Background & objectives: A longitudinal study was carried out to investigate the speciescomposition, seasonal abundance, parity and feeding preference of indoor sampled mosquitoes inAbeokuta, south-western Nigeria.Methods: The mosquitoes were sampled weekly from five stratified locations using Center forDisease Control (CDC) light-traps between August 2005 and July 2006. The mosquitoes wereexamined for abdominal condition and dissected for age composition. Microscopic and precipitintechniques were also employed for the determination of host blood source.Results: A total of 2969 mosquitoes which belong to 10 species of mosquitoes were collectedduring the study period. Mansonia africana (35.65%) constituted the most abundant species followedby Culex quinquefasciatus (32.23%) and Anopheles gambiae complex (13.52%). Other species indecreasing order of abundance were Coquilletidia maculipennis (8.2%), Aedes albopictus (5.9%),Ae. aegypti (1.93%), M. uniformis (1.81%), Cx. duttoni (0.25%), Cx. tigripes (0.25%) and An.funestus (0.25%). Seasonal abundance revealed a significant difference (p <0.05) in the populationof mosquito vectors collected during the wet season as compared to the dry season and theirabundance was positively correlated with rainfall. The results showed that the majority of thevector species collected were unfed and nulliparous. Moreover, the blood meal test was positive forhuman blood.Conclusion: The preponderance of mosquitoes observed in the study is of public health concernsince they serve as vectors of most tropical diseases including malari

