Abstract: The study was undertaken to ascertain the prevalence and microfilarial density of Mansonella perstans filariasis in 6 rural communities of the Lower Cross River Basin, Nigeria. Parasitological examination of 829 consenting subjects was carried out between January and November, 2002. Data collected was stratified by community, age and sex and analysed using the chi-square test. Overall, 131 (15.8%) of the subjects were infected with microfilariae of M. perstans. Prevalence varied significantly (p<0.05) within the communities (range 3.7-33.5%). Infection was significantly (p<0.05) higher in females (21.2%) than in males (12.1%) and occurred in all age groups with highest prevalence of (30.0%) in subjects within the age group of 21-40 years. The microfilarial density increased with age, reaching a peak (31.90 mf/50 μL blood) in persons within the age bracket of 21-40 years. The overall geometric mean density was 21.63 mf/50 μL blood and it was significantly (p<0.05) higher in males (28.24 mf/50 μL blood) than in females (15.02 mf/50 μL blood). In view of the fact that M. perstans has been implicated in the aetiology of many diseases, its infection could no longer be ignored.
INTRODUCTION
Mansonellosis due to Mansonella perstans is widely distributed in Africa, the Carribean, Central and South America (McMahon and Simonsen, 1996; Ottenson, 1990). The vectors are blood-sucking midges such as Culicoides grahamii, C. austeni and C. fulvithorax which thrive in the underbush and rotten banana stems (Crewe, 1977; Service, 2000; Agbolade, 2002). M. perstans is generally regarded as asymptomatic because in some regions up to 90% of infected native subject may exhibit little or no clinical manifestations. However, this filarial species is being implicated with clinical filariasis in some endemic areas. Serious symptoms that have bee reported in mansonellosis include pruritus, transient swelling (similar to Calabar swelling), fever, pain in the bursae and/or joint synovia, in serous cavities or in the liver region, elephantoid scrotum, extreme exhaustion, eosinophillia, monocytosis and neutrophilic toxic granulations (Adolph et al., 1962; Arene and Atu, 1986; Pierkersku, 1989; Ottenson, 1990; Udonsi, 1986; Agbolade, 2002; Agbolade and Akinboye, 2005).
M. perstans is arguably the most widespread human filarial infection in Nigeria (Uttah et al., 2005). There is high endemicity of mansonellosis in the coastal and mainland areas of Niger Delta. It is also the predominant filarial species in Calabar (Useh and Ejezie, 1995) from where it forms an epidemiological continuum with neighbouring rainforest Cameroon, which is reported to have high prevalence of 50% (Anderson et al., 1974). Similarly, high prevalence have been documented in other parts of Nigeria (Arene and Atu, 1986; Udonsi, 1986; Anosike et al., 1992, 2005).
There is paucity of information about mansonellosis in the Lower Cross River Basin Nigeria. This study was therefore conducted to ascertain the prevalence and intensity of M. perstans in Obanliku Local Government Area (LGA) of Cross River State, Nigeria.
MATERIALS AND METHODS
Study Site
The study was conducted in 6 rural communities of Obanliku Local Government
Area (LGA) of Cross River State, Nigeria between January and November 2002.
Cross River State is situated within the Cross River Basin located between latitude
5°311 and 4°271 North and longitudes 7°501
and 2°21 East and has an area of 23,074,425 square kilometers. The state
is dominated by the rainforest vegetation. However there are traces of derived
and guinea savanna in the Northern fringes of the state. The climate is classified
as hot-humid, with two main seasons namely: the rainy season (April-October)
and the dry season (November-March). An annual rainfall of about 160 and 220
mm is recorded in the hinterlands and coastal areas, respectively. Relative
humidity ranges between 80 and 90%, while temperature ranges between 23.4 and
27.9°C. The major occupation of the people are farming, fishing and hunting.
Crops cultivated are yams, cassava, maize, rice. Cattle rearing and bee farming
is largely practiced. A detailed description of the study site and area is documented
elsewhere (Opara et al., 2005; Ibanga, 2004).
Community Mobilization
Pre-survey visits were made with letters to the chairman of Obanliku Local
Government Area and the Primary Health Co-ordinator (PHC) to intimate them on
the proposed survey. The PHC then later introduced the research team to the
District Health Supervisors (DHS). Thereafter the villages were visited, where
the Village Heads, opinion leaders and chiefs, were informed about the aims
and objectives of the survey, consent was sought and obtained.
Ethical Consideration
The Cross River State Ministry of Health approved of the survey, informed
consent was sought and obtained from individuals and parents of persons less
than 18 years. Before commencement of the study, the inhabitants were informed
of the nature, scope and purpose of the study. Inconveniences of pains on pricking
the finger and late hours of blood collection was explained and individuals
were allowed voluntary participation.
Sample Collection and Examination
Blood samples for parasitological examinations were taken from every consenting
person of 1 year and above, between 22.00 and 02.00 h. The left thumb was cleansed
thoroughly with methylated spirit and allowed to air dry. Using a disposable
sterile lancet a deep prick was made and blood was collected using non-heparinized
capillary tubes, about 50 μL of blood was used to prepare a thick smear
on microscopic slides as described by Cheesbrough (1998). These were air dried
overnight, fixed in methanol stained with Geimsa, before microscopic examination.
