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.
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