1DEPARTMENT OF ZOOLOGY, UNIVERSITY OF PORT HARCOURT, NIGERIA

Wuchereria bancrofti is present in the Lower Imo River Basin with a prevalence of 6.7%. Microfilaraemia was higher among older than younger individuals and there was no significant difference in prevalence between sexes. Microfilarial intensities varied significantly between sexes and also between age groups. On the overall, prevalence of clinical filariasis was higher than prevalence of microfilaraemia. Clinical manifestations related to W. bancrofti were hydrocele, limb and scrotal elephantiasis in that order of importance. Limb elephantiasis was higher among females than among males. Chyluria was not found in the Lower Imo River Basin study population.

The earliest available study on lymphatic filariasis in Nigeria was perhaps by Annet et al. (1901), who reported W. bancrofti larvae in An. gambiae. Most of the early reports were from the northern part of the country (Courtney, 1923; Taylor, 1930 Kershaw et al., 1953). Bancroftian filariasis has also been reported from the southern parts including the Igwun River Basin (Udonsi, 1988a,b) and the Niger Delta area (Udonsi, 1986). There is no comprehensive epidemiological study on Filariasis. This work is part of a detailed work by the Danish Bilharziasis Laboratory and University of Port Harcourt Joint Project on Filariasis in the Imo River Basin.

The area is situated on a plain land, just a few kilometres from the Atlantic Ocean. The New Calabar River is the only river in the area. There are few stagnant water collections some of which are sacred. In addition to a pipe-borne water system, the river serves as a water source for domestic needs, and there are also some family owned water wells.

Farming is the main occupation in the area. Important agricultural crops grown in the area are cassava, yam, maize, groundnut, vegetables, cocoayam, three-leaf yam, banana, and pineapple. Among the trees, raffia palm tree and palm tree are the economically most important. It is worth noting that local gin is produced from raffia palm wine, and this is a common practice among men. Fishing is also a major occupation. Fish-smoking is a regular practice among the women and is a local traditional way of preserving fish.

There is a government-owned Community Health Centre, as well as a private hospital in the area. Self-medication is evident with the presence of some medicine stores. Educational facilities include two secondary schools, and six primary schools.

Local Government health authorities were contacted and their consent obtained before the actual work began. Furthermore, the local chiefs and leaders of town development unions were consulted and briefed about the project. During the Parasitological and Clinical surveys, health personnel from the Emohua Local Government Area were always present. Village assistants were recruited to assist in the project work in the various communities. Two workers in the local health centre who are trained nurses were recruited, and the resident doctor also participated in the clinical survey.

All individuals in the selected communities who were more than one year of age were included in the study population, which comprised natives as well as non-natives who had resided there for at least one year. Natives who were abroad or working somewhere else were excluded from the survey. The targeted sample size was 1000 people in each part of the basin.

Suitable central places suggested and provided by the community leaders were used for the clinical surveys. In the six communities in the lower part of the basin, Village halls or shady areas were used. The use of house numbers given during national census and the personal numbers assigned during our survey allowed for easy reference in completing our Filariasis Survey Form. It also made it possible to help check unregistered ineligible natives who had just returned from abroad, as well as to exclude intruders from neighbouring communities. A person was certified eligible when the above-mentioned numbers corresponded to his or her name and numbers in the Village Census Form.

The clinical examinations were conducted together with qualified medical doctors who were briefed well in advance about the purpose of the survey, and the clinical signs and symptoms for which to be looked. These were limb and scrotal elephantiasis, hydrocele, skin manifestations, visual manifestations, and among others. The various gradations of these clinical manifestations are shown in Table 1.

Table 1 Legends to stages of clinical manifestations

(*grading for limb elephantiasis, hydrocele, and scrotal elephantiasis).

The examinations took place in utmost privacy. Children and infants who needed help were accompanied by their parents or guardians. Informed oral consent was obtained from all adults, and from parents or guardians of children aged ≤ 15 years before any examination was carried out. Those who were sick with minor ailments were given medicine.

Vials were given out to volunteers for their early morning urine. These were collected and kept at room temperature in the laboratory in Port-Harcourt and examined for the presence of chyle. Some samples were brought to the DBL for confirmation.

Night blood samples for parasitological examination were taken from every consenting person of one year and above. This took place between 2200 hours and 0200 hours. The thick smear technique was the method chosen for use. Using sterile lancet, 50l of finger prick blood was taken from the left thumb, drawn into blood pipettes and used to prepare thick smears on the microscope slides. These were dried overnight and taken to the laboratory for staining with Giemsa before microscopic examination. Identification was according to the keys in Learning Bench Aid No.3 (Tropical Health Technology).