The microfilariae of M. perstans was identified and distinguished from
the microfilariae of other filarial pathogens especially Wuchereria bancrofti
and loa loa by their morphology as described by Cheesbrough (1998).
Clinical Examination
We did not investigate clinical manifestation because, M. perstans
co-exist with Wuchereria bancrofti and Loa loa in the study area
(Braide et al., 2003; TDR, 2001), a possible overlap with other filarial
pathogens in the aetiology of the clinical symptoms may exist.
Data Analysis
Differences in proportion were tested by chi-square. While variations in
microfilariae density were analysed by t-test.
RESULTS
Of the 829 persons examined 131 (15.8%) were infected with M. perstans. The prevalence varied significantly (p<0.05) among the communities ranging from 3.7% in Besenge to 33.5% in Utuhu (Table 1). The microfilariae prevalence in relation to sex is also presented in Table 1, of the 494 males and 335 females examined, 60 (12.1%) and 71 (21.2%), respectively were positive for M. perstans. There was significant difference (p<0.05) between prevalence and sex. Infection was recorded in all age groups. The highest infection rate was observed in the age group 21-40% (30.0%), while the least infection was recorded in subjects older than 60 years (1.6%). (Table 2). There was a significant difference (p<0.05) between prevalence and infection. The overall geometric mean intensity among microfilariae positive subjects was 21.63 mf/50 μL (range 1-170 mf/50 μL). Males (28.24 mf/50 μL) had a significantly (p<0.05) higher intensity than females (15.02 mf/50 μL) (Fig. 1).
| Table 1: | Sex-related prevalence of M. perstans infection in Obaniliku Local Government Area, Cross River State, Nigeria |
| Table 2: | Age-related prevalence of M. perstans infections in Obanliku LG, Cross River State |
| Table 2: | Continued |
| Fig. 1: | Geometric mean intensity of microfileriaemia among infected person |
DISCUSSION
This study reveals a relatively low prevalence of M. perstans infection among the people of Obanliku Local Government Area of Cross River State Nigeria. This result may be an underestimation of the actual prevalence of mansonellosis in this study area because of the volume of blood used in preparing the thick smear. It has been reported by Dreyer et al. (1996) that 20-60 μL blood films do not reliably detect microfilariaemic individuals with low parasitaemia level, in addition thick smears grossly underestimate the prevalence of filariasis (Faris et al., 1993). However, the 15.8% prevalence obtained in this study is similar to 14.6, 13.4 and 12.5% obtained by Wijeyarantne et al. (1982), Udonsi (1988) and Uttah et al. (2005), respectively. These values contrast with the lower values obtained by Anosike et al. (1992), Arene and Atu (1986), Agbolade and Akinboye (2001), all working in different parts of Nigeria. There was a significant variation in prevalence of mansonellosis in the different communities, this may be attributed to the uneven distribution of ecological factors that favour the breeding of culicoides vectors. The age-related prevalence shows that microfilariae were found in all age groups, indicating that the infections are acquired early in life. This is not entirely surprising considering the socio-cultural and economic practices of the people in the study area, they live in mud houses that are virtually unprotected from insect attacks and in close proximity to brooks. Most of these mud houses are surrounded by banana and plantain plantations, which are ideal breeding sites for the vectors. The prevalence in the age group of 20-40 years was higher than the others. This is because at the age of 21-40 years people are considered to be the most physically fit and able to do laborious work, hence spending most of their time doing outdoor work which leads to a higher risk of being bitten by the vectors, similar observations have been recorded by Anosike et al. (1992). Females were significantly more infected than males. This difference between sexes in prevalence has often been related to occupation leading to increased exposure to vectors. It was observed in the study area that women engage in late** evening story telling outside their houses and nocturnal outdoor meetings, in addition early morning and late evening market is also highly practiced among the women folks. It has been reported by Service (2000) that culicoides bite at anytime and late evening corresponding to peak period of these activities engaged by women. All these socio-cultural and economic activities expose the women folks to bites of the vector. Similar observation has been reported by Ufomadu and Ekejindu (1992).
We did not carry out any clinical investigation in this study, because in parts of Africa where several filarial diseases coexist, some confusion might arise with respect to their clinical manifestation since their might be possible overlap with other filarial pathogens in the aetiology of these conditions (Anosike et al., 2005). Since the study are is endemic for onchocerciais, bancroftian filariasis and Loa-loa, (Briade et al., 1980, 2003; TDR, 2001), carrying out clinical investigation might give a misleading result.
The findings from this study indicates that Perstans filariasis is an important serious health problem since it coexist with other filarial pathogens. Vector control measures do not seem practicable considering the peculiar breeding habitat of the culicoides vectors. The need for a well-articulated filariasis control programme for endemic rural communities whose major health problems are rooted in their socio-cultural and economic lifestyle and amplified by several contingencies of their physical environment and occupational imperatives cannot be over-emphasized. Since M. perstans has been implicated in the aetiology of many diseases, its infection in human subjects should no longer be ignored.
ACKNOWLEDGMENT
The study received financial support from Pro-health international. We are grateful to Cross River State Ministry of Health for support and cooperation. The authors are grateful to the Village Heads of the communities for mobilizing their support to participate in the study.