Biting on human body parts of Simulium vectors and its implication for the manifestation of Onchocerca nodules along Osun River, southwestern Nigeria.
Human onchocerciasis (river blindness) is a disease caused by a filarial nematode Onchocerca volvulus (1). The transmission of the infection is achieved through the bite of the fly belonging to the Genus Simulium. The adult worms of the parasite reside in the subcutaneous region of the human body where they produce millions of microfilariae (2,3). The microfilariae normally migrate in the intercellular spaces of the skin which usually leads to many reactions including leopard skin, premature ageing and blindness (1).
The congregation of the adult worms and subsequent fertilization to produce microfilariae in subcutaneous tissues has been known to elicit nodules (4). The examination of the palpable nodules in human body in the endemic communities has been adopted as a rapid diagnostic method of assessing onchocerciasis endemicity as it requires less ethical complications (5). The location of the Onchocerca nodules on human body has however been correlated with the biting preference of human parts by Simulium vectors6.
In west Africa, Simulium damnosum sensu lato has been reported to prefer biting the lower limbs of the body, therefore, producing high density of nodules and microfilariae at this location (7,8). Though, earlier report in Nigeria showed that head nodules were rare (9) but Onigbo10 and Ubachukwu6 later reported the prevalence of head nodules in southeastern Nigeria. Ubachuckwu (6) for example reported 41.2% prevalence of head nodules among primary school children in Uzo-Uwani, Enugu State, Nigeria. Since the geographical cytospecies of S. damnosum s.l. and O. volvulus strains exist, the differences in the epidemiological and clinical signs of onchocerciasis in the endemic regions had been attributed to these variations (6,11). It, therefore, becomes imperative to document the biting preference of the Simulium flies in different regions with the underlying aim of understanding the expected pattern of distribution of Onchocerca nodules and microfilariae and thus, better our knowledge on the epidemiology on the disease. This study investigates the biting pattern on human body parts by Simulium vectors along Osun river where S. soubrense Beffa form has been reported as the predominant vector (12) and its possible implications on the distribution of nodules and microfilariae in human populations in the area.
MATERIAL & METHODS
The study was conducted at two communities; Osun Eleja (in a derived savannah: Latitude 07[degree]16"N; longitude 04[degree]08"E) and Osun Budepo (in a rain forest: latitude 07[degree]04"N; longitude 04[degree]08"E) along Osun river system. The details of the study area have been previously described elsewhere (3).
Written consent was sought and obtained from the Ogun State Ministry of Health before the commencement of the study. Informed consent was also sought and obtained from the communities and subjects used for the study.
Experimental design for fly collection
The biting preference of adult S. damnosum s.l. to different human parts was investigated at Osun Eleja and Osun Budepo during the wet season (August-September 2008) and the dry season (November-December 2008). The data were collected twice every month. Two male fly catchers were positioned at each of the catching points. The fly catchers were instructed to wear short knickers and armless shirts. Only one person in sitting position exposed himself for every catching hour while the second person collected the flies perching on the exposed parts of his colleague. The sites of collection were categorized as: head/neck region, arms, calf and below the ankle. The human bait worked alternately between 0700 and 1800 hrs every catching day.
Palpation for Onchocerca nodules in human population The residents from 15 years and above were mobilized for the study in each community. The participants were examined for palpable nodules to the extent that decency permits.
The analysis of the data was carried out with t-test using SPSS version 16.0 after the data had been transformed by [square root of x] + 0.5 to standardize the variance.
The results showed that the number of flies collected below the ankle region was significantly higher than the number collected on other exposed parts (p <0.05) at Osun Eleja during the wet and dry seasons. Similar observations were made at Osun Budepo where significantly higher number of flies was collected below ankle while the least was collected on head/neck region during both the seasons (Table 1).
The results of the overall biting preference of S. damnosum s.l. to the exposed parts at both the locations are presented in Fig. 1. The region below the ankle recorded significantly higher number of flies than other exposed parts at both the study sites (p <0.05). However, there were no significant differences in the number of flies collected on arm and calf; and head and arm at both Osun Eleja and Osun Budepo (p >0.05).
