Onchocerciasis

1 - GENERAL

Onchocerciasis is a parasitic disease affecting the skin and eyes caused by Onchocerca volvulus which is transmitted through the bite of a fly of the genus Simulium. Recent eradication campaigns aimed at eliminating the vector (insecticide treatment of blackfly larvae breeding sites) and most importantly at the present time, the mass treatment of exposed populations with ivermectin, have led to a marked reduction in the incidence of the disease and its complications worldwide [1]. Onchocerciasis is consequently on the way to being eliminated as a major public health concern in South America. It has also become much less widespread in some historically endemic regions of sub-Saharan Africa (especially West Africa); however, areas of high prevalence still exist in central and East Africa.

A diagnosis of onchocerciasis should always be suggested in a patient resident in an endemic area and typically presenting with chronic, severe, diffuse itching affecting particular parts of the body (tops of the limbs, abdominal girdle and pretibial area) with various skin changes, starting with non specific lesions of the prurigo type; followed by hyperplasic, lichenified lesions or, conversely, skin atrophy, dyschromic manifestations, adenopathies and sub-cutaneous nodules containing the adult worms located mainly on the trunk and limbs. Potentially leading to blindness, the ocular symptoms and signs are rich and varied and include initially punctuate and then sclerotic anterior segment damage of the keratitis type and posterior pole damage resulting in chorioretinitis and optical atrophy. The diagnosis is confirmed by detection and numeration of dermal microfilariae in a skin snip. The Mazzotti test (skin or systemic reaction after intake of 6 to 50 mg of diethylcarbamazine) is dangerous in patients with ocular involvement or concomitant loa loa infection and is no longer used. The differential diagnosis is scabies or prurigo caused by insect bites. 

A so-called "primo-invasion" form has been described in European migrants at varying times after returning from an endemic area; the clinical picture consists of unilateral pruriginous oedema of a limb (most commonly the arm, known as "Cameroon swollen arm") combined with hypereosinophilia [3]. In such cases, the skin snip is often initially negative but the serology test is generally positive. The existence of endosymbiotic bacteria of the Wolbachia genus, which are also isolated in patients with lymphatic filariasis, suggests that doxycycline may be of use [1, 2]. This antibiotic sterilises adult females and has a macrofilaricide effect.