Onchocerca volvulus , a filarial nematode.
Through female blackflies (genus Simulium ), which typically bite during the day and breed near rapidly flowing rivers and streams.
Endemic to much of sub-Saharan Africa. Small endemic foci are also present in the Arabian Peninsula (Yemen) and in the Americas (Brazil and Venezuela). Foci center around blackfly breeding sites, which are located near rapidly flowing water. Most infections outside those in endemic populations occur in expatriate groups, such as missionaries, field scientists, and Peace Corps volunteers, though infection may sometimes occur in short-term travelers (<31 days).
Highly pruritic, papular dermatitis; subcutaneous nodules; lymphadenitis; and ocular lesions, which can progress to visual loss and blindness. Symptoms begin after patent infections are established, which may take 18 months. Symptoms in travelers are primarily dermatologic (rash and pruritus) and may occur years after departure from endemic areas. Subcutaneous nodules are more common in endemic populations.
Presence of microfilariae in superficial skin shavings or punch biopsy, 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 antifilarial antibody is available through the National Institutes of Health (301-496-5398) or CDC (www.cdc.gov/dpdx/; 404-718-4745; firstname.lastname@example.org).
Ivermectin is the drug of choice. Repeated annual or semiannual doses may be required to control symptoms, as the drug kills the microfilariae but not the adult worms. Some experts recommend treating patients with 1 dose of ivermectin followed by 6 weeks of doxycycline to kill Wolbachia , an endosymbiotic rickettsialike bacterium that appears to be required for the survival of the O. volvulus adult worm and for embryo genesis. Diethylcarbamazine is contraindicated in onchocerciasis, because it has been associated with severe and fatal posttreatment reactions. An expert in tropical medicine should be consulted to help manage these patients.
Avoid blackfly habitats (fast-flowing rivers and streams) and use protection measures against biting insects (see Chapter 3, Mosquitoes, Ticks & Other Arthropods).
CDC website: www.cdc.gov/parasites/onchocerciasis/
Sharon L. Roy, Christine Dubray