Of those examined, 6.7% were positive for W. bancrofti mf (6.9% for males and 6.6% for females). Microfilaraemia was rarely seen in young children and the oldest persons beyond 60 years old. The youngest mf positive boy was six years old and the youngest mf positive girl was five years old. The mf prevalence increased steadily with age to reach 12.9% in the penultimate age group (13.1% for males; 12.7% for females), followed by a marked decline in the 60+ years age group. The difference in the mf prevalence between the two sexes was not statistically significant (c ²test; p >0.05 for all tests) either overall or in any of the age groups.

The overall GMI among mf positive individuals was 198mf/ml blood (171 mf/ml for males, and 229 mf/ml for females). Females had higher GMI than males in all the age groups, except in the 1-9 years age group, but the differences were not statistically significant (t-test; p > 0.05). The GMI did not show a definite pattern with respect to age and sex. The youngest boys had the highest GMI (277 mf/ml) among males, while the oldest women had the highest GMI (339 mf/ml) among females. The GMI did not differ significantly with respect to age (one-way analysis of variance; p > 0.05).

Table 2 Prevalence of W. bancrofti microfilaraemia in the Lower Imo River Basin.

Figure 1. The W. bancrofti GMI in relation to age and sex in the Lower Imo River Basin study population.

The coverage for the clinical survey was high (84.3 % overall, 84.4% for males and 84.1% for females). Among the chronic manifestations examined for, three are related to bancroftian filariasis, namely hydrocele, limb elephantiasis and scrotal elephantiasis. Signs of acute lymphatic filariasis, namely lymphangitis, funiculitis, and orchitis, were not examined for because of differential diagnosis problems since the area is also endemic for malaria and other tropical diseases that could cause similar clinical signs.

The prevalence of hydrocele and its different stages in relation to age is presented in Figure 2. The total prevalence for all stages of hydrocele was 6.0% (0.8% for false hydrocele, i.e. the swelling of the spermatic cord, and 5.2% for stages II-IV combined, that is, true hydrocele). The youngest person found with the swelling of the spermatic cord was 20 years old. The prevalence of the swelling of the spermatic cord was unexpectedly low, and was observed in those in the middle and penultimate age groups only. The youngest person observed with true hydrocele was 21 years old. The prevalence of true hydrocele (stages II-IV) was higher than the prevalence of spermatic cord swelling (stage I) but the difference was not statistically significant (c 2-test; p > 0.05). No case of stage IV (> 15 cm) hydrocele was observed in those examined in the Lower Imo River Basin study population. Majority of the cases of true hydrocele were found among those in the penultimate age group. The prevalence of true hydrocele rose from 3.4% in the middle age group to reach 15.2% in the penultimate and to 16.7% in the 60+ years age group.

The prevalence of the different stages of limb elephantiasis in relation to age group and sex is presented in Figure 3. Of those examined, 3.7% had sings of limb elephantiasis. The overall prevalence in males (5.2%) was higher than in females (2.4%), and this difference was statistically significant (c ²-test; p < 0.01). Limb elephantiasis appeared late in life in the Lower Basin Imo River study population. The youngest male with elephantiasis was a 20-year-old boy, while the youngest female was 18 years old. Majority of cases of limb elephantiasis in both sexes were of stage I (61.4% of all cases; 55.3% of all cases in males, and 73.7% of cases in females). Males had higher prevalence of advanced stages of limb elephantiasis (stage II+III) than females. Only five individuals had stage III elephantiasis, and these were all males. The overall prevalence of limb elephantiasis for both sexes combined rose with increasing age to reach 10.7% in the penultimate age group before dropping to 6.8% in the oldest age group. Of the 57 cases of limb elephantiasis, only two were of the arm, while 55 were unilateral and two were bilateral. The scrotal elephantiasis was rare in the Lower Imo River Basin study population. Of all examined, only a 23-year-old man was observed with scrotal elephantiasis, and this was of stage II.

Figure 2. The prevalence of the four stages of hydrocele in relation to age in the Lower Imo River Basin study population.

Figure 3. The prevalence of limb elephantiasis stages in relation to age in the Lower Imo River Basin study population.

RELATIONSHIP BETWEEN WUCHERERIA BANCROFTI MICROFILARAEMIA AND CLINICAL MANIFESTATIONS

The relationship between W. bancrofti microfilaraemia and hydrocele, limb and scrotal elephantiasis was analysed. The relationship between W. bancrofti microfilaraemia and hydrocele in males, presented in Table 3, shows that of the 44 persons with hydrocele, 17 (38.6%) had W. bancrofti mf. Thirty-two (4.8%) of the 670 males who were hydrocele negatives, were positive for W. bancrofti microfilaraemia. The difference in mf prevalence between those with and without hydrocele was statistically significant (c ²-test; p < 0.001). All cases of hydrocele were seen in males aged 20 years and above. In this age group, the mf prevalence of those with hydrocele was significantly higher than that of those without hydrocele (c ²-test; p < 0.001). The overall mf GMI among those with hydrocele, as well as the mf GMI of those with hydrocele ≥ 20 years old, among those with hydrocele was significantly higher than the mf GMI of those without hydrocele (t-test; p < 0.01 for both tests).

