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Mosquito-borne diseases pose a major threat to both human populations and diversity of indigenous fauna throughout the world where they transmit diseases to more than 700 million people annually (Adeleke, Mafiana, Idowu, Sam-Wobo & Idowu, 2010). Species tend to breed in both natural habitats and artificial containers such as ground pools, gutters, domestic run-offs, tree holes, septic tanks (Mafiana et al., 1989; Okogun, Nwoke, Okere, Anosike & Esekhegbe, 2003; Aigbodion & Anyiwe, 2005). Environmental and climatic factors directly influence the distribution of mosquito larvae (Mafiana, Anaeme & Olatunde, 1998).
Studies to determine the distribution and abundance of mosquito larvae in various habitats have been carried out in various parts of Nigeria (Adeleke, Mafiana, Idowu, Adekunle & Sam-Wobo, 2008; Olayemi, Ukubuiwe & Oyibo-Usman, 2014; Akpan & Nwabueze, 2015). Species encountered in the various studies include Anopheles gambiae, Aedes aegypti, Culex quinquefasciatus, Mansonia africana, etc. These studies will help to ascertain the present state of the different mosquito species in each locality as this is necessary in mosquito control. For any vector control measures to be successful, knowledge of the breeding ecology of mosquitoes including, the types and preferences for larval habitats, spatial and temporal distribution of breeding sites, as well as, the physical, biological and chemical characteristics of the habitats are required (Olayemi, Omalu, Famotele, Shegna & Idris, 2010; Egbuche, Ezihe, Aribodor & Ukonze, 2016; Goselle et al., 2017). The continued transmission of mosquito borne diseases is associated with climatic factors and the vast larval habitats that exist in a particular environment that necessitates breeding of the vectors (Oluwasogo et al., 2016). Studies to identify mosquito larval habitats have been carried out in several parts of Nigeria including Abeokuta (Mafiana et al., 1998; Adeleke et al., 2008), Ekpoma (Okogun et al., 2003), Uyo (Usip & Edem, 2003) and Awka (Mbanugo & Okpalononuju, 2003). Therefore, this study was to determine seasonal distribution and abundance of mosquito larvae at Nkporo and Abiriba communities of Ohafia, Abia State, south eastern Nigeria.
Materials and methods
Study Area: This study was carried out in two communities namely Nkporo and Abiriba at Ohafia Local Government Area (Latitude 05o37″N - 5o617 ″N and longitude 7o 50 ″E and 7o833 ″E) of Abia State, southeastern Nigeria. The study area is within the tropical rainforest zone of Nigeria with two clearly distinguishable annual seasons - wet and dry seasons. The annual rainfall ranges from 1 750 to 2 000mm and a mean annual temperature range of 26°C to 28°C.The entire Local Government Area has a population estimation of 350 000 inhabitants (National population commission, 2006).
Two communities were selected purposely selected to represent rural (Nkporo) and Peri-urban (Abiriba) settlements in the local Government Area. The communities were selected due to their different population size which affects human activities.
Methodology: Investigations were carried arbitrarily in selected streets, roads, private and public residences and open places to determine seasonal distribution and abundance of mosquito larvae. Larval sampling was done monthly between 07.00 and 11.00 hours for eight months (from November 2014 to June 2015) representing (four months each of wet and four dry seasons). The larvae in the ground pools, containers, gutters and domestic run-offs were collected with the aid of plastic dippers while pipette was used for the collection of larvae from tree holes. Dippers could not be used in containers. Therefore, the water in them were emptied into the bowls and larvae collected according to (Service, 1993). The larvae collected were poured through a fine sieve into a white bowl to concentrate them. All collected larvae were transferred into labelled specimen bottles and transported to laboratory for rearing and identification.
Habitat characteristics recorded included the presence of vegetation, the intensity or duration of sunlight and the depth of aquatic microhabitat, submerged, emergent and terrestrial vegetation. Algal cover and debris were estimated as percentage of the habitats using a square grid.
In the laboratory, larvae were kept in bowls covered with a fine nylon mesh, to prevent emerging adults from flying away. They were subsequently transferred into rearing cage.
The cages were stood in tomatoes tins containing water to deny ants and other predators access to the larvae. The larvae were fed with a mixture of yeast and biscuits. The set up was monitored daily until the emergence of adults.
