Description

Human onchocerciasis is caused by the prelarval (microfilaria) and adult stages of the filarial nematode Onchocerca volvulus. The disease is transmitted by the bite of certain species of female Simulium flies (black flies) that bite by day and are found near rapidly flowing rivers and streams (1,2).

Occurrence

Onchocerciasis is endemic in more than 25 nations located in a broad band across the central part of Africa. Small endemic foci are also present in the Arabian Peninsula (Yemen) and in the Americas (Brazil, Colombia, Ecuador, Guatemala, southern Mexico, and Venezuela) (1).

Risk for Travelers

Persons traveling for short periods in onchocerciasis-endemic regions appear to be at very low risk for acquiring this condition. However, travelers who visit or live in endemic regions for >3 months and live or work near black fly habitats are at greater risk for infection. Infections tend to occur in expatriate groups such as missionaries, field scientists, and Peace Corps volunteers.

Clinical Presentation

Infection with O. volvulus can result in a highly pruritic, papular dermatitis; subcutaneous nodules; lymphadenitis; and ocular lesions, which can progress to visual loss and blindness (1,2). Symptoms in travelers are almost always dermatologic (3) and may occur months to years after departure from endemic areas. Immigrants from endemic areas may present with skin and/or ocular disease. Diagnosis is made by finding the microfilariae in superficial skin shavings or punch biopsy, adult worms in histologic sections of excised nodules (4), or characteristic eye lesions (5). The results of serologic testing are generally nonspecific outside the research setting; however, there are several research laboratories in the US where test results, when placed within the context of an appropriate exposure history and physical findings, can be quite helpful when microfilariae are not identifiable.

Prevention

No vaccine and no effective chemoprophylaxis are available. Protective measures include avoidance of black fly habitats and the use of personal protection measures against biting insects (the vectors of onchocerciasis bite by day), such as those outlined in the section on Protection against Mosquitoes and Other Arthropods in Chapter 2.

Treatment

Ivermectin (150-200 µg/kg orally once or twice per year) (6) is the drug of choice for onchocerciasis. Repeated annual or semiannual doses may be required (5,7,8), because the drug kills the microfilariae but not the adult worms, which can live for many years (1). If subcutaneous nodules are present, they should be excised if their anatomic location allows that to be done safely (4). Travelers who have a diagnosis of onchocerciasis should be advised to consult with a specialist in infectious diseases or tropical medicine.

References

1. World Health Organization. Onchocerciasis and its control: report of a WHO expert committee on onchocerciasis control. World Health Organ Tech Rep Ser. 1995;852:1-104.
2. Burnham G. Onchocerciasis. Lancet. 1998;351:1341-6.
3. Murdoch ME, Asuzu MC, Hagan M, Makunde WH, Ngoumou P, Ogbuagu KF, et al. Onchocerciasis: the clinical and epidemiological burden of skin disease in Africa. Ann Trop Med Parasitol. 2002;96:283-96.
4. Albiez EJ, Buttner DW, Duke BO. Diagnosis and extirpation of nodules in human onchocerciasis. Trop Med Parasitol. 1988;39 Suppl 4:331-46.
5. Abiose A. Onchocercal eye disease and the impact of Mectizan treatment. Ann Trop Med Parasitol. 1998;92 Suppl 1:S11-22.
6. Anon. Drugs for parasitic infections. The Medical Letter on Drugs and Therapeutics. 2004 Apr:1-12.
7. Brieger WR, Awedoba AK, Eneanya CI, Hagan M, Ogbuagu KF, Okello DO, et al. The effects of ivermectin on onchocercal skin disease and severe itching: results of a multicentre trial. Tropical Med Int Health. 1998;3:951-61.
8. Tielsch JM, Beeche A. Impact of ivermectin on illness and disability associated with onchocerciasis. Tropical Med Int Health. 2004;9 (Supp):A45-56.