Onchocerciasis is one of the four major causes of blindness in the world, affecting some 270000 people, with an additional 500000 with severe visual impairment. It is generally a cumulative
The most important determinant of the burden of infection in a community is the infective density of the vector. A measure of the biting density of blackflies can be made by regular dawn-to-dusk catches, using human bait at selected sites (Figure 18b.4). This allows the
calculation of the monthly biting rate (MBR), a theoretical estimate of the total number of bites an individual could receive if maximally exposed at that site:
MBR = Number of blackflies caught x Number of days in month/Number of catching days in month
The annual biting rate (ABR) is the sum of 12 consecutive MBRs.
The estimation by dissection of the parous rate and the number of third stage infective (L3) larvae permits the calculation of the monthly transmission potential (MTP), a theoretical estimate of the number of infective larvae that could be transmitted to a maximally exposed individual (Duke, 1968b; Walsh et al., 1978):
MTP = MBR x Number of Onchocerca volvulus L3 larvae detected/Number of flies dissected
The annual transmission potential (ATP) is the sum of 12 consecutive MTPs.
In practice, these indices give only a rough guide to the level of transmission. For example, not all third-stage larvae are truly infective, only about 80% of L3 larvae are transmitted during a blood meal, no one individual is likely to be maximally exposed to bites, and the infective larvae of other worms may be hard to distinguish from O. volvulus. However, it has been shown that, in the forest zone of West Africa, the prevalence and intensity of infection and clinical features in humans are related to the numbers of biting flies and measured transmission potentials (Duke et al., 1972a, b).
The pattern of clinical features of the disease shows geographical variation throughout its range. For example, the majority of nodules in African patients occur on the pelvic girdle (Albiez et al., 1988). In young children in Africa, relatively more nodules are found on the upper part of the body, especially the head. In Guatemala and Mexico, nodules are more frequent on the head in all age groups, while in South America nodules are not common, but are mostly found on the lower half of the body. This variation is thought to be related to parasite strain differences and vector-parasite relationships. This has been studied most closely in West Africa, where the main differentiation is into three strains: a forest strain with low ocular pathogenicity, associated with high nodule numbers and severe skin disease; a dry savanna strain, with high ocular pathogenesis and an associated high rate of blindness; and a humid savanna strain, with an intermediate pattern (Anderson et al., 1974b). This pattern of lower prevalences of ocular lesions for a given ocular microfilarial load in the forest, compared to savanna areas, has been confirmed by other workers (Dadzie et al., 1989), but McMahon et al. (1988) have argued that this could be explained by higher ultraviolet radiation in savanna regions, causing more microfilarial deaths in the eye and hence more anterior segment eye disease, rather than different invasiveness or pathogenicity of putative parasite strains, as suggested by Duke and Anderson (1972b). Microfilariae are generally morphologically indistinguishable throughout their range, but in West Africa at least four different patterns of acid phosphatase staining are found, supporting the hypothesis that a number of biological strains exist. Use of DNA probes has been reported which can distinguish O. volvulus parasites from forest and savanna areas of West Africa (Erttmann et al., 1987; Zimmerman et al., 1992). It is postulated that local populations of microfilariae have become adapted to the specific sibling-species of vector in their area. This could lead to incompatibility between parasite strains and vectors from different areas (WHO, 1981; Duke et al., 1966). However, recent work shows that the relationship between savanna vectors and the blinding strain of the parasite is not clear-cut (Toe et al., 1997).
Research is under way into developing improved molecular tools to identify vector and parasite populations (Tang et al., 1995). These include the heteroduplex technique, which allows identification based on hybridization with mito-chondrial DNA fragments, and microsatellite DNA analysis, which provides polymorphic markers for populations or even individuals.
Adult blackflies are small (under 4 mm long) dipterans, found in all parts of the world except for a few islands. They are squat, heavy-bodied flies with a pronounced humped thorax. They are usually black and may have black, white or silvery hairs on the body. The wings are short and broad, with well-developed anterior veins but otherwise membranous. Males can be distinguished from females by the compound eyes. In males the eyes occupy almost all the head and meet anteriorly and superiorly (holoptic), whereas the eyes are separated on top in females (dichoptic). The rasp-like mouthparts are short and broad and do not penetrate deeply into the host's skin.
