Contributor:  Kevin R. Kazacos
Ocular larva migrans is a zoonotic infection caused by the intraocular migration of larvae of various helminth parasites of domestic and wild animals, especially dogs, cats, and raccoons. It is specific type of larva migrans in which helminth larvae produce clinical disease within ocular tissues. The term ocular larva migrans is often used synonymously for lesions produced by Toxocara canis [toxocariasis], however, numerous helminths of lower animals (such as ascarids of domestic and wild canids, cats, raccoons, skunks, badgers, bears, opossums, snakes, raptors, and other carnivores) can potentially causes larva migrans in humans, with possible invasion of the eye. The parasites include the larvae of numerous ascarids (Toxocara, Toxascaris, Baylisascaris, Lagochilascaris, Hexametra, Ophidascaris, Porrocaecum), hookworm (Ancylostoma), gnathostomes (Gnathostoma), trematodes (Alaria, Paragonimus) and cestodes (Spirometra), and pentastome nymphs (Linguatula, Armillifer). The most common causes of ocular larva migrans in North America and Europe are Toxocara canis of dogs, Toxocara cati of cats, and Baylisascaris procyonis of raccoons. Diffuse unilateral subacute neuroretinitis is a type of ocular larva migrans caused by at least three different helminths (Toxocara, Baylisascaris, Alaria). Humans become infected with ascarid larvae and pentastome nymphs by accidentally ingesting infective eggs from soil or other articles contaminated with the feces of infected dogs, cats, raccoons and other animals and possibly through the ingestion of larvae in lower vertebrates (intermediate or paratenic hosts). Infection with hookworm larvae is via skin penetration. Humans become infected with gnathostomes, Alaria, Paragonimus, and Spirometra by ingesting larvae in improperly cooked lower animals, including fish, frogs, snakes, rodents, birds, and crayfish. Most human infections with these parasites are sporadic and of low incidence; however, human infection with Toxocara is common, as indicated by seroprevalence rates of 2->10%. Human infection with Baylisascaris is also probably common, based on the widespread occurrence of the parasite in raccoons and the high level of human exposure.
Ocular larva migrans is an inflammatory condition caused by larval migration in the retina, choroid, vitreous, and associated structures. Larvae usually gain access to the eye via the retinal or choroidal vasculature, or rarely (Alaria) through direct penetration of the globe. In acute or active infection, there may be eosinophilic retinochoroiditis, vitreitis, retinal vasculitis, focal hemorrhages, iritis, and/or endophthalmitis. Early migration-related lesions in the retina consist of retinal disruption, with associated pyknosis of nuclei, hyperplasia of the pigment epithelium, and varying degrees of retinitis, vasculitis, and choroiditis. Larvae may be seen at any depth of the retina, including subretinal or embedded in the outer layers. More advanced lesions associated with migration tracks consist of total retinal disruption and necrosis, with intense eosinophilic retinitis, necrotizing vasculitis, perivascular sheathing by eosinophils, hyperplasia and migration of the pigment epithelium, and associated vitreitis and choroiditis. There may be little left of the retina but the internal limiting membrane, and diffuse eosinophilic retinitis radiates from the main lesion. Larger larvae, such as Baylisascaris and Alaria, produce greater migratory damage, but all can stimulate eosinophilic inflammation. In more chronic infections, larvae become encapsulated in granulomatous masses, often located in the posterior or peripheral retina but also possible in the choroid or vitreous. Larval granulomas in the retina often develop fibrous traction bands radiating 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 occasionally is bilateral in heavy primary infections. Clinical disease is more serious with involvement of the macula and optic disc and in cases of eosinophilic endophthalmitis with retinal detachment. In the latter, the inflammatory process often organizes into a white, retrolental mass with development of a cyclitic membrane, posterior synechias, and other abnormalities. In DUSN, there is neuroretinitis and vitreitis, which progress to optic atrophy, narrowing of retinal vessels, and widespread pigment epithelial damage, leading to profound visual loss. As larval granulomas become well-organized, the eye may become quiescent, however, fibrous traction band contraction may still lead to retinal folds or detachment.
The retina is most frequently affected by larval migration damage and granulomas, followed by the choroid, vitreous, and other structures. Larval granulomas are found most often in the posterior or peripheral retina.
Appearance. The initiating complaint may be decreased or blurred vision, occurring suddenly or intermittent over time, leukokoria, and/or strabismus. The patient may note the worm moving in their visual field. Various degrees of retinitis and choroiditis are present, 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. Larvae will vary in size, shape, and movement depending on their major group and genus (Toxocariasis, Baylisascariasis, Alariosis). Signs of DUSN include multiple gray-white retinal lesions, vitreitis, optic disc swelling or pallor, attenuation of retinal vessels, and focal derangements of the retinal pigment epithelium. Chronic lesions are of more well-defined, elevated white or gray masses in the retina, choroid, or vitreous. Traction bands often extend from the lesion to the optic disc or macular area, and there may be dragging of the retina toward the mass producing a falciform fold. In endophthalmitis, the vitreous is often hazy or turbid and a yellow-white mass may be seen in the peripheral retina; in advanced cases with retinal detachment, the pupillary opening is obscured by a white, retrolental mass and cyclitic membrane.
Ocular larva migrans usually occurs as a distinct entity, without concomitant systemic manifestations, i.e., signs of visceral or neural larva migrans. However, 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, pulmonary signs) and occasionally CNS disease, the latter more likely with baylisascariasis.
Definitive diagnosis involves identification of a larva in or from the eye, so in most cases diagnosis is presumptive and indirect. Morphometric analysis of the worm in the eye is often highly suggestive. Serologic testing is available for Toxocara, less readily available for Baylisascaris, and unavailable for Alaria and most of the other parasites. Since ocular larva migrans usually involves low infection levels, a negative ELISA would not exclude the diagnosis in a patient with compatible clinical signs and exposure history. Ocular fluids have higher antibody levels than serum and are often positive when serum is negative. Aqueous humor or vitreous cytology often demonstrates eosinophils. Human ocular larva migrans and diffuse unilateral subacute neuroretinitis may also be caused by larvae of other zoonotic helminths, the most common being Toxocara canis of dogs.