1. Introduction

Loiasis is a tabanid-transmitted helminth disease of Central and Western Africa[1, 2]. The diurnally periodic microfilariae are usually absent from peripheral blood of people at night, but appear during the day, particularly in the morning. The microfilariae are therefore readily picked up by the primary tabanid vectors which bite during the day[3]. Loiasis has been established as endemic and widespread in Akwa-Ibom State, Nigeria[4-6]. There have been anecdotal records of tabanids in the State. A study was therefore undertaken over a 1-year period to determine variations in species composition, relative abundance and distribution of tabanids in rural and urban locations across the eco-vegetational zones in the State.

2. Materials and Methods

2.1. Study Area

Akwa Ibom State is located in the south eastern region of Nigeria, 4.32⁰ -5.33⁰N and 7.25⁰ - 8.25⁰E. There are four vegetation zones: mangrove swamp forest, fresh water swamp forest, Lowland rainforest and moist savanna woodland. The mean annual rainfall is 2000-2500mm and the temperature range, 27 - 32℃. There are two seasons: rainy (April - October) and dry (Nov - March).

2.2. Study Sites

Two Local Government Areas (LGAs), each from an urban and a rural location were randomly selected in each of the four eco-vegetational zones; the LGAs were: Eket, Onna, Etinan, Nsit Atai, Uyo, Ibesikpo Asutan, Ikot Ekpene and Ibiono-Ibom. The geographical coordinates of the study towns and villages appear in Table 1.

2.3. Vector Collection, Handling and Identification

The Malaise trap, effective for the collection of tabanids[7] was used. Tabanids were collected fortnightly, 07.00-19.00hrs; data were pooled to obtain monthly relative abundance.

Table 1. Study Sites and their Geo-Coordinates

During collections, the collecting jar was sprayed with pyrethrum-based insecticide, opened after one minute and insects harvested. Tabanids were transferred to containers with soaked cotton wool on filter paper. The top of the container was closed and labeled with date and study site. The container was transferred to a cooler containing ice-packs. The cooler was taken to the laboratory and species identified by standard keys[8-10]. The Identified species were compared with type-specimens at the Insect Museum, Department of Animal and Environmental Biology, University of Port Harcourt, Port Harcourt.

2.4. Data Analyses

SPSS (Graph path) was used for data analyses. The Students’t-test and the Analysis of Variance (ANOVA) were used to analyze seasonal, rural-urban and vegetational data. All statistical tests were performed at 5% (0.05) level of significance.

3. Results

3.1. Tabanid Distribution across Eco-vegetational Zones

The total number of tabanids collected in the 4 vegetation zones was 2790. The mangrove swamp produced the highest number 1650 (59.1%); the least was from Lowland rainforest, 70 (2.5%) (Table 2). Differences in numbers across eco-vegetational zones were not significant (F = 0.61 < 4.35, df 3, 4; P = 0.05).

3.2. Distribution of Tabanids in Rural and Urban Locations

More Tabanids were collected from the rural areas, constituting 63.0% of all collections. This pattern was observed in all species, except C. fuliginosus which was confined to urban locations (Table 3). However, the difference in rural-urban numbers was not significant (t = 1.98 < 2.131; df = 15, P > 0.05).

3.3. Seasonal Abundance of Tabanids

In the wet season, April, - September, 2329 (83.5%) were collected, while 461 (16.5%) were recorded in the dry season (October-March). This seasonal activity pattern was observed in all species, except Chrysops fuliginosus (Table 4). Some species were absent in some months of the dry season: C. univattatus (February), C. silacea(December-February). C. longicornis (February -March); C. fuliginosus (January-June), H. exiguicornatum (January-February); H. decora (November-February) and Hippocentrum vesicolor (December-February). However, the overall seasonal differences were not significant (t = 1.64 > 2.447; df = 6, P > 0.05).

4. Discussion

The preponderance of tabanids in the mangrove and freshwater swamps is not surprising because the oviposition sites overhang or are adjacent to the larval habitats, which are often muddy, aquatic or semi-aquatic habitats[3]. They are also known to breed in brackish swamps[11]. The more complex rural locations in these eco-vegetational zones were also preferred[2]. Species richness of tabanids was found to negatively correlate with large open habitats and positively with patch-shape complexity[12]. Rains provide ideal breeding grounds and the higher numbers in the wet season have also been observed in other studies[13-17]. The population peak of most species including those with higher vector potential suggests that the rainy season can be considered as the period of potentially higher risk of transmission[12].

Table 2. Numbers of Tabanids collected across Eco-vegetational Zones

Table 3. Tabanid species at Rural and Urban locations across Eco-vegetational Zones

Table 4. Seasonal abundance of Tabanidae (April 2002 – March 2003)

The primary vectors of Loa loa were incriminated in a series of studies in the 1950s in the Congo Region of equatorial Africa, which includes parts of the Democratic Republic of Congo, Congo-Brazzaville, Gabon, Cameroon and Southern Nigeria[18]. Chrysops silaceus and Chrysops dimidiatus particularly attracted to people near fire were the primary vectors. One of these, C. silaceus, was recorded in the study. It was also the only species that was distributed across the four eco-vegetational zones. The high dispersal ability of tabanids[13] would enhance their role in transmission. Further studies are planned in the State to determine the infection rates of known and potential vectors of Loa loa.

5. Conclusions

The favourability of muddy habitats for tabanids oviposition is reflected in their preponderance in swamp forests. The patterns of spatial distribution of Chrysops silacea suggest that the disease may be more prevalent in rural communities in the wet season. The endemicity and widespread occurrence of loiasis in Akwa Ibom State is probably maintained by the widely distributed C. silacea and other vectors, enhanced by their high dispersal ability.

ACKNOWLEDGEMENTS

Dr V. I. Itina acknowledges with gratitude the support of staff of the Ministry of Health, Akwa Ibom State, and advice from Prof O.C. Umeozor. The assistance of Mr A. J. Akpan in field collections and mosquito identification is highly appreciated. The cooperation of community leaders, guides, field escorts and health workers at the various health centers at study locations is gratefully acknowledged.