Population dynamics of indoor sampled mosquitoes and their implication in disease transmission in Abeokuta, south-western Nigeria.
Mosquito-borne diseases are major health problems in Nigeria as in other parts of sub-Saharan Africa. Statistics show that malaria accounts for >300,000 deaths from >20 million clinical cases annually while 10-20% of hospital admissions are due to malaria (1). Apart from malaria, other mosquito-borne diseases have also accounted for huge economic loss, social disgrace, low productivity, absenteeism, sleeplessness, etc. in many parts of the country (2). Mosquitoes have been observed to have temporal and spatial distribution depending on the species and the prevailing climatic and environmental conditions (3,4). Several factors have been reported to influence the vectorial role of mosquitoes in disease transmission like abundance, biting behaviour, host preference and longetivity (5). A full understanding of these factors is a prerequisite in planning effective vector control measures.
Abeokuta is one of the urban areas in Nigeria which has witnessed tremendous growth in terms of human population and resource development in recent time (6). The increase in environmental modification as a result of urbanization is usually being accompanied by creation of more breeding sites for mosquitoes which most often lead to the increase in the incidence of mosquito-borne diseases (7). The previous epidemiological studies showed that Abeokuta is prone to mosquito-borne diseases (8,9) and the environmental condition of the city supported the prolific breeding of mosquito vector species.
The present study therefore provides information on species composition, seasonal abundance and parity of the indoor sampled mosquitoes with the view of understanding the possible implications on the transmission of mosquito-borne diseases and in planning effective control strategies in Abeokuta, Ogun State Nigeria.
Material & Methods
Study area: The study locations comprise Ago-Ika, Ijaye, Kugba, Ibara and Obantoko which are within Abeokuta City, located on approximately latitude 7[degrees]10' N and longitude 3[degrees]21' E in the transitional zone between the tropical rainforest and derived Savannah zone in the south-west Nigeria. Abeokuta, the state capital of Ogun State is rocky with rapid construction of roads and drainage systems across the city. It experiences two seasons, the dry season (November to March) and the wet season (April to October). Goats, dogs and chickens could be found in few areas but not commonly seen in town. The ratio of human to other vertebrates is 1: 500.
Sampling of adult mosquitoes: Adult mosquitoes were collected in three randomly selected houses in each of the study areas once a week using Center for Disease Control (CDC) light-traps model 512 between August 2005 and July 2006. Only one trap was used in each house on each catching night. The trap was suspended from the roof about 1.5 m above the floor and 0.5 m from a bed occupied by an adult sleeper using untreated bednet. Each trap was operated with 6 volt rechargeable battery every week. The sleeper in each house was instructed to switch on the trap at 2000 hrs and switch it off at 0500 hrs after the neck of the collection bag has been properly tied. The mosquitoes trapped were identified using gross morphological keys as described by Gillet (10). Only the female mosquitoes were retained, the males were subsequently discarded since they do not transmit diseases.
The gonotrophic stages of the species were examined according to the external appearance of the stomach contents using hand lens as described by WHO (11). The female mosquitoes were classified as fed, unfed and gravid. The ovaries of the unfed and freshly fed mosquitoes were dissected using entomological dissecting pin. The wings and the legs of each mosquito were removed while the remaining part was dissected by placing it on a slide containing distilled water. The degree of coiling of ovarian tracheoles was then observed to determine if the female is parous or nulliparous. The blood meal source was identified by microscopic examination and precipitin technique using human antiserum as described earlier (11).
Rainfall data from August 2005 to July 2006 were collected from the Department of Water Resources and Agro-meteorology of the University of Agriculture, Abeokuta.
Statistical analysis: Analysis of variance was used to test significant difference in abundance and monthly collection of mosquito species. Correlation was also used to determine the association between rainfall and mosquito abundance.
In all, 2693 mosquitoes were caught between August 2005 and July 2006 in five locations within Abeokuta metropolis. The mosquitoes caught belong to five genera, namely; Aedes, Culex, Anopheles, Mansonia and Coquilletidia. Out of the 10 species encountered, M. africana was the most abundant species (35.65%), followed by Cx. quinquefasciatus (32.23%). Other species encountered in decreasing order 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%). The difference in abundance was statistically significant (p <0.05). Mansonia africana occurred and dominated other vector species for most part of the year except April, May and June when Cx. quinquefasciatus and An. gambiae s.l. predominated. Apart from M. africana, Cx. quinquefasciatus, An. gambiae s.l., Ae. albopictus and Coq. maculipennis also occurred throughout the year. Anopheles funestus was caught only in April, June and September while Cx. duttoni and Cx. tigripes occurred only in October and November respectively (Table 1).
