Introduction

Infection by the filarial parasite, Wuchereria bancrofti, is the most common cause of lymphatic filariasis (LF), accounting globally for approximately 90% of all infections (Lenhart et al. 2006). The disease is transmitted by species of mosquitoes of the genera Aedes, Anopheles, Culex and Mansonia (Nissen et al. 2002). Worldwide, over 120 million people are infected with lymphatic filariasis, with 20% of the global population (over 1.1 billion people) at risk for infection (WHO, 1997a). In Africa, the prevalence of lymphatic filariasis is especially striking, affecting over 40 million people in the sub-Saharan region alone (Dunyo et al. 2005). Overall, Africa is thought to account for 40% of all cases of lymphatic filariasis in the world (Gyapong and Amuyunzu-Nyamongo, 1999). Lymphatic filariasis is a major public health problem and strikes vulnerable people of all ages and both sexes (Anosike et al. 2005).The disease has been established to cause considerable socio-economic burden to affected communities in many tropical and sub-tropical countries (WHO, 1997a).

The common clinical manifestations of lymphatic filariasis are acute attacks of adenolymphangitis and disfiguring conditions such as hydrocoele and lymphoedema/elephantiasis (Ahurlu et al. 2001). Because of the debilitating nature of these manifestations and the large numbers of individuals affected, lymphatic filariasis has been identified as one of the leading causes of permanent and long – term disability in the world (WHO, 1995). The disease results in loss of work, productivity, direct and indirect economic loss and functional impairment (Pani et al. 1995; Ramaiah et al. 1996c, 1997a,b; Ramu et al. 1996).

The World Health Assembly targeted lymphatic filariasis for elimination mainly through a strategy of mass drug administration (MDA) (Abel et al. 2002). The effectiveness of the lymphatic filariasis elimination depends on upon the consumption of the recommended drug by the affected population (Mariappan, 2007). However, implementation of MDA led to diverse problems in some communities (urban areas, remote areas, migrant population and minority groups), with high rates of non-compliance having caused low treatment coverage (Gyapong and Twum-Danso, 2006). Although, MDA alone has been shown to suppress transmission of lymphatic filariasis in many areas where it has been implemented, it is often accompanied by resurgence once there is residual infection in the population. Therefore, sustainability of transmission suppression of lymphatic filariasis could be achieved only through integration of different strategies of vector control along with MDA (Mariappan, 2007). Besides, monitoring of the success of the lymphatic filariasis elimination programme depends on entomological studies of the mosquito vectors that transmit the disease in endemic communities (Das and Ramaiah, 2002). This investigation is an attempt to determine the abundance of different species of mosquitoes as well as identify those responsible for the transmission of lymphatic filariasis in three endemic villages in Kano State, Nigeria.

Materials And Methods

The Study Area

Survey of Vectors of Lymphatic Filariasis in the Three Villages

Collection and Dissection of Mosquitoes

Identification and Dissection of Mosquitoes

The female mosquitoes collected from each household were dissected under a dissecting microscope immediately after collection in the field. A mosquito was placed on the middle of a none-grease slide and identified into genus or species level based on morphological features outlined in Gillett, (1972) under an x40 microscope. The mosquito was separated into head, thorax and abdomen. The head and abdomen were placed on either sides of the slide respectively while the thorax was placed at the middle of the slide. The last two abdominal segments were removed to determine the parity status of the mosquito. The parity status measures whether a female mosquito has completed at least one cycle of reproduction which starts with a blood meal and ends in ovipositoin as against the nulliparous that are yet to lay eggs. Each of the parts was put into a drop of normal saline and squashed by means of the two dissecting pins. The content of the squashed three parts (head, thorax and abdomen) were observed under a light microscope (using x40 objective) for the presence of larval stages L1/L2 and L3 and microfilariae. Identification of the third stage larvae were on the morphological features outline in WHO, (1997b). Observations were carefully recorded in a record book.

The data generated was using simple frequencies and percentages; and then presented in tabular forms. Infection and infectivity rates respectively were calculated using the formulae below:

Figure 1

Results

Collection of Mosquitoes

A total of 1,604 mosquitoes comprising 1,291 females and 513 males were collected from 196 houses in the three villages (Table, I). More mosquitoes were collected in Gunduwa (852 from 105 houses), followed by Marke (448 from 54 houses) and Buda (348 from 37 houses). The overall mean number of mosquitoes caught per house was 8.2; Buda had the highest 9.4, followed by Marke 8.3 and the least was in Gunduwa 8.1. However, the abundance of mosquitoes was observed to depend on the nature of settlements, whether or not animals are reared at the back yard and perhaps the weather prevalent at the time of collection. For example, the high abundance of mosquitoes recorded in Gunduwa was because there is a hamlet, Wailari that has scattered type of settlement and thatched buildings and in most cases animals were raised at the back yards of the houses. In that hamlet alone, 653 mosquitoes were caught from 21 compounds indicating a mean number of mosquitoes per house to be 31 as against 199 mosquitoes from the other four hamlets combined from 84 houses with mean number of mosquitoes caught per house to be 2.3. In Buda, mosquito abundance declined with a break in the rainfall.

