Causative agent of onchocercosis
This filarial species causes onchocercosis, a disease that manifests mainly in the form of skin alterations, lymphadenopathy, and eye damage, which latter is the reason for the special importance of the disease.
Occurrence. Onchocerca volvulus is endemic in 30 countries in tropical Africa (from the Atlantic coast to the Red Sea) and southern Arabia (Yemen) as well as in six countries in Central and South America (Mexico, Guatemala, Columbia, Venezuela, Brazil, Ecuador). About 17.6 million persons are currently infected and 267 000 are blind due to onchocercosis (WHO, 1998, 2000). The WHO has been coordinating successful control programs in 11 African countries since 1974, and in six Latin American countries since 1991 (see below).
Life cycle. The adult Onchocerca live in the connective and fatty tissues, usually tightly coiled in connective tissue nodules in the subcutis or deeper tissue layers (Fig. 10.16c). Sexually mature parasites can live as long as 15 years.
Female Onchocerca produce microfilariae that live in the tissue within the nodules or skin. Starting from the site of the female worms, the microfilariae migrate through the deep corium of the dermis into other skin regions and can also affect the eyes—especially if the nodule is located on the head or upper body. Through the lymphatics, the microfilariae can penetrate into the bloodstream and also appear in urine, sputum, and cerebrospinal fluid. The relatively large microfilariae have no sheath (Table 10.4); their lifespan in human hosts is from six to 30 months.
Simuliidae (blackflies) are the intermediate hosts and vectors. The development of the infective larvae, transmitted by a blackfly to a human host, takes many months before the nematodes reach sexual maturity. Microfilar-iae are usually be detected in skin after 12-15 months (seven to 24 months) (prepatent period).
Epidemiology. Humans are the sole parasite reservoir of O. volvulus. Onch-ocercosis occurs in endemic foci along the rivers in which the larvae and pupae of the blackflies develop. Therefore, the blindness caused by onchocer-cosis is designated as "river blindness."
Pathogenesis and clinical manifestations. Pathological reactions are provoked by adult parasites and by microfilariae. These reactions are influenced by the immune status of infected individuals.
■ Reactions to adult parasites: enclosure of adult filariae in fibrous nodules (onchocercomas), usually 0.5-2 cm (sometimes up to 6 cm) in diameter in the subcutis along the iliac crest, ribs, scalp, etc., more rarely in deeper tissues. Nodulation occurs about one to two years after infection and is either asymptomatic or causes only mild symptoms (Fig. 10.16c).
■ Reactions to microfilariae: microfilariae appear in the skin about 12-15 (seven to 24 months p.i.). Initial symptoms occur after about 15-18 months: for example, pruritus, loss of skin elasticity with drooping skin folds, papules, depigmentation, and swelling of lymph nodes; blood eosinophilia may also be present.
■ Eye changes: "snowflake" corneal opacities, in later stage sclerosing ker-atitis, the main cause of blindness, chorioretinitis and ocular nerve atrophy; tendency toward bilateral damage (Fig. 10.16d).
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