M.A. Adeleke, C.F. Mafiana, A.B. Idowu, S.O. Sam-Wobo & O.A. Idowu

Directory of Open Access Journals

J Vector Borne Dis 47, March 2010, pp. 33–38Population dynamics of indoor sampled mosquitoes and theirimplication in disease transmission in Abeokuta, south-western NigeriaM.A. Adelekea,b, C.F. Mafianaa, A.B. Idowua, S.O. Sam-Woboa & O.A. IdowuaaDepartment of Biological Sciences, University of Agriculture, Abeokuta, Ogun State; bPublic Health Division, NigerianInstitute of Medical Research, Yaba, Lagos, NigeriaAbstractBackground & objectives: A longitudinal study was carried out to investigate the speciescomposition, seasonal abundance, parity and feeding preference of indoor sampled mosquitoes inAbeokuta, south-western Nigeria.Methods: The mosquitoes were sampled weekly from five stratified locations using Center forDisease Control (CDC) light-traps between August 2005 and July 2006. The mosquitoes wereexamined for abdominal condition and dissected for age composition. Microscopic and precipitintechniques were also employed for the determination of host blood source.Results: A total of 2969 mosquitoes which belong to 10 species of mosquitoes were collectedduring the study period. Mansonia africana (35.65%) constituted the most abundant species followedby Culex quinquefasciatus (32.23%) and Anopheles gambiae complex (13.52%). Other species indecreasing order of abundance were Coquilletidia maculipennis (8.2%), Aedes albopictus (5.9%),Ae. aegypti (1.93%), M. uniformis (1.81%), Cx. duttoni (0.25%), Cx. tigripes (0.25%) and An.funestus (0.25%). Seasonal abundance revealed a significant difference (p <0.05) in the populationof mosquito vectors collected during the wet season as compared to the dry season and theirabundance was positively correlated with rainfall. The results showed that the majority of thevector species collected were unfed and nulliparous. Moreover, the blood meal test was positive forhuman blood.Conclusion: The preponderance of mosquitoes observed in the study is of public health concernsince they serve as vectors of most tropical diseases including malaria.Key words Mosquitoes; Nigeria; parity; seasonal abundanceIntroductionMosquito-borne diseases are major health problemsin Nigeria as in other parts of sub-Saharan Africa.Statistics show that malaria accounts for >300,000deaths from >20 million clinical cases annually while10–20% of hospital admissions are due to malaria1.Apart from malaria, other mosquito-borne diseaseshave also accounted for huge economic loss, socialdisgrace, low productivity, absenteeism, sleepless-ness, etc. in many parts of the country2. Mosquitoeshave been observed to have temporal and spatial dis-tribution depending on the species and the prevail-ing climatic and environmental conditions3,4. Sev-eral factors have been reported to influence the vec-torial role of mosquitoes in disease transmission likeabundance, biting behaviour, host preference andlongetivity5. A full understanding of these factors isa prerequisite in planning effective vector controlmeasures.Abeokuta is one of the urban areas in Nigeria whichhas witnessed tremendous growth in terms of hu-man population and resource development in recent J  VECTOR  BORNE  DIS  47, MARCH 2010 34time6. The increase in environmental modification asa result of urbanization is usually being accompa-nied by creation of more breeding sites for mosqui-toes which most often lead to the increase in the in-cidence of mosquito-borne diseases7. The previousepidemiological studies showed that Abeokuta isprone to mosquito-borne diseases8,9 and the envi-ronmental condition of the city supported the pro-lific breeding of mosquito vector species.The present study therefore provides information onspecies composition, seasonal abundance and parityof the indoor sampled mosquitoes with the view ofunderstanding the possible implications on the trans-mission of mosquito-borne diseases and in planningeffective control strategies in Abeokuta, Ogun StateNigeria. Material & MethodsStudy area: The study locations comprise Ago-Ika,Ijaye, Kugba, Ibara and Obantoko which are withinAbeokuta City, located on approximately latitude7°10’ N and longitude 3°21’ E in the transitional zonebetween the tropical rainforest and derived Savan-nah zone in the south-west Nigeria. Abeokuta, thestate capital of Ogun State is rocky with rapid con-struction of roads and drainage systems across thecity. It experiences two seasons, the dry season (No-vember to March) and the wet season (April to Oc-tober). Goats, dogs and chickens could be found infew areas but not commonly seen in town. The ratioof human to other vertebrates is 1: 500.Sampling of adult mosquitoes: Adult mosquitoeswere collected in three randomly selected houses ineach of the study areas