The main vector of onchocerciasis, while always a blackfly, varies in different parts of the world. In most of Africa and Arabia blackflies of the Simulium damnosum species complex are the main vectors, while in East Africa the Simulium neavei complex are the principal vectors, and in
Central America Simulium ochraceum is the main vector. S. exiguum, S. metallicum, S. guianense, S. oyapockense are the primary vectors in South America. In West Africa, different sibling species of Simulium damnosum have been identified in different geographical areas (Crosskey, 1987). For example, in dry savanna areas the dominant vectors are Simulium damnosum sensu stricto and Simulium sirbanum, while in forest areas the main vectors are S. sanctipauli, S. soubrense, S. squamosum and S. yahense.
S. damnosum blackflies lay their eggs on trailing vegetation in fast-flowing, well-oxygenated water, while S. neavei lay their eggs on amphibious Potamonautes crabs. Each egg batch may contain 100-900 eggs, which are laid with a secretion of mucus that is immediately wetted and cements the eggs to the substrate. The eggs hatch after 1-2 days, the emerging larvae remaining attached to their substrate and filtering the water for food. It is their feeding and oxygen requirements that restrict blackfly breeding to rapidly flowing, well-oxygenated water (Figure 18b.5). The larvae
develop into pupae after 5-10 days, and adult flies emerge to the surface in a gas bubble 2 days later. Swarming and mating usually occur soon after emergence. Female blackflies need mate only once during their life.
All blackflies feed on plant juices and sugar solutions. Only the female feeds on blood and this process has to be repeated for each ovarial cycle. For Simulium damnosum, feeding occurs in daylight, with peak biting activity soon after sunrise and a lesser peak in the late afternoon (Garms, 1973). Location of a blood host is probably related to odour, CO2 output, movement, colour or outline (Wenk, 1981). In West Africa, most blackfly bites on humans are on the ankles and calves, with few bites more than 50 cm above the ground (Renz and Wenk, 1983). If these areas are not exposed, then the blackfly will probably seek an alternative host, and the relative efficiency of a species of blackfly as a vector for O. volvulus therefore depends partly on the degree of anthropophily and the persistence with which a blackfly will seek a human, rather than an animal, blood meal. Simulium damnosum normally takes 5 minutes to feed to full engorgement on humans (Crosskey, 1962). The labrum stretches the skin, which is then penetrated by the maxillae and the rapid scissor-like action of the paired mandibles to a depth of about 400 pm. These tear a ragged hole in the host's skin, blood pools in the wound from capillaries and is then sucked up by the blackfly. The bite is notoriously painful and often continues to bleed for some time.
Only a few ingested microfilariae will penetrate the mid-gut wall and reach the flight muscles of the blackfly, so that the average number of infective larvae per infected fly is only two in the savanna and five in the forest. However, once larvae have started to develop, their mortality is fairly low. The survival rate of the blackfly is not adversely affected unless parasitised with more than 20 microfilariae (Duke, 1962). The gono-tropic cycle is usually about 3-6 days, but the extrinsic cycle of O. volvulus is 6-8 days. The parasitic cycle therefore cannot be completed between two successive blood meals; thus, a female infected at her first blood meal cannot transmit larvae until her third meal. The longevity of blackflies in the wild was estimated at about 15 days (Duke, 1968a), with a theoretical maximum of four infective bites, but more recent reports estimate the maximum life span at 4 weeks (WHO, 1987), and studies of the movement of waves of migrating flies suggest that survival is possible for as long as 7-10 weeks (Baker et al, 1990).
There are normally major differences in black-fly populations in the dry and wet seasons, related to the height of rivers and availability of sites for larvae. This can vary at individual sites. Some rivers will dry up completely in the dry season and only support breeding during the rainy season. Other rivers may have potential breeding sites that are completely submerged when water levels are high and are only uncovered during the dry season. Bimodal variation may occur when rapid changes in water levels disrupt breeding sites by submerging or drying out.
Dispersal of blackflies is normally linear in savanna areas along the course of rivers, although nulliparous females usually display more radial dispersal than parous flies, which characteristically remain close to the river banks. Consequently, the risk of infection is higher close to the river, which explains the lay term for onchocerciasis—'river blindness'. In forested areas, more radial dispersal is observed, with higher proportions of infective blackflies found away from the breeding river (Duke, 1975), particularly during the rainy season (Garms, 1973). Under field conditions the natural infectiveness of blackfly populations is low. Near emergence sites in the savanna, it averages 3-5% of the total population and 6-10% of the parous population. Exceptionally, the theoretical maximum of 25% infective parous flies is reached (Philipon, 1977). Migration, as distinct from dispersal, of blackflies is probably mainly a wind-borne phenomenon. In West Africa this may be of long distances, up to 500 km (Baker et al., 1990), and may be crucial for the survival of S. damnosum populations during the dry season.
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