The relationship between W. bancrofti microfilaraemia and limb elephantiasis, presented in Table 4, shows that among 57 individuals who had limb elephantiasis, 24 (42.1%) were also positive for W. bancrofti microfilaraemia with mf GMI of 274 mf/ml. Among 1430 individuals who had no limb elephantiasis, 76 (5.3%) were positive for W. bancrofti microfilaraemia with mf GMI of 178 mf/ml. The mf prevalence among those with limb elephantiasis was significantly higher than among those without limb elephantiasis (c ²-test; p < 0.001). Similarly the mf GMI among those with limb elephantiasis was significantly higher than among those without limb elephantiasis (t-test; p < 0.001). Most cases of limb elephantiasis were seen in individuals aged 20 years and above. Among these, the mf prevalence of those with limb elephantiasis was significantly higher than among those without limb elephantiasis (c ²-test; p < 0.001). Similarly, the mf GMI of those with limb elephantiasis was significantly higher than the mf GMI of those without limb elephantiasis (t-test; p < 0.001). The only case of scrotal elephantiasis was reported in the Lower Basin study population was negative for W. bancrofti microfilaraemia.

Table 3 The relationship between W. bancrofti microfilaraemia and hydrocele in males in the Lower Basin study population.

* Only indicated if there are ≥ 3 mf positive cases.

Table 4 The relationship between W. bancrofti microfilaraemia and limb elephantiasis in the Lower Basin study population

The mf prevalence of W. bancrofti was higher among older persons than among the younger persons. This is consistent with Wijers (1974) and with results previously reported from the coastal Niger Delta area, (Udonsi, 1986), and from other parts of Nigeria (Anosike and Onwuliri, 1994; Akogun, 1992; Wijeyaratne et al, 1982); and the coastal areas of Ghana (Dunyo et al, 1996). This difference in mf prevalence between older and younger individuals may not be strictly attributed to differences in level of exposure given the night-biting habit of the vector and sleeping arrangements of the people (Day et al., 1991). It can be thus be argued that adults present greater surface area to biting female mosquitoes. The age-related differences may also reveal that the initially very light infections are difficult to detect with available parasitological techniques. Thus detection by antigenaemia shows that children are becoming infected during early and that the prevalence of positive antigenaemia among children correlates with the degree of transmission in an area (Graham, 1997). Furthermore, although people, including children are continually exposed to mosquito transmitted infective larvae (Day et al., 1991), the rate of gain of infection exhibits a convex age profile peaking in the 16-20 year old age class (Vanamail et al., 1989). The decline in older age group is considered due to resistance to new infection (Day et al., 1991). Another reason for lower mf prevalence among children is the need for one to be bitten repeatedly by infective mosquitoes before microfilariae appear in the blood (White, 1971). Wijers (1974) stated that the often people are bitten by infective mosquitoes, the younger are those found to harbour microfilariae and show signs of the disease.

Although the difference in prevalence between males and females was not statistically significant, prevalence in males was slightly higher than in females. This is consistent with reports from other endemic regions in Nigeria (Wijeyaratne, et al., 1982; Akogun, 1992; Anosike and Onwuliri, 1994) from Ghana (Dunyo, et al, 1996) from East Africa (Estambale et al, 1994; Meyrowitsch et al, 1995; Simonsen et al, 1995;). The comparable prevalence in both sexes may reflect that both males and females engage equally in activities involving an exposure risk, such as nocturnal out-door meetings and story telling. Exposure during sleep is also comparable in males and females with peak vector biting between 22.00 - 02.00 hours. In the Nile Delta, microfilaraemia prevalences were same in males and females (Gad et al, 1994).

The microfilarial intensities varied between both sexes and between individuals of various age groups which is consistent with results from studies elsewhere (Simonsen et al, 1995; Meyrowitsch et al, 1995). The microfilarial intensity increased to 10-19 years age group, followed by a decline and another marked peak in the oldest age group. Day et al. (1991) observed an increase in worm burden in individuals during childhood to reach a plateau from the age of 20 years and onwards. Thus a balance is maintained between establishment and death of worms. Variation in microfilaria counts between individuals decreases by age, reflecting development of acquired immunity (Das et al, 1990; Michael and Bundy, 1997), and development of this immunity is a function of exposure rate. Our study and studies from other endemic areas in southeastern Nigeria (Udonsi, 1986, 1988a), did not show any reduction in microfilaraemia level among women of reproductive age as has been reported in northern Nigeria and elsewhere (Kazura, 1987; Brabin, 1990;Anosike and Onwuliri, 1994).

