Background

Mosquitoes are the most common blood sucking arthropods and the most important vectors of human disease including malaria, filariases, and numerous viral diseases, such as dengue, Japanese encephalitis, West nile virus, yellow fever and Zika virus.

Mosquitoes feed on human, amphibians, reptiles, birds, mammals. Some species exhibits considerable host specificity, while others are more encompassing. Adult female mosquitoes differ generally in their behaviour especially biting habits including host blood meal preferences, time and place of biting and resting sites, which are very important in the epidemiology of disease transmission. Sometimes mosquitoes are strictly zoophilic (feeding on animals only), or strictly anthropophilic (biting only humans), while others bite both man and animals indiscriminately. Also some bite and rest indoors (endophagic and endophilic) but others bite and rest outdoors (exophagic and exophilic). Furthermore, some are crepuscular and nocturnal and others are diurnal (Service, 1980).

Mosquitoes have diverse habitats that allow them to colonize different kinds of environments. The immature stages of mosquitoes are found in a variety of aquatic habitats e.g., ponds, streams, ditches, swamps, marshes, temporary and permanent pools, rock holes, tree holes, crab holes, lake margins, plant containers (leaves, fruits, husks, tree holes, bamboo nodes), artificial containers (tires, tin cans, flower vases, bird feeders), and other habitats (Rueda, 2008).

Mosquitoes though have worldwide distribution, but are more abundant in the tropical countries of the world (Darsie, 1985). They have different breeding grounds with collection of water, which may includes, containers, tree holes, floor containers, street gutters, abandoned fish ponds, old tiers, forest canopy as well as swamps and refuse dump (Okogun, 2003).

The refuse dumps thus compose vector, vermin and other nuisance organisms capable of transmitting or causing diseases such as typhoid, diarrhea and cholera in humans and animals (Siboe, 1996). Mosquitoes breed in refuse dump and frequently visit human homes where they bite human and their survival and capacity to transmit diseases are directly linked to the availability of breeding sites (Onyido, 2006).

Prior to this study, the number of species of mosquitoes breeding in dump sites in any part of the rainforest region of Nigeria has not been fully elucidated because each of the breeding sites represent a micro-habitat for the mosquitoes. Thus, in order to provide information on mosquito species associated with breeding around refuse dumps within Enugu, Nigeria, the study was carried out to assess the species breeding on different dump sites and their abundance in dump sites within Enugu municipal.

1 Materials and Methods

1.1 Study area

The study was conducted in Enugu Municipal between March and October 2014. Enugu Municipal is located between longitudes 7^o 6’E and 7^o 54’E and latitudes 5^o 56’N and 6^o 52’N. The annual rainfall varies between 100 -200 meters with its peak occurring between March and September. The lowest rainfall of about 0.16 cubic centimetres (0.0098 cu in) is normal in February, while the highest is about 35.7 cubic centimetres (2.18 cu in) in July. Enugu is a city which covers an area of 85 sq. km with a population of about 722,664 (Federal Republic of Nigeria Official Gazette, 2007).

Enugu State Waste Management Authority (ESWAMA) responsible for management of waste was established through Enugu State Law (ENSL) No 8 and 12 of 2004 which also dissolved the Enugu State Environmental protection Agency. On establishment, ESWAMA was given the mandate to take the responsibility of the general cleanliness of the urban areas such as Enugu, Nsukka, 9th Mile Corner, Oji River, Obollor-Affor, Awgu and any other settlement as may be designated as urban by the state Government from time to time. In other to ensure thoroughness and high efficiency of service, ESWAMA functions in partnership with the private sector participants (PSPs) which it supervises.

1.2 Study sites

The field surveillances were carried out in five zones according to ESWAMA classifications, and these zones are: New haven, Emene, G.R.A, Independence Layout, and Abakpa zone (Figure 1). Within these zones, two refuse dumpsites were randomly selected based on the proximity to human dwelling and how often they are filled.

1.3 Mosquito collection, processing, identification and analysis

The eggs of different species of mosquito were collected around the different dump sites using the Ovitrap collection technique and larval sampling were conducted around the dump sites. The ovitraps were set at strategic places close to the refuse dump site as well as under the refuse dump can and were retrieved after 48 hours. The retrieved white ribbons were checked for those positive with mosquito eggs using compound microscope. The ribbons were dried under room temperature for 2 days and were soaked. The total number of eggs that hatched was recorded according to the areas of collection. The larvae collected using the dippers (ten dips per site) around the dumpsites were transferred into the larval container. Yeast was used to feed the immature stages before they emerged as adult. The adults were identified and recorded.

