Parasitic Retinal Disorders
Inflammation of the retina caused by infection with parasites such as Onchocerca volvulus (the pathogen that causes onchocerciasis), Toxocara canis or Toxocara cati (nematode larvae that are normally intestinal parasites of dogs and cats), Taenia solium, (pork tapeworm), and other parasites.
Onchocerciasis, like trachoma and leprosy, is one of the mostfrequent causes of blindness worldwide. However, like the other parasitic dis-eases discussed here, it is rare in Europe and North America.
Onchocerca volvulusis transmitted by the bite of black flies. Thisallows the larvae (microfilaria) to penetrate the skin, where they form fibrous subcutaneous nodules. There they reach maturity and produce other micro-filaria, which migrate into surrounding tissue. The danger of ocular infiltra-tion is particularly great where there are fibrous nodules close to the eye.
Toxocara canis or Toxocara cati (eggs of nematodes infesting dogs andcats) are transmitted to humans by ingestion of substances contaminated with the feces of these animals. The eggs hatch in the gastrointestinal tract, where they gain access to the circulatory system and may be spread through-out the entire body. The choroid can become infested in this manner.
Taenia solium: The pork tapeworm infestation can occur from eating porkcontaminated with larvae or other substances contaminated with tapeworm eggs. Mature tapeworms can also release eggs into the intestine. The larvae travel through the bloodstream to various organs and can also infest the eye.
Ophthalmoscopy will revealintraocular inflammation. Onchocerciasis has been known to be associated with posterior uveitis as well as keratitis and iritis. Histologic examination will demonstrate microfilaria in the retina. Visceral larva migrans, Toxocaracanis, or Toxocara cati can cause complications involving endophthalmitisand retinal detachment. Subretinal granulomas and larval inflammation of the retina have been known to occur. The larvae of different species of worms can produce diffuse unilateral subacute neuroretinitis with the typical clini-cal picture of grayish white intraretinal and subretinal focal lesions. Fly larvae can also invade the subretinal space in ophthalmomyiasis.
Other causes of retinal inflammation and subretinalgranulomas should be excluded.
Laser photocoagulation or surgical removal of the worm larvaemay be indicated.
It is not uncommon for these disorders tolead to blindness.