Ichimori, 1989; Kitthawee et al.1990), including An. gambiae (P. Billingsley, un-published data). For the low gametocyte densitiesencountered in human infections this leads to apositive correlation between size of the mosquito,number of ingested gametocytes, and number ofoocysts (Carter & Graves, 1988; T. Ponnudurai,personal communication).Intermediate-sized mosquitoes were most likely toharbour sporozoites. The peak sporozoite infectionwas observed for mosquitoes close to the mean sizein the population. This result is contrary to theexpectation. Because adult survival generally increas-es with body size (Nasci, 1986; Packer & Corbet,1989; Landry, Defoliart & Hogg, 1988; Haramis,1985), the proportion of mosquitoes surviving theparasite's incubation period is expected to be highestin the largest mosquitoes, which would lead to thehighest prevalence of sporozoites in the largestmosquitoes (Macdonald, 1958). The fact that preva-lence decreased in the largest mosquitoes thusimplies increased mortality in large mosquitoes. Wecould speculate that this increased mortality was dueto the large number of oocysts found in the largemosquitoes. Although investigations of the patho-genicity of malaria parasites in mosquitoes are notconclusive, recent studies (Gad, Maier & Piekarski,1979; Klein et al. 1982) have shown that survival isreduced in infected mosquitoes, in particular inthose mosquitoes harbouring more than 10 oocysts(Klein et al. 1986).Whatever the processes leading to the observedpatterns, mosquito size has a strong effect on thenumber of gametocytes that develop into oocysts andsporozoites. Thus, mosquito size might influence therate at which malaria parasites are transmitted.We thank Derek Charlwood, Jo Lines, Mike Packer andAndy Read for helpful discussions and Drs Chris Dye,Willem Takken, Peter Billingsley for a critical review of anearlier version of the manuscript. The entomology team atSTIFL helped with mosquito collections. E. Lyimo wassupported by the Directorate General for InternationalCooperation of the Government of the Netherlands. Thestudy was undertaken as part of the Kilombero MalariaProject, a joint project of the World Health Organizationand of the Governments of Tanzania, Switzerland, and theNetherlands.REFERENCESBAQAR, w., HAYES, T. & AHMED, T. (1980). The effect oflarval rearing conditions and adult age on thesusceptibility of Culex tritaeniorhynchus to infectionwith West Nile virus. Mosquito News 40, 165-71.BIRO, s. (1987). Investigations on the bionomics ofAnopheline vectors in the Ifakara area (KilomberoDistrict, Tanzania). Ph.D. thesis, University of Basel.CAMPBELL, G. H., BRANDLING-BENNETT, A. D., ROBERTS,J. M., COLLINS, F. H., KASEJE, D. C. O., BARBER, A. M. &TURNER, A. (1987). Detection of antibodies in humansera to the repeating epitope of the circumsporozoiteprotein of Plasmodium falciparum using the syntheticpeptide (NANP)3 in an enzyme-linkedimmunosorbent assay (ELISA). American Journal ofTropical Medicine and Hygiene 37, 17-21.CARTER, R. & GRAVES, p. M. (1988). Gametocytes. InMalaria: Principles and Practice of Malariology (ed.Wernsdorfer, W. H. & McGregor, I.), pp. 253-305.Edinburgh: Churchill Livingstone.DYE, c. & HASIBEDER, G. (1986). Population dynamics ofmosquito-borne disease: effects of flies which bitesome people more frequently than others. Transactionsof the Royal Society of Tropical Medicine and Hygiene80, 69-77.FISH, D. (1985). An analysis of adult size variation withinnatural mosquito populations. In Ecology ofMosquitoes: Proceedings of a Workshop (ed. Lounibos,L. P., Rey, J. R. & Frank, J. H.), pp. 419-429.Florida Medical Entomological Laboratory.GAD, A. M., MAIER, W. A. & PIEKARSKI, G. ( 1 9 7 9 ) .Pathology of Anopheles stephensi after infection withPlasmodium berghei berghei. I. Mortality rate.Zeitschrift fur Parasitenkunde 60, 249-61.GILLIES, M. T. & DE MEILLON, B. (1968). The Anophelinaeof Africa South of the Sahara, 2nd edn. Publicationno. 