1.4 Collection and fixing of water samples for Physico-chemical analysis

Water samples for physico-chemical analysis were collected from the five habitat types investigated using 500 ml capacity specimen bottles. The water was fixed immediately using standard procedures (American Public Health Association, 1998) in preparation for laboratory analysis. However, water temperature was determined at the sites during the larval collection using mercury thermometer.

1.5 Physico-chemical analysis of water from the breeding habitats

Water samples were analysed for the following physico-chemical parameters: Dissolved Oxygen (DO), pH, Temperature, Copper, Lead, Zinc, and Iron.

1.6 Statistical analysis

Tests of statistical differences were carried out using correlation, one way analysis of variance and Kruskal-Wallis test. One way analysis of variance was used to compare the abundance of different mosquito species collected as larvae and eggs. Kruskal- Wallis test was used to compare the abundance of mosquito larvae and eggs at different locations. Correlation analysis was used to know the effects of different chemical and physical characteristics of the breeding habitats on mosquito abundance. The statistical package used was SPSS version 21.0.

2 Results

From the five zones in the municipal of Enugu, twenty refuse dumpsters were selected. Table 1 shows the selected dumpsites, frequently encountered content of the dump sites and the number of tin for refuse disposals.

A total of 1,140 (100%) mosquito eggs were collected using the ovitraps, 820 (71.9%) were positive with mosquito eggs. Out of this number positive with mosquito eggs, 168 mosquito eggs (20.5%) were collected from New Haven, 199 mosquito eggs (24.3%) from G.R.A, 186 mosquito eggs (22.7%) from Abakpa, 163 mosquito eggs (19.8%) from Emene and 104 mosquito eggs (12.7%) from Independent layout. From the mosquito species collected, Culex quinquefasciatus accounted for 33.3% of the eggs collected while Aedes albopictus and Aedes aegypti accounts for 28.5% and24.6% respectively. Of all the species of mosquitoes collected, the eggs of Culex quinquefasciatus, Aedes albopictus and Aedes aegypti were the most abundant (P < 0.05). The abundance of other species was low and occurred at the same level. The eggs of Anopheles species were not collected from the selected zone. From the sites sampled, the highest collections were made in G.R.A followed by Abakpa while Independence layout recorded the least collection of mosquito eggs. The statistical analysis carried out on the mosquito occurrence at and the various locations studied showed that there is no significant difference between sites studied (Figure 2).

A total of 362 (100%) mosquito larvae comprising of seven species were collected in the study during the larval sampling. Of the collections, larvae of Culex quinquefasciatus (36.7%) were predominant in the collection followed by Aedes albopictus (28.2%), Aedes aegypti (21.3%), Aedes africanus (5.2%), Culex tigripes (4.1%) and Anopheles gambiae (0.8%). The number of larvae collected in a site showed a distinct habitat adaptation to the site they were collected. All the species collected were recorded across all sites except Anopheles gambiae that was encountered only in G.R.A and Emene. The majority of the larvae collected were in GRA (25.7%) while Independence layout (11.3%) recorded the least (Figure 3).

The abundance of mosquito species in a particular site was determined as the mean frequency of occurrence per 500 ml of water. The relationship between larval abundance and physico-chemical parameters was assessed using Linear Coefficient Correlation (Table 2). Differences in physico-chemical properties among habitat types were determined using ANOVA. The mean physico-chemical characteristics of water in the five larval breeding habitat types are shown in Table below. There was no significant correlation (P < 0.05) between the mean percentage abundance of mosquito larvae collected and each of the physical or chemical characteristics (Temperature, pH, Dissolved O₂, Copper, Lead, Zinc, and Iron) of the breeding habitats.

3 Discussion

Human Induced environment such as refuse dumps, water sachets, over-grown bushes, abandoned tires can hold water long after rain and in turn provides breeding sites for mosquitoes in Enugu municipal. Knowledge of mosquito breeding habitats and why they prefer such site is a key to mosquito control as well as the disease they disease they transmit. The observation that mosquitoes breeds in refuse dumps in this study is in agreement with the findings of (Okpalaononuju and Mbanugo, 2003), (Ozumba and Nwosu, 2003) and (Onyido et al., 2009).

Refuse dumping activities in Enugu Metropolis regulated by the Enugu State Waste Management Agency (ESWAMA) and one of their statutory function is to collect, remove, process, treat and safe disposal of waste. The result of the study therefore highlights the hazard associated with indiscriminate dumping of refuse.

From the study, 842 positive eggs of different species of mosquitoes were collected of which the eggs of Culex quinquefasciatus were the most collected. The content of the refuse dumps (rotten fishes, spoilt fruits and food and other decomposing materials) encourages breeding of the Culex mosquitoes. Weinstein et al. (1997) noted that availability of decaying matters and other stagnant water also helps in the proliferation of the species which is a well-known breeder in stagnant, dirty and artificial container.

