The distribution of onchocerciasis is linked to that of its vector (Simulium), which reproduces near rapidly flowing rivers in intertropical Africa (99% of cases), Latin America (Guatemala, Mexico, Ecuador, Colombia, Venezuela, Brazil) and Yemen.
In endemic areas, the following signs, alone or in combination, are suggestive of onchocerciasis:
– Onchocercomas: painless subcutaneous nodules containing adult worms, usually found over a bony prominence (iliac crest, trochanters, sacrum, rib cage, skull, etc.), measuring several mm or cm in size, firm, smooth, round or oval, mobile or adherent to underlying tissue; single, or multiple and clustered
– Acute papular onchodermatitis: papular rash, sometimes diffuse but often confined to the buttocks or lower extremities, intensely itchy, associated with scratch marks, often superinfected (« filarial scabies »)1 . This arises from dermal invasion by microfilariae.
– Late chronic skin lesions: patchy depigmentation on the shins (« leopard skin »), skin atrophy or areas of dry, thickened, peeling skin (lichenification; “lizard skin”).
– Visual disturbances and ocular lesions: see Onchocerciasis, Chapter 5.
– Detection of the microfilariae in the skin (skin snip biopsy, iliac crest).
– If the skin biopsy is positive, look for loiasis in regions where loiasis is co-endemic (mainly in Central Africa).
– Diethylcarbamazine is contra-indicated (risk of severe ocular lesions).
– Doxycycline PO (200 mg once daily for 4 weeks; if possible 6 weeks) kills a significant percentage of adult worms and progressively reduces the number of O. volvulus microfilariae2 . It is contraindicated in children < 8 years and pregnant or breast-feeding women.
– Ivermectin PO is the drug of choice: 150 micrograms/kg single dose; a 2nd dose should be administered after 3 months if clinical signs persist. Repeat the treatment every 6 or 12 months to maintain the parasite load below the threshold at which clinical signs appear3 . Ivermectin is not recommended in children < 5 years or < 15 kg and pregnant women.
– In case of co-infection with Loa loa or in regions where loiasis is co-endemic, ivermectin should be administered with caution (risk of severe adverse reactions in patients with high L. loa microfilarial load):
• If it is possible to test for Loa loa (thick blood film):
Confirm and quantify the microfilaraemia. Administer the appropriate treatment according to the microfilarial load (see Loiasis).
• If it is not possible to perform a thick film examination, take a history from the patient:
If the patient has received a previous treatment with ivermectin without developing serious adverse reactions (see Loiasis), administer the treatment.
If the patient has never received ivermectin nor developed signs of loiasis (migration of an adult worm under the conjunctiva, or « Calabar » swellings), administer the treatment.
If the patient already has developed signs of loiaisis and if onchocerciasis has a significant clinical impact, administer ivermectin under close supervision (see Loiasis) or use an alternative (doxycycline, as above).
– In the case of concomitant lymphatic filariasis: administer ivermectin first then start treatment for lymphatic filariasis with doxycycline PO (see Lymphatic filariasis) one week later.
Nodulectomy (surgical removal of onchocercomas)
Nodules are benign, often deep, and their ablation does not treat onchocerciasis. Thus, nodulectomy is reserved for cranial nodules (their proximity to the eye is a risk factor for visual compromise) or nodules which are cosmetically unacceptable. In other cases, refrain from nodulectomy. Nodulectomy is performed under local anaesthesia, in an appropriately equipped facility.
|1||Differential diagnosis is sarcoptic scabies (Scabies, Chapter 4).|
|2||Elimination of Wolbachia reduces the longevity and fertility of the macrofilariae, and thus the production of new microfilariae within the organism.|
|3||Ivermectin kills microfilariae and disrupts production of microfilariae by adult worms. However the treatment must be administered at regular intervals since it does not kill adult worms.|