Onchocerciasis clinical features and management
Onchocerciasis clinical features and management
Infection with the filarial parasite Onchocerca volvuluscauses chronic intense pruritus with a papular eruption dueto the death of microfilariae in the skin. Death of microfilariaein the eye causes a range of ocular disease; microfilarialdeath in the cornea over a prolonged period resultsin scarring of the cornea and unilateral or bilateral blindness.
Aetiology and transmission Infective larvae escape on to the skin from the proboscisof an infected Simulium blackfly when it takes a bloodmeal, and enter the skin through the bite site.The infection is maintained in the vectors when adultblackflies ingest microfilariae in blood meals. Thesedevelop into infective larvae over about 9 days. In humansthe larvae develop into adults over 6 months and then startto shed microfilariae into the skin. These migrate out fromthe adult, and only when the larvae die do symptoms startto occur. Adult worms survive for up to 15 years and mayshed microfilariae for most of that time.
Distribution Tropical Africa and Central America are the main endemicareas, with foci of disease in Yemen, Colombia, Venezuelaand the northern Amazonian region of Brazil. In Africa,two distinct areas are affected: the savannah area throughoutWest Africa, where ocular involvement is common, andthe tropical rainforest area where ocular involvement isless common.
Epidemiology The distribution of the disease relates closely to theecology of the Simulium vector, which requires fast-flowingfreshwater streams and rivers for breeding. Most humancases occur in people who live close to such water sourcesand use them to collect water, wash clothes or play.
Pathology and pathogenesis The changes in the skin and eyes are caused by the death ofmicrofilariae. Following an acute response to microfilarialdeath there is granuloma formation and fibrosis. Over anumber of years the result is destruction of elastic fibres,fibrosis, and thickening in the affected skin. Microfilariaedie in the cornea, leading to irreversible corneal scarring.Worms also may die in the iris and ciliary body, causinguveitis with secondary glaucoma as a further complication.Choroidoretinitis and optic atrophy are also recognized.Some heavily infected patients have no symptoms, whereas others have marked symptoms and signs. Antibody and eosinophils are the important effector cells inkilling microfilariae.
Clinical features The earliest symptom is pruritus, which may be madeworse by a hot bath. Examination shows a papular eruptionwith excoriation. When the arm or leg is affectedthe limb may be obviously swollen in the early phase ofthe infection. As this process goes on over years there isdestruction of the elastic fibres in the skin, giving theappearance of premature ageing.The lateral corneoscleral junction should be examinedwith a slit lamp for keratitis, which appears first as smalldiffuse white spots. Slit-lamp examination may showmicrofilariae in the anterior chamber. The posterior segmentof the eye should be examined for retinal lesions.The adult worms are found in nodules that consist of theadult female surrounded by fibrous tissue of host origin. These nodules are felt over bony prominences such as theskull, shoulder blades, ribs, pelvic girdle and knees. Whenthey are not palpable they must lie deep to scapulae andmuscles of the gluteal region, for example.The papular rash is less apparent in chronic cases wherefibrosis has produced inelastic thickened skin. There maybe patchy destruction of the pigment layer in a black skin,giving a 'leopard skin' appearance. Destruction of theelastic fibres of skin in the inguinal region creates pouchescontaining enlarged inguinal glands, and predisposes toinguinal hernia.
Diagnosis The parasitological diagnosis is confirmed by examiningskin shavings from affected areas for microfilariae. Thesecan be obtained using a needle and scalpel blade, but skinsnips taken using a corneal punch is perhaps easier for the patient. Eosinophilia and a positive serological test forfilariasis are usually found in the peripheral blood but are not diagnostic for onchocerciasis.
Differential diagnosis Scabies and lichen planus are common or relativelycommon itching skin diseases in the tropics and shouldbe considered. Nodules and ocular disease are not found,and the appearance and distribution of lesions in both conditionsis different. Loa loa infection causes transientsoft tissue swelling that produces short-lived pruritus.The lymphatic filarial infections cause unilateral limbswelling with regional adenopathy but no pruritus orpapular rash.
Treatment Ivermectin is now the drug of first choice because itexacerbates clinical signs much less than does diethylcardiethylcarbamazine(DEC). Ivermectin is given in a single dose of200 ug/kg body weight; most adults receive 9 mg. There maybe some local swelling of affected skin, but much less thanthat with DEC.Like DEC this drug only kills microfilariae, not adultworms, and so relapse is likely in about 50% of cases overthe next year; these patients are then retreated. Diethylcarbamazineis given with a starting dose of 50 mg, increasingthe dose on alternate days to a maximum of 200 mgthrice daily for 21 days. When there is marked local reactionwith exacerbation of ocular involvement, topical orsystemic steroids may be needed.Severe and even fatal reactions are seen in malnourishedand sick patients given DEC, and so it is essential thatintercurrent infection and nutritional status are improvedbefore DEC is given.Suramin (Antrypol) is effective against adult worms andmicrofilariae but is used mainly for its ability to kill adultworms when ocular involvement is present. Anaphylaxis tothe drug can occur and the drug is nephrotoxic. Excisionof onchocercal nodules is recommended, particularly thoseon the head and shoulders.
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