Data were analysed using Chi-Square SPSS version 20.0
Results
Mosquito breeding habitats and mosquito abundance at Ohafia L.G.A: The mosquito larvae habitats identified from the study area were ground pools 14%, containers 51%, domestic run-offs 12%, gutters 13% and tree holes 10 %. Containers had the highest breeding habitats followed by ground pools while the least was tree hole. Similarly containers housed the highest number of mosquito larvae, followed by ground pools then domestic runoff, while tree holes contained the least number (Table 1). (X2=1,036; df=4, p>0,05).
Table 1: Percentage of mosquito breeding habitat type’s abundance of larvae in Nkporo and Abiriba communities in Ohafia, Abia State.
| Location | Ground Pools | Containers | Domestic Run-offs | Gutter | Tree holes | Total | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| Nkporo | 13,7 | 49,3 | 11,5 | 15,42 | 10,1 | 54,3 | ||||
| Abiriba | 14,1 | 52,9 | 12,0 | 10,47 | 10,5 | 45,7 | ||||
| 13,9 | 51,0 | 11,7 | 13,2 | 10,3 | ||||||
Distribution of mosquito larval species collected in the study area: The abundance and distribution of species of mosquitoes in the study area is shown in Table 2. Culex quinquefasciatus recorded the highest population (40%), followed by Ae. aegypti (27%), then Ae. albopictus (17%), while the number of An. gambiae collected was (14%). The least larval species collected was An. funestus (2%) and it was found to breed only at Nkporo community.
The percentage of An. gambiae collected from Nkporo (60%) were significantly higher compare with those collected from Abiriba (40%) respectively, whereas 100% of the An. funestus were caught in Nkporo community. In contrary, a higher population of Ae aegypti were collected in Abiriba community than in Nkporo community i.e. 52% and 48% respectively. Similarly, a higher population of Ae. albopictus were collected in Abiriba (53%) community, while 48% were in Nkporo community. Higher population of Culex (55%) were caught in Nkporo community while 45% were caught in Abiriba community. Statistical analysis shows that there was significant different (X2= 81,335, df = 4, p<0,05) in the distribution of mosquito larval species caught in the study area.
Table 2: Distribution of mosquito larval species collected in the study area.
| Mosquito Larval Species | Nkporo (%) | Abiriba (%) |
|---|---|---|
| Culex quinquefasciatus | 55 | 45 |
| Aedes aegypti | 48 | 52 |
| Aedes albopictus | 48 | 53 |
| Anopheles gambiae | 60 | 40 |
| Anopheles funestus | 100 | 0 |
| Total | 54 | 47 |
Seasonal/monthly distribution of mosquito larval species in the study area: In Nkporo community, the highest collection (56%) was made in November during the dry season, followed by (30%) in December. Then the month of January (10%) and the least collections were made in the month of February (4%). In the month of November, C. quinquefasciatus were the highest larvae caught, (40%). While An. funestus were the least larvae collected. Same trend was seen in all the other months of dry season in Nkporo community. However, statistical analysis shows that there was statistical difference (X2= 56,865, df = 12, p<0,05) in the monthly distribution of mosquito larval species during dry season in Nkporo community.
During the rainy season in Nkporo community, highest collection were made in the month of June (53%), followed by the month of May (26%), then the month of April (17%). The least collections were made in the month of March (5%). Ae. Aegypti had the highest collection in the month of March (26%), while An. funestus were the least (0.00%). In the month of April, the larvae collected in highest number were C. quinquefasciatus (42%), while An. funestus were the least (4%). This trend also was observed in other months of the rainy season in Nkporo community. The month of May recorded (40%) for C. quinquefasciatus. While An. funestus (4%). However in the month of June, C. quinquefasciatus also recorded the highest collection of (40%) while An. funestus was the least (4%). Statistical analysis shows that there was significant difference (X2= 22,241, df = 12, p<0,05) in the monthly distribution of mosquito larval species during rainy season in Nkporo community.
During dry season in Abiriba community, the month of November contained highest number of collections (51%) followed by December (30%), then January (17%) and the month of February had the least number (3%). An. funestus were not caught in any of the months of the dry season. While C. quinquefasciatus were the highest mosquito larval species caught in all the month of the dry season in Abiriba community. Statistical analysis shows that there was significant difference (X2= 31,776, df = 12, p<0,05) in the monthly distribution of mosquito larval species during dry season in Abiriba community.
