Contributor: Kevin R. Kazacos
Ocular toxocariasis is a non-contagious infectious disease caused by the intraocular migration of the canine nematode larvae of Toxocara canis and less frequently by larvae of the cat Toxocara cati. Humans become infected by accidentally ingesting infective Toxocara eggs from soil or other articles contaminated with the feces of infected dogs and cats. Ocular toxocariasis is seen most often in older children and adults, who do not have concomitant visceral larva migrans or other signs of infection. Human infection with Toxocara is common. Ocular toxocariasis is a pet-associated zoonosis, contracted by ingesting infective Toxocara eggs from environments contaminated with the feces of infected dogs and cats.
Ocular toxocariasis is an inflammatory condition caused by larval migration in the retina, choroid, vitreous, and associated structures. Inflammatory reactions are directed against larval enzymes, cuticular proteins, and metabolic wastes released by the larvae during migration. Larvae gain access to the eye via the retinal or choroidal vasculature. In acute or active infections, there is eosinophilic retinochoroiditis, vitritis, and/or endophthalmitis. Toxocara appears to be the main cause of the small nematode variant of diffuse unilateral subacute neuroretinitis. In chronic infections, larvae become encapsulated in granulomatous masses in the retina or other locations. Retinal granulomas often develop fibrous traction bands to the optic disc.
Larval migration can produce extensive retinal damage and eosinophilic inflammation, leading to visual deficits and blindness. The condition is most often unilateral, but is occasionally bilateral in heavy primary infections. Clinical disease is more serious with involvement of the macula and optic disc and in cases of eosinophilic endophthalmitis [endophthalmitis - eosinophilic] with retinal detachment. In the latter, the inflammatory process often organizes into a white, retrolental mass with development of a cyclitic membrane, posterior synechiae, and other abnormalities. In diffuse unilateral subacuteneuroretinitis, there is neuroretinitis and vitreitis, which progresses to optic atrophy, narrowing of retinal vessels, and widespread pigment epithelial damage, leading to profound visual loss. Over time, larval granulomas become well-organized and the eye may become quiescent, however, fibrous traction band contraction may still result in retinal folds or detachment.
The retina is most frequently affected by larval migration and granulomas, followed by the choroid, vitreous, and other structures. Larval granulomas are found most often in the posterior or peripheral retina.
The initiating complaint may be decreased vision, leukokoria, and/or strabismus. Various degrees of retinitis and choroiditis are noted, as hazy, ill-defined white lesions, with associated inflammatory cells in the vitreous. Retinal lesions may include meandering gray-white or pigmented migration tracks and occasionally a larva is visualized, which measures 350-450Ám long by 18-21Ám in diameter and moves in a sinuous manner. Chronic lesions are of more well-defined, elevated white or gray masses in the retina. Traction bands may extend from the lesion to the optic disc and there may be dragging of the retina toward the mass producing a falciform fold. In toxocaral endophthalmitis, the vitreous is often hazy or turbid and a yellow-white mass may be seen in the peripheral retina; in advanced cases, the pupillary opening is obscured by a white, retrolental mass and cyclitic membrane. In diffuse unilateral subacute neuroretinitis, there are evanescent crops of gray-white retinal lesions, vitritis, optic disc swelling, attenuation of retinal vessels, and areas of pigment epithelial derangement.
Ocular toxocariasis usually occurs as a distinct entity, without concomitant systemic manifestations of visceral larva migrans or neural larva migrans. In heavy primary infections with somatic dissemination, ocular larva migrans may be seen in conjunction with signs of visceral larva migrans (hepatomegaly, high peripheral eosinophilia, hypergammaglobulinemia) and occasionally central nervous system disease.