once a week using Center forDisease Control (CDC) light-traps model 512 be-tween August 2005 and July 2006. Only one trapwas used in each house on each catching night. Thetrap was suspended from the roof about 1.5 m abovethe floor and 0.5 m from a bed occupied by an adultsleeper using untreated bednet. Each trap was oper-ated with 6 volt rechargeable battery every week.The sleeper in each house was instructed to switchon the trap at 2000 hrs and switch it off at 0500 hrsafter the neck of the collection bag has been prop-erly tied. The mosquitoes trapped were identifiedusing gross morphological keys as described byGillet10. Only the female mosquitoes were retained,the males were subsequently discarded since they donot transmit diseases.The gonotrophic stages of the species were exam-ined according to the external appearance of thestomach contents using hand lens as described byWHO11. The female mosquitoes were classified asfed, unfed and gravid. The ovaries of the unfed andfreshly fed mosquitoes were dissected using ento-mological dissecting pin. The wings and the legs ofeach mosquito were removed while the remainingpart was dissected by placing it on a slide containingdistilled water. The degree of coiling of ovarian tra-cheoles was then observed to determine if the fe-male is parous or nulliparous. The blood meal sourcewas identified by microscopic examination and pre-cipitin technique using human antiserum as describedearlier11.Rainfall data from August 2005 to July 2006 werecollected from the Department of Water Resourcesand Agro-meteorology of the University of Agricul-ture, Abeokuta.Statistical analysis: Analysis of variance was usedto test significant difference in abundance andmonthly collection of mosquito species. Correlationwas also used to determine the association betweenrainfall and mosquito abundance.ResultsIn all, 2693 mosquitoes were caught between Au-gust 2005 and July 2006 in five locations withinAbeokuta metropolis. The mosquitoes caught belongto five genera, namely; Aedes, Culex, Anopheles,Mansonia and Coquilletidia. Out of the 10 speciesencountered, M. africana was the most abundant spe-cies (35.65%), followed by Cx. quinquefasciatus(32.23%). Other species encountered in decreasing 35 ADELEKE ET AL: POPULATION DYNAMICS OF INDOOR RESTING MOSQUITOESorder of abundance were An. gambiae s.l.(13.52%), Coq. maculipennis (8.2%), Ae.albopictus (5.9%), Ae. aegypti (1.93%), M.uniformis (1.81%), Cx. duttoni (0.25%), Cx.tigripes (0.25%) and An. funestus (0.25%). Thedifference in abundance was statistically signifi-cant (p <0.05). Mansonia africana occurred anddominated other vector species for most part ofthe year except April, May and June when Cx.quinquefasciatus and An. gambiae s.l. predomi-nated. Apart from M. africana, Cx. quinque-fasciatus, An. gambiae s.l., Ae. albopictus andCoq. maculipennis also occurred throughout theyear. Anopheles funestus was caught only inApril, June and September while Cx. duttoni andCx. tigripes occurred only in October and No-vember respectively (Table 1).The seasonal abundance of main vector speciesshowed that the abundance of An. gambiae s.l.,Cx. quinquefasciatus, Ae. albopictus, M.africana, Ae. aegypti and M. uniformis increasedas the season progressed from January with adrastic decline between June and July and fur-ther increased in August and September and fi-nally declined between October to December.However, M. uniformis was only encounteredbetween January and July while the populationof Ae. aegypti declined to nil between Decemberand February (Fig. 1). Positive correlation andsignificant relationship was observed between therainfall pattern and the seasonal abundance of thespecies (p <0.05).The results of abdominal condition of the fliesrevealed that majority of the vector species col-lected were unfed except An. gambiae s.l. andAe. albopictus. Most of the An. gambiae s.l.trapped were either fed (66.64%) or gravid(22.19%). Majority of Ae. albopictus collectedwere blood fed (66.22%) but only small numberwere gravid (4.05%) (Table 2). Moreover, theparous rate of the mosquitoes dissected showedthat majority was nulliparous. However, An.gambiae s.l. recorded the highest number of pa-Table 1. Species diversity of mosquitoes collected  between August 2005 and July 2006 in AbeokutaMosquito speciesAugSepOctNovDecJanFebMarAprMayJunJulTotalMansonia africana1101571771287542567644171068960Mansonia uniformis0000031515734249Culex quiquefasciatus74959670623224501131076184868Culex duttoni0070000000007Culex tigripes0007000000007Anopheles gambiae2324252195122647507646364Anopheles funestus0400000020107Aedes aegypti284600083127252Aedes albopictus19183117224614141912158Coquilletidia macullipennis182030292111131922190613221Total246326370278169951242002522221842272693Percentage9.1312.1113.7410.326.283.534.607.439.368.246.838.43 