Onchocerciasis is caused by onchocercal filariae transmitted by the bite of the blackfly (genus Simulium), which lays its eggs on vegetation in fast-flowing rivers (hence the common name for the disease, river blindness) and is endemic in parts of sub-Saharan Africa, the Middle East, and Latin America. Microfilariae penetrate the skin and mature in subcutaneous nodules formed at the site of the bite for approximately 1 year, after which mating produces microfilariae offspring (≈300 μm in length)—up to 1500 a day per female (100 cm in length). These worms can live as long as 15 years in the human host; thus, diagnosis can be made with skin snips demonstrating the microfilariae.
Figure 10-7 Acanthamoeba cyst (arrows). (Diff-Quik stain, original magnification ×100.)
(Courtesy of Elmer Y. Tu, MD.)
Migration of microfilariae to the skin and eye results in clinical onchocerciasis, and subsequent blackfly bites can carry the organism to other individuals. Microfilariae enter the peripheral cornea, where they can be visualized by slit-lamp examination, and may reach the inner eye. Keratitis (including punctate keratitis and “snowflake” and sclerosing peripheral corneal opacities), anterior uveitis, and chorioretinitis occur upon death of the microfilariae. The intense, blinding inflammatory keratitis has been shown to be a reaction less to the microfilariae than to a bacterial endosymbiont, Wolbachia, which is essential to filariae reproduction. Antifilarial therapy can produce a systemic inflammatory response, but prior treatment with systemic doxycycline has been shown to reduce this response. Treatment by nodulectomy or with oral ivermectin (the discoverers of which were awarded the Nobel Prize in Physiology or Medicine 2015) and control of local blackfly populations have been successful in reducing the incidence of onchocerciasis in selected areas.
Loa loa larvae enter the skin at the bite of an infected deer fly (genus Chrysops). Adult worms may grow to 6 cm in length and migrate through the connective tissues, causing transient hypersensitivity reactions. Loa loa may also appear beneath the conjunctiva.
Visceral larval migrans is a multisystem disease in young children that is caused by the migrating larvae of Toxocara canis and Toxocara cati, natural residents of dogs and cats, respectively. Toxocara larvae develop and mate in the intestines of their natural host; human ingestion of fertilized eggs in pet feces results in infection. Toxocara larvae in the human intestine do not receive the proper environmental signals and consequently migrate throughout the body, invading and destroying tissues as they go. Ocular larval migrans occurs in older children, and the viscera are typically spared.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.