Five out of 33 people examined at Osun Eleja had nodules while 13 out of the 61 participants examined at Osun Budepo harboured palpable nodules. The lower trunk was the most common site (60%) for nodule location at Osun Eleja followed by upper trunk (40%). Nodules were not found in the head and limb regions. At Osun Budepo, the upper trunk was the most common site of the nodule location (53.8%) followed by the lower trunk (38.5%) and head region (7.7%).
[FIGURE 1 OMITTED]
The present investigations have revealed that the appetitive Simulium flies could utilize any part of the body to obtain the blood meal needed for the maturation of the eggs as flies were caught on all exposed human parts. In Africa, the biting of S. damnosum s.l. and S. neavei have been reported to be occurring in the ankle region, therefore, producing higher density of nodules and microfilariae in the lower part of the body while in central America, S. ochraceum bites mostly in the head and neck regions producing higher density of nodules and microfilariae in the upper part of the body (6,8).
Though, the larger proportion of the flies was caught at the ankle region, the biting of other exposed human parts like arms and head/neck region is of interest and could have accounted for the pattern of distribution of nodules in the study area. Coincidentally, nodules were found at the head and upper trunk in one of the study communities which contradicts with the general information documented on the distribution of nodules in Africa. It has been known in Africa that nodules are largely confined to the lower trunk especially around the hip region (7,8,13). Earlier report by Crosskey (9) showed that head nodules were rare in Nigeria but the prevalence of head nodules was later documented in eastern part of the country (6,10). The present study has also confirmed the presence of head nodules in southwestern Nigeria. This observation has many epidemiological implications as Simulium vectors could obtain microfilariae from any part of the body thus increasing the risk of onchocerciasis transmission.
The variation in the location of Onchocerca nodules in human populations in the two communities may not be attributed to variation in biting species of Simulium flies as S. soubrense Beffa is the predominant vector in both the locations. The variation could be a reflection of the level of body exposure of the residents or variation in strains of O. volvulus harboured by human hosts in the study area. It could also be as a result of other human factors that are not assessed in this study. This observation is, however, recommended for further investigation.
The authors thank the residents and fly capturers at the study communities for their support.
Received: 30 January 2012
Accepted in revised form: 23 August 2012
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Correspondence to: Dr M.A. Adeleke, Public Health Entomology and Parasitology Unit, Department of Biological Sciences, PMB 4429, Osun State University, Osogbo, Nigeria. E-mail: email@example.com
Monsuru Adebayo Adeleke , Sammy Olufemi Sam-Wobo , Olaoluwa Pheabian Akinwale , Ganiyu Olatunji Olatunde  & Chiedu Felix Mafiana 
 Public Health Entomology and Parasitology Unit, Department of Biological Sciences, Osun State University, Osogbo;  Department of Biological Sciences, University of Agriculture, Abeokuta;  Molecular Parasitology Laboratory, Nigerian Institute of Medical Research, Yaba, Lagos;  National University Commission, Abuja, Nigeria
Table 1. Seasonal biting preference of Simulium damnosum complex to exposed human parts at the study sites Human parts Flies caught in percent Budepo Eleja Wet season Dry season Wet season Dry season Head 9.0 (c) 6.7 (c) 12.2 (c) 0 (d) Arm 19.1 (b) 1.0 (d) 16.0 (c) 7.69 (c) Calf 27.7 (b) 23.3 (b) 26.3 (b) 15.5 (b) Below the ankle 44.2 (a) 60 (a) 45.5 (a) 76.9 (a) Values with different letters are significant at p <0.05.
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|Author:||Adeleke, Monsuru Adebayo; Sam-Wobo, Sammy Olufemi; Akinwale, Olaoluwa Pheabian; Olatunde, Ganiyu Ola|
|Publication:||Journal of Vector Borne Diseases|
|Date:||Sep 1, 2012|
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