Author(s): Paul Cantey, Sharon Roy

Parasitological diagnosis: DPDx

Infectious Agent

Onchocerca volvulus, a filarial nematode, causes onchocerciasis, also known as river blindness.

Transmission

Transmission occurs through female blackfly (genus Simulium) bites. Simulium vectors typically bite during the day and breed near rapidly flowing rivers and streams.

Epidemiology

Onchocerciasis is endemic to much of sub-Saharan Africa. Small endemic foci also are present in the Arabian Peninsula (Yemen) and in the Americas (Brazil and Venezuela). Foci center around blackfly breeding sites, located near rapidly flowing water. Most transmission occurs in rural areas, but some transmission occurs in semi-urban and urban areas. Infections diagnosed in the United States are most commonly in immigrants, people visiting friends and relatives in endemic areas, long-term travelers to endemic areas, and expatriates. Although rare, infection can occur in short-term (< 1 month) travelers, particularly to areas with intense exposure. The incidence of infections identified outside endemic areas might be declining due to successful implementation of ivermectin mass drug administration in many endemic areas.

Clinical Presentation

Clinical signs and symptoms include highly pruritic, papular dermatitis; subcutaneous nodules; lymphadenitis; and ocular lesions, which can progress to vision loss and blindness. Symptoms begin after patent infections are established, which can take 18 months. Symptoms in travelers are primarily dermatologic, typically acute erythematous papular rash and pruritus, sometimes just edema, occurring years after departure from endemic areas. Signs of chronic skin changes are rare in travelers. Subcutaneous nodules and chronic skin changes (e.g., depigmentation, lichenification, hyperpigmented flat-topped papules), are more common in endemic populations. Peripheral eosinophilia is often present in symptomatic travelers and migrants.

Diagnosis

The standard method of diagnosis is examination of a skin snip biopsy to determine the presence of microfilariae. The diagnosis also can be made by identifying adult worms in histologic sections of excised nodules or characteristic eye lesions. Serologic testing is most useful for detecting infection when microfilariae are not identifiable. Determination of serum filarial antibody is available through the National Institutes of Health (301-496-5398) or the Centers for Disease Control and Prevention (CDC). See instructions on how to submit a serum specimen to CDC for testing. Assistance with parasitological diagnosis can be obtained through DPDx (www.cdc.gov/DPDx). General assistance with the diagnosis or treatment of onchocerciasis can be obtained by contacting CDC’s Parasitic Diseases Branch (parasites@cdc.gov; 404-718-4745).

Treatment

Ivermectin is the drug of choice to relieve symptoms. Patients might require repeated annual or semiannual doses to control symptoms, because the drug kills the microfilariae but not the adult worms. Some experts recommend treating patients with 1 dose of ivermectin, then 6 weeks of doxycycline to kill Wolbachia, an endosymbiotic rickettsia-like bacterium that appears to be required for the survival of the O. volvulus adult worm and for embryogenesis. Individuals at risk for co-infection with Loa loa should have blood evaluated to assess for the presence of Loa loa microfilariae. If co-infected, enlist the aid of a tropical medicine expert for management due to the risk of Loa loa–related fatal post-treatment reactions associated with ivermectin.

Diethylcarbamazine is contraindicated as a treatment for onchocerciasis because it leads to microfilarial death and, in some cases, systemic reactions associated with an increased risk for causing blindness in some patients with eye disease.

Prevention

Travelers should avoid blackfly habitats (e.g., fast-flowing rivers and streams) and use protective measures against biting insects (see Sec. 4, Ch. 6, Mosquitoes, Ticks & Other Arthropods).

CDC website: Onchocerciasis (also known as River Blindness)