The most common clinical manifestation seen among men in the study area was hydrocele, which was predominant among individuals in the older age groups (≥ 40 years). True hydrocele was more common than the swelling of the spermatic cord. A similar pattern was observed in Ghana (Dunyo et al., 1996) and in East Africa (Meyrowitsch, 1995; Wegesa et al., 1979). Prevalence of hydrocele was lower than that found in studies in East Africa (Jordan, 1954; McMahon 1979; McMahon et al., 1981; Matola, 1984; Meyrowitsch, 1995). Prevalence of hydrocele correlated well with the community prevalence of microfilaraemia in Ghana (Gyapong et al., 1998), and this was true for our study area.

Limb elephantiasis was more common among males than females in the area, while the reverse was the case in Ghana (Dunyo, et al., 1996). In the Northern Malumfashi district no limb elephantiasis case was observed, and is reportedly uncommon in the region (Wijeyaratne et al., 1982). Scrotal elephantiasis was not common in the Lower Imo River Basin; only two individuals had it. Similarly, in some coastal areas of West and East Africa a low prevalence of scrotal elephantiasis has been reported (Meyrowitsch, 1995; Dunyo et al, 1996). A considerably higher prevalence of scrotal elephantiasis was reported in Igwun River Basin (Udonsi, 1988b), and high rate of genital filariasis was reported among attendees at a hospital in Enugu (Nwafo et al., 1981). In Gongola State, elephantiasis of the scrotum was rather common, involving both W. bancrofti and O. volvulus (Akogun, 1991). In the coastal areas of Niger Delta namely, the Okirika-Eleme and Opobo areas, the prevalence of hydrocele and elephantiasis was close to 20% (Udonsi, 1986). This is higher than reported elsewhere in South-eastern Nigeria. It is noteworthy that the prevalence of hydrocele and elephantiasis in the Igwun River Basin was about twice higher than observed in the Lower Imo River Basin.

The clinical incubation period of lymphatic filariasis is currently considered as being two and 10 years (Partono, 1987). This explains, among other reasons, the high frequency of chronic clinical manifestations of bancroftian filariasis among the older age groups than among the younger age groups in the Imo River basin study area.

Udonsi (1988a) showed that chyluria was common in the Igwun Basin population. This condition was not seen in the present study. A milk-coloured urine sometimes containing blood was common, but did not show compartmentalization characteristic of chyluria after settlement. This condition may be attributable to bacterial infection or urinary schistosomiasis, which is endemic in the Imo River Basin.

On the whole the number of individuals with clinical manifestations was higher than the proportion of individuals with microfilaraemia. According to Bundy et al (1991) this is the norm, and is explained by the fact that although the point-prevalence could be low, the total proportion of the population who eventually experience infection (cumulative prevalence) is substantial, and that development of disease is an inevitable sequel to infection. Some of those once infected may become amicrofilaraemic but may retain their steady drift to chronic pathology, and Gyapong et al. (1998) stated that infection prevalence and disease prevalence are likely to achieve some dynamic equilibrium in areas where no intervention is carried out.

The association between hydrocele and microfilaraemia is not fully understood. In the study area, microfilaraemia was common among males who had hydrocele which is in line with findings from studies elsewhere (Wegesa et al, 1979; Abaru, 1980; Weller et al, 1982; Estambale et al, 1994; Meyrowitsch, 1995), but not in line with Meyers et al. (1976) whose viewpoint is that microfilariae are lost among individuals with chronic filarial pathologies. There was significant difference between level of microfilaraemia among males with or without hydrocele. Meyrowitsch et al (1995) observed no significant difference in microfilaraemia prevalence in males aged 20 years or above between those with and without hydrocele, and could not point out any association between hydrocele microfilaraemia. In areas with multifilarial endemicity, relationship between microfilariae and clinical manifestations appear complex. In the Malumfashi district, microfilariae of W. bancrofti and M. perstans, and in some cases also of O. volvulus, were found in the hydrocele fluids (Wijeyaratne at al., 1982).

Wuchereria bancrofti microfilaraemia was common among individuals who had limb elephantiasis in the study area. This is not in line with the findings of Meyrowitsch (1995) in Tanzania. This disparity may be due to epidemiological differences. The causative role of other filarial species in the Lower Imo River Basin could change the epidemiological picture. These could be complications also of mansonellosis (Arene and Atu, 1986), which is present in the study area.

The Danish International Development Assistance (DANIDA) through the Danish Bilharziasis Laboratory (DBL), Charlottenlund, Denmark, funded this work.