During rainy season in Abiriba community the month of November was when the highest collections were made (54%) and the least collections were made in the month of March (4%). Ae. aegypti had the collections of the collection made in the month of March (39%), while An. funestus was the least (0.00). Infact, An. funestus was not found during rainy season in Abiriba community. Apart from the month of March, all other months, April (43%), May (40%) and June (32%), C. quinquefasciatus recorded the highest number of collections. Statistical analysis shows that there was no significant difference (X2= 8,830, df = 12, p>0,05) in the monthly distribution of mosquito larval species during rainy season in Abiriba community. (Table 3)
Table 3: Seasonal/monthly distribution of mosquito larval species in the study area (%)
| Location | Season | Month | Anopheles gambiae | Anopheles funestus | Aedes aegypti | Aedes albopictus | Culex quinque-fasciatus | Species Total |
|---|---|---|---|---|---|---|---|---|
| NKPORO | DRY | Nov | 15,4 | 7,0 | 26,3 | 11,0 | 40,4 | 55,5 |
| Dec | 13,7 | 6,5 | 13,7 | 19,4 | 46,8 | 30,2 | ||
| Jan | 19,1 | 0,0 | 4,8 | 14,3 | 61,9 | 10,2 | ||
| Feb | 23,5 | 0,0 | 5,9 | 29,4 | 41,2 | 4,1 | ||
| WET | Mar | 32,0 | 0,0 | 26,0 | 18,0 | 24,0 | 5,0 | |
| April | 18,1 | 3,6 | 27,1 | 9,6 | 41,6 | 16,6 | ||
| May | 17,0 | 3,5 | 23,6 | 16,2 | 39,8 | 25,9 | ||
| June | 13,5 | 4,0 | 27,0 | 15,4 | 40,1 | 52,5 | ||
| ABIRIBA | DRY | Nov | 10,7 | 0,00 | 28,4 | 18,1 | 42,8 | 50,7 |
| Dec | 12,8 | 0,00 | 30,4 | 16,0 | 40,8 | 29,5 | ||
| Jan | 8,6 | 0,00 | 27,1 | 11,4 | 52,9 | 16,5 | ||
| Feb | 7,14 | 0,00 | 57,1 | 7,1 | 28,6 | 3,3 | ||
| WET | Mar | 12,9 | 0,0 | 38,7 | 12,9 | 35,5 | 100,0 | |
| April | 7,7 | 0,0 | 31,9 | 17,6 | 42,9 | 3,9 | ||
| May | 11,55 | 0,0 | 31,9 | 16,3 | 40,2 | 11,30 | ||
| June | 14,81 | 0,0 | 29,9 | 23,4 | 31,9 | 31,2 |
Discussion
This study revealed that five species from three genera of mosquito larvae were present in the study area. Two Anopheles species larvae were collected (An. gambiae and An. funestus), two Aedes species (Ae. aegypti and Ae. albopictus) and one Culex species (Cx. quinquefasciatus). This is probably due to the presence of suitable breeding habitats such as domestic water containers, run-offs, gutters and other breeding places created by man. This results are similar to reports by different researchers from different parts of Nigeria (Oguoma & Ikpeze, 2008; Umaru, Akogun & Owuama, 2006; Onyido, Ezike, Ozumba, Nwankwo & Nwankwo, 2009). Of the five breeding habitats sampled during the study in the two communities, it was observed that containers had the highest number of mosquito larvae (60,02%) while the tree holes recorded the least number of mosquito larvae (6,40%). This agrees with the work done by Awolola, Oyewole, Koekemoer and Coetzee (2005), Okogun (2005), Anosike et al. (2007) and Adeleke et al. (2010), who noticed that domestic containers were the preferred breeding habitats of mosquitoes. Okogun et al. (2005) also noted that the greater number of larvae observed in containers was due to the absence of or reduced larvae predation by mosquito natural enemies in such habitat. These were also seen in the result of this study. According to Mafiana et al. (1998), tree holes can only retain water for short period of time and dry up at the time of no rain. This may be the reason to explain the low contribution of tree holes to the breeding of mosquito in the study.
