Onchocerciasis (River Blindness)
Humans are the only host for Onchocerca volvulus parasite. As the vector, female black flies that breed near rivers bite an infected human and ingest microfilariae. The infective larvae are then transmitted to another human with a future bite. Onchocerciasis is endemic in many areas of sub-Saharan Africa and in isolated foci in Central and South America. Worldwide, at least 25 million people are infected, of whom almost 300,000 are blind and 800,000 are visually impaired. Microfilariae probably reach the eye by various routes:
direct invasion of the cornea from the conjunctiva
penetration of the sclera, both directly and through the vascular bundles
hematogenous spread (possibly)
Microfilariae can be observed swimming freely in the anterior chamber. Live microfilariae can be seen in the cornea; dead microfilariae cause a small stromal punctate keratitis. Anterior uveitis may lead to synechiae, secondary glaucoma, and cataract. In the posterior segment, RPE disruption and focal atrophy may occur. Later, severe chorioretinal atrophy develops. Optic atrophy is common in advanced disease (Fig 11-42).
Diagnosis is based on clinical appearance and a history of pathogen exposure in an endemic area and confirmed by finding microfilariae in small skin biopsies or in the eye. Ivermectin, the treatment of choice, is given every 3–6 months as long as there is evidence of skin or eye infection. Topical corticosteroids can be used to control any anterior uveitis.
Figure 11-42 Fundus photograph montage showing extensive chorioretinal scars involving both the periphery and the posterior pole as well as optic nerve pallor secondary to onchocerciasis.
(Courtesy of H. Nida Sen, MD.)
Ivermectin safely kills the microfilariae but does not have a permanent effect on the adult worms. Concomitant doxycycline therapy helps kill the adult worms by eradicating the symbiotic partner—Wolbachia bacteria. Lastly, patients coinfected with onchocerciasis and Loa loa are at risk of a fatal encephalitic reaction to ivermectin, so infectious disease consultation should be procured for such individuals.
Ejere HO, Schwartz E, Wormald R, Evans JR. Ivermectin for onchocercal eye disease (river blindness). Cochrane Database Syst Rev. 2012;8:CD002219.
Winthrop KL, Furtado JM, Silva JC, Resnikoff S, Lansingh VC. River blindness: an old disease on the brink of elimination and control. J Glob Infect Dis. 2011;3(2):151–155.
Excerpted from BCSC 2020-2021 series: Section 9 - Uveitis and Ocular Inflammation. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.