J  VECTOR  BORNE  DIS  47, MARCH 2010 36rous females, while M. uniformis recorded the low-est. Despite the high number of M. africana encoun-tered, the large proportion of the mosquitoes wasnulliparous (Table 2). The blood meal obtained fromthe abdomen of all fed mosquitoes was of humanhost, therefore, the human blood index of each spe-cies was 1.DiscussionMosquito-borne diseases still remain the major pub-lic health problem in Africa and their transmission isbecoming frequent on a daily basis due to widespreadof the insects. The presence of five genera of mos-quitoes, Aedes, Culex, Anopheles, Mansonia andCoquilettidia in this study is an indication that theclimatic and environmental conditions of Abeokutaare condusive to support the survival and develop-ment of wide range of mosquitoes. The seasonalabundance of the mosquitoes showed that most ofthe vectors were collected during the wet seasonwhich could be associated to the availability of morebreeding sites created by the rainfall. This observa-tion may invariably suggest that the incidence ofmosquito-borne diseases will be high during the wetseason than the dry season. However, there was re-duction in abundance of the mosquitoes between Mayand June which increased back in July. The markedreduction in numbers during this period may be dueto the heavy rainfall. Heavy rainfall is likely to flushbreeding sites, strand larvae, cause mechanical dam-age and egg mortality, therefore, reduces the abun-dance of adult mosquitoes3.The high abundance of the bancroftian vectors en-countered in the present study should be a source ofconcern as this could translate to the risk ofbancroftian filariasis by the residents. Mansoniaafricana, M. uniformis and Cx. quinquefasciatushave been incriminated as efficient vectors of filari-asis in Ogun state4. Anopheles gambiae s.l., the ma-jor malaria vector in Africa has also been found inrecent time as an efficient transmitter of filariasis2.Moreover, the proportion of Ae. aegypti and Ae.albopictus collected by light-trap during this studyalso have many epidemiological implications. Thesespecies would have been caught during the earlierpart of the night as Aedes are generally regarded asday biters but their high proportion had also beenreported in night catches in a suburb of Abeokuta.Fig. 1: Seasonal abundance of mosquito vectors collectedat the study locations in AbeokutaTable 2. Gonotrophic stages and parity rate of mosquito species at the study locations in AbeokutaSpecies Unfed (%) Fed (%) Gravid (%) Parity rate (No. dissected)Mansonia africana 479 (49.8) 379 (39.4)   102 (10.63) 14 (855)Mansonia uniformis 25 (51.02) 19 (38.78)    5 (10.20) 11 (44)Culex quiquefasciatus 435 (49.77) 346 (39.48)    93 (10.64) 15 (775)Anopheles gambiae 59 (16.16) 225 (61.64)    81 (22.19) 33 (284)Aedes aegypti 28 (50.19) 20 (36.36)    07 (12.72) 21 (47)Aedes albopictus 44 (29.73) 98 (66.23)    06 (4.05) 2 (142) 37 ADELEKE ET AL: POPULATION DYNAMICS OF INDOOR RESTING MOSQUITOESAe. aegypti and Ae. albopictus are known vectors ofyellow fever in Africa3,5,6,10. Though, yellow feverepidemic has not been recorded in recent time, buthistory had it that the residents of Abeokuta had oncewitnessed the epidemic of the disease9. Apart fromtransmitting yellow fever virus, Ae. aegypti has beenincriminated in harbouring filarial worms and couldas such transmit the infective stage to the potentialhost10,12.The preponderance of unfed mosquitoes might havebeen influenced by some host-seeking factors. Thereproductive success of mosquitoes depends on hostdefensive behaviour13. Most mosquitoes could havebeen trapped indoor while searching for host aftertheir emergence from the breeding sites. However,the high number of fed mosquitoes observed in somespecies contradicts the earlier report on mosquitoesin some rural areas of Ogun State7. The differencesin human sleeping habits could have been respon-sible for this. In urban areas, many residents usuallykeep indoors as early as 1900 hrs, therefore, expos-ing them to indoor appetitive mosquitoes before go-ing to bed unlike the rural dwellers who usually spendthe larger part of the night outdoors. Moreover, itcould plausibly be that the fed females were attractedto the light source on their exit of the house duringovipositional flight14. This could also be the reasonaccounting for the appreciable proportion of gravidmosquitoes collected by the trap.The dissection of the species for parity also showedthat the majority of the mosquitoes was nulliparouswhich indicates a low survival rate and thus, low vec-torial capacity in transmitting disease as the only pa-rous flies could transmit disease. On the other hand,the high number of nulliparous mosquitoes