54. Johannesburg: South African Institute forMedical Research.GRIMSTAD, P. R. & HARAMIS, L. D. (1984). Aedes triseriatus(Diptera: Culicidae) and La Crosse virus. III.Enhanced oral transmission by nutrition-deprivedmosquitoes. Journal of Medical Entomology 21,229-62.HARAMIS, L. D. (1985). Larval nutrition, adult body size,and the biology of Aedes triseriatus. In Ecology ofMosquitoes: Proceedings of a Workshop (ed. Lounibos,L. P., Rey, J. R. & Frank, J. H.), pp. 431-437.Florida Medical Entomological Laboratory.HOVANITZ, w. (1947). Physiological factors whichinfluence the infection of Aedes aegypti withPlasmodium gallinaceum. American Journal of Hygiene45, 67-81.ICHIMORI, K. (1989). Correlation of mosquito size, bloodmeal size and malarial oocyst production. JapaneseJournal of Sanit Zoology 2, 81-5.KINGSOLVER, j . G. (1987). Mosquito host choice and theepidemiology of malaria. American Naturalist 130,811-27.KITTHAWEE, S., EHMAN, J. D. & SATTABONGKOT, J. ( 1 9 9 0 ) .Evaluation of survival potential and malariasusceptibility among different size classes oflaboratory-reared Anopheles dirus. American Journal ofTropical Medicine and Hygiene 43, 328-32.https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0031182000061667Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 13:18:10, subject to the Cambridge Core terms of use, available atMosquito size and malaria infection 237KLEIN, T. A., HARRISON, B. A., ANDRE, R. G., WHITMIRE,R. E. & INLAO, i. (1982). Detrimental effects ofPlasmodium cynomolgi infections on the longevity ofAnopheles dims. Mosquito News 42, 265-71.KLEIN, T. A., HARRISON, B. A., GROVE, J. S., DIXON, S. V. &ANDRE, R. G. (1986). Correlation of survival; rates ofAnopheles dirus A (Diptera: Culicidae) with differentinfection densities of Plasmodium cynomolgi. Bulletin ofthe World Health Organization 64, 901-7.KOELLA, j . c. (1991). On the use of mathematical modelsof malaria transmission. Ada Tropica 49, 1-25.LANDRY, S. V., DEFOLIART, G. R. & HOGG, D. B. ( 1 9 8 8 ) .Adult body size and survivorship in a field populationof Aedes triseriatus. Journal of the American MosquitoControl Association 4, 121-8.MACDONALD, G. (1957). The Epidemiology and Control ofMalaria. London: Oxford University Press.NASCI, R. s. (1986). The size of emerging and host-seeking Aedes aegypti and the relation of size to blood-feeding success in the field. Journal of the AmericanMosquito Control Association 2, 61-2.PACKER, M. j . & CORBET, p. s. (1989). Size variation andreproductive success of female Aedes punctor (Diptera:Culicidae). Ecological Entomology 14, 297-309.REISEN, w. K. (1975). Intraspecific competition inAnopheles stephensi Liston. Mosquito News 35, 473—82.SAS. (1988). SAS/STAT User's Guide, Release 6.03Edition. SAS Institute Inc., Cary, NC, USA.TAKAHASHI, M. (1976). The effects of environmental andphysiological conditions of Culex tritaeniorhynchus onthe pattern of transmission of Japanese encephalitisvirus. Journal of Medical Entomology 13, 275—84.TANNER, M., DEGREMONT, A., DE SAVIGNY, D., FREYVOGEL,T., MAYOMBANA, c. & TAYARI, s. (1987). Longitudinalstudy on the health status of children in Kikwawilavillage, Tanzania: study area and design. Acta Tropica44, 137-74.https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0031182000061667Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 13:18:10, subject to the Cambridge Core terms of use, available at

Relationship between body size of adult Anopheles gambiae s.l. and infection with the malaria parasite Plasmodium falciparum

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