Figure 2

Table I: Mosquitoes collected from the study area

Dissection and Identification of Mosquitoes

Of the 1,291 female mosquitoes collected from the three villages, 718 were dissected (Table, II). Only dissected mosquitoes were identified to species level. The parity status of mosquito species composed of 39 nulliparous, 643 gravid and 36 parous. Of the 718 mosquitoes dissected, the most abundant species was Anopheles gambiae, (325) and the least was An. Zimani, (1) (Table, III). Only one mosquito, Culex quinquefasciatus was found to be infected with four larvae (L3) in its thorax at Buda village. The overall infection and infectivity rates were 0.14% and 0.0% respectively. The infection and infectivity rates for Buda where the infection was observed in only one mosquito were 0.7% and 0.0% respectively.

Figure 3

Table II: Number of mosquitoes dissected from the study sites

Figure 4

Table III: Species composition of dissected mosquitoes

Discussion

The abundance of mosquitoes, which translates into high or low harvest, was observed to depend on the weather prevalent at the time of collection, the number of houses sampled, the nature of the settlements in the villages and hamlets that make up the villages and animal husbandry practices. High harvest was seen when it rained and in general thatched houses seems to harbour more mosquitoes than unthatched. For instance, Gunduwa village had the highest abundance of mosquitoes because more houses were sampled and the nature of settlements at one of the hamlets, Wailari. The village has scattered type of settlement and majority of the houses are of the thatched type. In addition, animals (cows and goats) are reared at the backyards of most of the houses. The decline in mosquito abundance at Buda was due to a break in rainfall at the time of the mosquito collection. In order of increasing abundance the following mosquitoes were identified and dissected: Anopheles zimani, Aedes sp., An. funestus, Culex quinquefasciatus and An. gambiae. The Anopheles species were more abundant in Marke because it is more of a rural setting and that the village is located near a large water body, which provides excellent breeding sites. On the other hand Culex quinquefasciatus was more abundant in Buda because the village is more of a semi-urban setting which provides breeding sites for this species.

An. gambiae, An. funestus and Culex quinquefasciatus, which have been encountered in these villages, have been incriminated as vectors of lymphatic filariasis in various part of Africa. For example, An gambiae, An. funestus and Culex quinquefasciatus have been incriminated as vectors in an irrigation community in southern Ghana by Dzodzomenyo et al. (1999) and in Ebonyi State, Nigeria (Anosike et al. 2005). An. funestus and An.gambiae were found to be the vectors in three communities in Uganda by Onapa et al. (2001) while Merelo-Lobo et al. 2003 identified An. arabiensis, An. gambiae, An. funestus and Mansonia sp. as vectors in Lower Shire Valley, Sourthern Malawi. Udonsi, (1988) identified An. gambiae and Culex pipiens as vectors of lymphatic filariasis in Igwun River Basin, Nigeria. Lenhart et al. (2007) implicated An. gambiae, An. funestus and An. arabiensis as major vectors from 13 villages in central Nigeria.

In this study, infection was seen in only one Culex quinquefasciatus, this goes to confirm the increasing importance of this species in the transmission of lymphatic filariasis in Africa. Moreover, Culex transmitted filariasis have already been reported in Bauchi State by Anosike (1996) which share a similar climatic condition with Kano State. As observed by Anosike et al. (2005), this finding is potentially worrying for many areas of West Africa where W. bancrofti is endemic and C. quinquefasciatus is rapidly becoming more abundant but where local compatibility between the parasite and the mosquitoes has not yet appeared.

The low infection rate recorded in this study may be because mosquito collection and dissection was not done during the period of peak transmission. It could also be because the prevalence of lymphatic filariasis is low which an indication of light transmission in the villages. In addition, fewer mosquitoes from spray catches would be infective than those from human landing catches as infective larvae are lost during feeding (McMahon et al. 1981). In this study, majority of the mosquitoes collected through the spray method were blood-fed and gravid females.

This investigation apart from given an insight into the abundance of mosquito species, has incriminated Culex quinquefasciatus as a vector. It also identified An. gambiae, An. funestus and Aedes that have been found to serve as vectors in different parts of Nigeria. This therefore provides a baseline information for integrating vector control into the nation’s Lymphatic filariasis elimination programme. However, investigations covering longer period of time and involving more villages is recommended.

Acknowlegdements