The eggs of the Yellow fever, Dengue and Zika virus vectors {Aedes albopictus, Aedes aegypti} abound in all the locations sampled and this affirms the finding of (Onyido et al., 2009) that arbovirus vectors are abundant in Enugu. Government Residential Area (G.R.A) recorded a high number of Aedes africanus which breeds in tree-holes. The collection of the tree-hole breeders in G.R.A point towards the availability of tall trees in those areas and this was also observed in the studies of Onyido et al. (2006). Other mosquito eggs collected from the study includes: Eretmapodite chrysogaster, Culex tigripes and Toxorhynchites which are mostly mosquito predators at larval stage.

Egbuche et al. (2016) noted the preference of Anopheles mosquitoes being selective in their oviposition sites as they prefer breeding in open drains and ground pools as this could be the reason for not collecting the mosquito species in this study.

Mosquito larvae were recorded more in Abakpa zone followed by New haven zone. Abakpa tends to be more populated than other locations and Eguniobi (1996) observed that the number of people living in an area is directly proportional to the waste they generate and this sequentially creates breeding habitat for mosquitoes. Mutero et al. (2004) pointed out that various chemical properties of larval habitats observed in different breeding sites are linked to vegetations and physico-chemical characteristics of water in the sites ranging from the pH, temperature, total suspended solids and heavy metals have been observed to affect larval development and survival. The physico-chemical parameters in the breeding sites were recorded to be ranging from 26.9°C-28.1°C, 6.03-6.89, 8 mg/L-16 mg/L, 0.6 mg/L-1.1 mg/L, 0.1 mg/L-1 mg/L, 0.1 mg/L, and 0.52 mg/L-0.79 mg/L respectively for temperature, pH , dissolved Oxygen, Copper, lead, Zinc and Iron. The reading of the physico-chemical parameters were higher compared to the U.S Environmental Protection Agency (EPA) standards. Pelizza et al. (2007) stated that pH of 7 is neutral and water with a pH of 6.8-7.2 is preferable for breeding mosquito of any species and the pH reading observed in this study was in that range (6.03-6.89). The result of the pH has no correlation (r=-0.695) with abundance of the larvae. The mean temperature seen in this study which ranged from 26.9°C-28.1°C is in agreement with that observed in the study of Bradley and Kutz (2006) where the temperature reading was 16°C-32°C. Water temperature affects both oviposition and growth of larvae and pupae (Sanford, 2005). The collections in this study shows strong correlation (r= 0.5) with water temperature as this suggests that as the temperature increases, mosquito abundance likely increases as these was confirmed by the findings of Afolabi et al. (2010). Tadesse et al. (2011) observed that optimum dissolved oxygen contributed to the survival and breeding of the mosquito larvae and from the study, it was observed that the breeding sites in Abakpa holds the highest dissolved oxygen of 16mg/l as against the standard (4 mg/l-6 mg/l) by American Public Health Association (1998). Though our finding seems to contradict the report of Muturi et al. (2008) in having negative correlation (r= -0.337) between dissolved oxygen and the larval abundance but the findings of Oyewole et al. (2009) support the idea that optimum dissolved oxygen might have contributed for survival and breeding of mosquito larvae. However, there was no significance correlation (P>0.05) between the heavy metals and the mosquito larval abundance even though Copper (Cu) and Lead (Pb) showed strong negative (r= -860) and strong positive (r=0.6440) correlation respectively. The result showing no significant correlation between the metals and the mosquito species despite their concentrations of the metals indicates that the larval populations have adapted very well to the heavy metal levels. The finding in this study corresponds to the findings of Taiwo et al. (2014) that other factors apart from the heavy metals affects the predominance of the mosquito larvae.

The abundance of the disease vectors in Enugu municipal calls for the attention of the State Ministry of Health to roll out programs on how to checkmate the abundance of these vectors .The agency responsible for waste disposal and collections (ESWAMA) are on their best form but additional intensive efforts are needed both from the populace, media houses and the ministries of Health and Environment on the public health importance of proper waste disposal and to discourage indiscriminate refuse dumping around the provided dumpsters in the metropolis. From the study sites, it was observed that areas with heavy social and human activities such as markets, hospitals and churches tend to have increased relative abundance of the mosquito species. The observation was as well recorded in the study of Simon-Oke et al. (2012). The sanitary state of an area is chiefly influenced by proper handling of waste and safe waste evacuation and disposal. As we know that indiscriminate waste disposal results into public health menace, economic and other welfare loses. To avert disease outbreaks caused by vectors breeding in dump sites or around it in Enugu municipal, public awareness on refuse disposal as well as more dumpsters with coverlid should be provided to prohibit waste been disposed indiscriminately.

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