Different mosquito species use different habitats for breeding as well as water quality (Onyido et al., 2009). This observation agreed with this study. Some species are restricted to a single type of breeding habitat and water quality. Others possess a larger adaptability, but their presence in a given type of breeding site was due to oviposition habits of the female mosquito which is the main determining factor of the presence of larvae in different types of larval habitats. Worthy to note in this study, was the high prevalence of Cx. quinquefasciatus in all the habitats. This is not a surprise because of its preference for polluted water (Mafiana et al., 1998). It was also observed in the current study that Cx. quinquefasciatus was not only caught in large quantities from dirty domestic runoffs and gutters. Anopheles gambiae was another species of mosquito that was found in all the habitats in high quantities. The high prevalence of An. gambiae can be explained by the fact that the female species of An. gambiae preferentially select these habitats for oviposition and this is also in agreement with the works of Bentley and Day (1989). Furthermore, it was observed that An. gambiae had high occurrence in containers, domestic run offs and gutters shows that An. gambiae can utilize other habitats apart from ground pools which was originally known (Igbinosa, 1989). Although it was observed that the Anopheles species lacked the ability to breed in containers but, evidence from this work showed that Anopheles species can utilize containers as well as other habitats as breeding sites. This result corroborates the work of Opoku, Ansa-Asare and Amoako (2005) which reported that anopheles species also occur in a wide range of habitats, but with relatively low nutrient status and high oxygen level, and which also in agreement with the findings of Aigbodion and Odiachi (2003), who documented large numbers of An. gambiae in containers, domestic run-offs and gutters. Ae. aegypti was also found to be high in this study, mostly their remarkably high prevalence in tree holes. The indiscriminate breeding habit displayed by Ae. aegypti has been reported by Adeleke et al. (2008). Also, according to Sattler et al. (2005) the species preference of breeding site was not only for the site in question but also for the water quality: depth, light and vegetation which served as sources of food and shelter appropriate for their survival and development. According to findings from this study, it was observed that all the mosquito genera preferred shallow water to deep type. It was observed that light does not have much effect on the abundance of Culex and Aedes while anopheles preferred mostly sunlight and semi-shaded water. In addition, all the mosquito genera found in the study area existed in the habitat without vegetation and in the presence of algae. While it was observed that they lack preference for water with floating plants.
Seasonal variations were also observed in the abundance and distribution of mosquito larvae in the study area. Collections made during the rainy season were about two-third of all the mosquito larvae collected in both rainy and dry season. These findings agreed with that of Bunza, Suleiman, Yusuf and Bala (2010). From this study, the month of June which had the highest amount of rainfall recorded the highest number of larvae encountered during the study. High rate in abundance of mosquito larvae in the month of June and other months of wet seasons showed that rainfall is a key factor in determining mosquito breeding rate. The study in Kenya also showed that the rainy season presents favourable environmental conditions that enhance mosquito breeding and survival through the proliferation of larval habitats and improved humidity respectively (Minakawa, Pamela & Yan, 2002). Also, similar findings by Jaensen, Niklasson and Henriksson (1986) on influence of weather and hydrological conditions on mosquito abundance have been demonstrated. C. quinquefasciatus was the most encountered species in all of the eight months of this study. This might be attributed to the presence of open drains, cracked and open septic tanks,flooded drains and natural containers that were mostly polluted with organic matter. while Ae. aegypti was another species that was encountered in a high number during both dry and rainy seasons.
This is an indication that the climatic and environmental conditions of Abiriba and Nkporo communities are conducive to support the survival and development of the mosquito vector. Surprisingly, An. funestus was only caught in some months of dry season (November and December) and some months of rainy season (April, May and June) in Nkporo community, but was not found at all in all the months of both dry and rainy seasons in Abiriba community. Anopheles funestus breeds normally in fairly clean, sunlit, small collections of standing water. This was in existence in some areas of Nkporo community and was probably the reason behind the presence of A. funestus in that area.
The outcome of this study indicates that there was a high vector breeding sites and therefore, vector control measures should be properly applied in the area. This requires proper environmental planning and surveillance. This is important to avoid the risk of outbreak of mosquito-borne diseases in the area.
This study has provided vital information to the distribution and abundance of mosquito larvae within Ohafia Local Government Area. A combination of factors of abundant rainfall, tropical temperatures and high relative humidity, breakdown of public pipe borne water systems that cause people to store up water in containers, increase in human development efforts, indiscriminate disposal of tins and cans used for food, all these and many more accounted for mosquito breeding both in dry and wet period of the study. Eliminating such mosquito breeding areas can be an extremely effective and permanent way to reduce mosquito population without resorting to insecticides.
