Onchocerca species microfilariae, like other filarial worms, are transmitted by insect intermediate hosts, specifically gnats (Culicoides species) or black flies (Simulium species). The microfilariae are transmitted to a mammalian host while the insect is feeding; the microfilariae then migrate
to specific sites in the host's body, most commonly to the skin and the ocular subconjunctiva and conjunctiva. In these sites,
the microfilariae mature, mate, and release more microfilariae that circulate in the blood of the mammal and infect another
blood-feeding insect when it feeds. In horses, the adults often inhabit the nuchal ligament.
Microfilariae form microfilarial nodules in the deep subcutaneous tissues and ligaments of large mammals, but in the eye,
microfilariae may cause conjunctivitis, conjunctival depigmentation, uveitis, and keratitis (Figures 5 & 6). Microfilariae may also lead to exophthalmos due to retrobulbar granulomas.
Figure 5 : A dog (not the dog in this case report) with ocular onchocerciasis. The right eye exhibits hyperemic conjunctiva
overlying a mass at the temporal limbal area. Moderate chemosis is also present. Photo courtesy of Dr. David Williams.
New research into river blindness strongly suggests that the disease is caused largely by the host's reaction to the microfilariae's
endosymbiotic bacteria Wolbachia species rather than to the Onchocerca species microfilariae themselves.7 This research has important implications for the treatment of this disease because the nematodes need these bacteria to reproduce.
Killing the bacteria with antibiotics interrupts the parasite's life cycle.8 Experimentally, extracts of Wolbachia species proteins cause significantly more ocular damage than extracts of Onchocerca species microfilariae alone cause.9 Tetracyclines are effective against Wolbachia species.9
Figure 6 : A dog (not the dog in this case report) with severe ocular onchocerciasis. The right eye has two subconjunctival
masses, severe conjunctival hyperemia, and corneal edema and neovascularization. Photo courtesy of Dr. David Williams.