The seasonal abundance of main vector species showed that the abundance of An. gambiae s.l., Cx. quinquefasciatus, Ae. albopictus, M. africana, Ae. aegypti and M. uniformis increased as the season progressed from January with a drastic decline between June and July and further increased in August and September and finally declined between October to December. However, M. uniformis was only encountered between January and July while the population of Ae. aegypti declined to nil between December and February (Fig. 1). Positive correlation and significant relationship was observed between the rainfall pattern and the seasonal abundance of the species (p <0.05).
The results of abdominal condition of the flies revealed that majority of the vector species collected were unfed except An. gambiae s.l. and Ae. albopictus. Most of the An. gambiae s.l. trapped were either fed (66.64%) or gravid (22.19%). Majority of Ae. albopictus collected were blood fed (66.22%) but only small number were gravid (4.05%) (Table 2). Moreover, the parous rate of the mosquitoes dissected showed that majority was nulliparous. However, An. gambiae s.l. recorded the highest number of parous females, while M. uniformis recorded the lowest. Despite the high number of M. africana encountered, the large proportion of the mosquitoes was nulliparous (Table 2). The blood meal obtained from the abdomen of all fed mosquitoes was of human host, therefore, the human blood index of each species was 1.
[FIGURE 1 OMITTED]
Mosquito-borne diseases still remain the major public health problem in Africa and their transmission is becoming frequent on a daily basis due to widespread of the insects. The presence of five genera of mosquitoes, Aedes, Culex, Anopheles, Mansonia and Coquilettidia in this study is an indication that the climatic and environmental conditions of Abeokuta are condusive to support the survival and development of wide range of mosquitoes. The seasonal abundance of the mosquitoes showed that most of the vectors were collected during the wet season which could be associated to the availability of more breeding sites created by the rainfall. This observation may invariably suggest that the incidence of mosquito-borne diseases will be high during the wet season than the dry season. However, there was reduction in abundance of the mosquitoes between May and June which increased back in July. The marked reduction in numbers during this period may be due to the heavy rainfall. Heavy rainfall is likely to flush breeding sites, strand larvae, cause mechanical damage and egg mortality, therefore, reduces the abundance of adult mosquitoes (3).
The high abundance of the bancroftian vectors encountered in the present study should be a source of concern as this could translate to the risk of bancroftian filariasis by the residents. Mansonia africana, M. uniformis and Cx. quinquefasciatus have been incriminated as efficient vectors of filariasis in Ogun state (4). Anopheles gambiae s.l., the major malaria vector in Africa has also been found in recent time as an efficient transmitter of filariasis (2). Moreover, the proportion of Ae. aegypti and Ae. albopictus collected by light-trap during this study also have many epidemiological implications. These species would have been caught during the earlier part of the night as Aedes are generally regarded as day biters but their high proportion had also been reported in night catches in a suburb of Abeokuta. Ae. aegypti and Ae. albopictus are known vectors of yellow fever in Africa (3, 5, 6, 10). Though, yellow fever epidemic has not been recorded in recent time, but history had it that the residents of Abeokuta had once witnessed the epidemic of the disease (9). Apart from transmitting yellow fever virus, Ae. aegypti has been incriminated in harbouring filarial worms and could as such transmit the infective stage to the potential host (10, 12).
The preponderance of unfed mosquitoes might have been influenced by some host-seeking factors. The reproductive success of mosquitoes depends on host defensive behaviour (13). Most mosquitoes could have been trapped indoor while searching for host after their emergence from the breeding sites. However, the high number of fed mosquitoes observed in some species contradicts the earlier report on mosquitoes in some rural areas of Ogun State (7). The differences in human sleeping habits could have been responsible for this. In urban areas, many residents usually keep indoors as early as 1900 hrs, therefore, exposing them to indoor appetitive mosquitoes before going to bed unlike the rural dwellers who usually spend the larger part of the night outdoors. Moreover, it could plausibly be that the fed females were attracted to the light source on their exit of the house during ovipositional flight (14). This could also be the reason accounting for the appreciable proportion of gravid mosquitoes collected by the trap.
The dissection of the species for parity also showed that the majority of the mosquitoes was nulliparous which indicates a low survival rate and thus, low vectorial capacity in transmitting disease as the only parous flies could transmit disease. On the other hand, the high number of nulliparous mosquitoes could plausibly be an indication of high productivity of the mosquito breeding sites. However, the relatively high parous rate of An. gambiae s.l. could have probably accounted for high prevalence rate of malaria in Abeokuta (8) since only aged mosquitoes have chance of transmitting parasites.