couldplausibly be an indication of high productivity of themosquito breeding sites. However, the relatively highparous rate of An. gambiae s.l. could have probablyaccounted for high prevalence rate of malaria inAbeokuta8 since only aged mosquitoes have chanceof transmitting parasites.Though, all the blood meals of the fed mosquitoes inall the dominant species were of human origin, theearlier reports on some of these species showed dif-ferent view. Apart from An. gambiae s.l. that feedsprimarily on human blood, other species have beenreported to be catholic in the host-seeking, utilizingboth man and animal bloods12,13. The ability of thesemosquitoes to utilize both man and animal bloodscould be related to host availability and acceptabil-ity. Though all the blood meal tested was positivefor human blood, this may not indicate that the mos-quitoes were completely anthropophilic in their feed-ing but could be a reflection of availability of humanhost as compared to other alternative hosts. How-ever, the high human blood index observed in themosquitoes is of vectorial importance.We, therefore, recommend that the large-scale dis-tribution of insecticide-treated nets should be insti-tuted in Abeokuta and the residents should be en-couraged to avoid man-mosquito contact. TheGovernment of Ogun State should also embrace cleanenvironment policy and clearing of blocked drain-ages as part of the measures to reduce mosquitobreeding sites. All these measures will go a long wayto reduce the incidence of mosquito-borne diseasesin the city.AcknowledgementThe authors are grateful to Messers Akindele Adeyi,Junaid Qazim, Funmilayo Ola and Samuel Bankolefor their assistance in field and laboratory analysis ofmosquitoes. The contribution of Late (Dr) A.A.S.Amusan to the study is greatly acknowledged.References1. Nigeria Medical Association makes a case for the freetreatment of malaria. Nigeria: New Nigeria NewspaperLtd.,  31 July  2000.2. Anosike JC, Onwuluri COE, Nwoke BEB, Dozie INS.Laboratory investigation of the infection rates of Anoph-eles gambiae and Anopheles funestus in the transmissionof  Wuchereria bancrofti.  Nigeria J Parasitol 2003;24:153–8.3. Amusan AAS, Mafiana CF, Idowu AB, Oke OA. A sur- J  VECTOR  BORNE  DIS  47, MARCH 2010 38vey of adult mosquitoes in the hostels of the University ofAgriculture, Abeokuta, Ogun State, Nigeria. Nigeria JParasitol 2003; 24: 167–72.4. Wanji S, Mafo FF, Tendongfor N, Tanga MC, TchuenteF, Bilong Bilong CF, Njine T. Spatial distribution, envi-ronmental and physicochemical characterization ofAnopheles breeding sites in the Mount Cameroon region.J Vector Borne Dis 2009; 46: 75–80.5. Noutcha MAE, Anumudu CI. Entomological indices ofAnopheles gambiae Sensu lato at a rural community insouth-west Nigeria. J Vector Borne Dis 2009; 46: 43–51.6. Adeleke MA, Mafiana CF, Idowu AB, Adekunle MF,Sam-Wobo SO. Mosquito larval habitats and public healthimplications in Abeokuta, Ogun State, Nigeria. TanzaniaJ Health Res 2008; 10(2): 103–8.7. Amusan AAS, Mafiana CF, Idowu AB, Olatunde GO.Sampling mosquitoes with CDC light-traps in rice fieldand plantation communities in Ogun state, Nigeria. Tan-zania Health Res Bull 2005; 7: 111–6.8. Ojo DA, Mafiana CF. Evaluation of fever in the presump-tive diagnosis of malaria endemicity. Nigerian J Parasitol2001; 22: 35–42.Corresponding author: M.A. Adeleke, Public Health Division, Nigerian Institute of Medical Research, P.M.B. 2013,Yaba,Lagos, Nigeria.E-mail: healthbayom@yahoo.comReceived: 22 July 2009 Accepted in revised form: 15 January 20109. Ozumba NA, Ezekie VI, Nwundo KS, Adetunji AA,Ozumba UC. Field epidemiological observations on hu-man activities that promote the breeding of yellow fevermosquitoes in some parts of Nigeria. Nigerian J Parasitol2001; 22: 143–8.10. Gillet JD. Common African mosquitoes and their medi-cal importance (with colour illustrations). London: Will-iam Heinemann Medical Books Ltd. 1972; p. 236.11. Guide on Medical Entomology on Malaria. Pt. II. Geneva:World Health Organization 1994; p. 226.12. Amusan AAS. Distribution of mosquitoes (Diptera: Cu-licidae) and disease transmission patterns in Ogun state.Ph.D. thesis. University of Agriculture, Abeokuta, OgunState 2004; p. 336.13. Hayes RO, Tempelis CH, Hess AD, Reeves RC. Mosquitohost preference studies in Hale county, Texas. Am J TropMed Hyg 1973; 22: 270–7.14. Davis JR, Hall T, Chee ES, Majala A, Minjas J, Shiff CJ.Comparison of sampling Anopheles mosquitoes by light-trap and human-bait collections indoors at Bagamoyo,Tanzania. Med Vet Entomol 1995;  9: 249–55.

Population dynamics of indoor sampled mosquitoes and their implication in disease transmission in Abeokuta, south-western Nigeria

Abstract

Similar works

Full text

Available Versions

Directory of Open Access Journals