Though, all the blood meals of the fed mosquitoes in all the dominant species were of human origin, the earlier reports on some of these species showed different view. Apart from An. gambiae s.l. that feeds primarily on human blood, other species have been reported to be catholic in the host-seeking, utilizing both man and animal bloods (12,13). The ability of these mosquitoes to utilize both man and animal bloods could be related to host availability and acceptability. Though all the blood meal tested was positive for human blood, this may not indicate that the mosquitoes were completely anthropophilic in their feeding but could be a reflection of availability of human host as compared to other alternative hosts. However, the high human blood index observed in the mosquitoes is of vectorial importance.
We, therefore, recommend that the large-scale distribution of insecticide-treated nets should be instituted in Abeokuta and the residents should be encouraged to avoid man-mosquito contact. The Government of Ogun State should also embrace clean environment policy and clearing of blocked drainages as part of the measures to reduce mosquito breeding sites. All these measures will go a long way to reduce the incidence of mosquito-borne diseases in the city.
The authors are grateful to Messers Akindele Adeyi, Junaid Qazim, Funmilayo Ola and Samuel Bankole for their assistance in field and laboratory analysis of mosquitoes. The contribution of Late (Dr) A.A.S. Amusan to the study is greatly acknowledged.
Received: 22 July 2009
Accepted in revised form: 15 January 2010
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M.A. Adeleke (a, b), C.F. Mafiana (a), A.B. Idowu (a), S.O. Sam-Wobo (a) & O.A. Idowu (a)
(a) Department of Biological Sciences, University of Agriculture, Abeokuta, Ogun State; (b) Public Health Division, Nigerian Institute of Medical Research, Yaba, Lagos, Nigeria
Corresponding author: M.A. Adeleke, Public Health Division, Nigerian Institute of Medical Research, P.M.B. 2013,Yaba, Lagos, Nigeria.
Table 1. Species diversity of mosquitoes collected between August 2005 and July 2006 in Abeokuta Mosquito species Aug Sep Oct Nov Dec Mansonia africana 110 157 177 128 75 Mansonia uniformis 0 0 0 0 0 Culex quiquefasciatus 74 95 96 70 62 Culex duttoni 0 0 7 0 0 Culex tigripes 0 0 0 7 0 Anopheles gambiae 23 24 25 21 9 Anopheles funestus 0 4 0 0 0 Aedes aegypti 2 8 4 6 0 Aedes albopictus 19 18 31 17 2 Coquilletidia macullipennis 18 20 30 29 21 Total 246 326 370 278 169 Percentage 9.13 12.11 13.74 10.32 6.28 Mosquito species Jan Feb Mar Apr May Mansonia africana 42 56 76 44 17 Mansonia uniformis 3 15 15 7 3 Culex quiquefasciatus 32 24 50 113 107 Culex duttoni 0 0 0 0 0 Culex tigripes 0 0 0 0 0 Anopheles gambiae 5 12 26 47 50 Anopheles funestus 0 0 0 2 0 Aedes aegypti 0 0 8 3 12 Aedes albopictus 2 4 6 14 14 Coquilletidia macullipennis 11 13 19 22 19 Total 95 124 200 252 222 Percentage 3.53 4.60 7.43 9.36 8.24 Mosquito species Jun Jul Total Mansonia africana 10 68 960 Mansonia uniformis 4 2 49 Culex quiquefasciatus 61 84 868 Culex duttoni 0 0 7 Culex tigripes 0 0 7 Anopheles gambiae 76 46 364 Anopheles funestus 1 0 7 Aedes aegypti 7 2 52 Aedes albopictus 19 12 158 Coquilletidia macullipennis 06 13 221 Total 184 227 2693 Percentage 6.83 8.43 Table 2. Gonotrophic stages and parity rate of mosquito species at the study locations in Abeokuta Species Unfed (%) Fed (%) Mansonia africana 479 (49.8) 379 (39.4) Mansonia uniformis 25 (51.02) 19 (38.78) Culex quiquefasciatus 435 (49.77) 346 (39.48) Anopheles gambiae 59 (16.16) 225 (61.64) Aedes aegypti 28 (50.19) 20 (36.36) Aedes albopictus 44 (29.73) 98 (66.23) Parity rate Species Gravid (%) (No. dissected) Mansonia africana 102 (10.63) 14 (855) Mansonia uniformis 5 (10.20) 11 (44) Culex quiquefasciatus 93 (10.64) 15 (775) Anopheles gambiae 81 (22.19) 33 (284) Aedes aegypti 07 (12.72) 21 (47) Aedes albopictus 06 (4.05) 2 (142)
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|Author:||Adeleke, M.A.; Mafiana, C.F.; Idowu, A.B.; Sam-Wobo, S.O.; Idowu, O.A.|
|Publication:||Journal of Vector Borne Diseases|
|Date:||Mar 1, 2010|
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