Contributor: Kevin R. Kazacos
Baylisascariasis is an inflammatory nematode disease caused by a migration of raccoon roundworm larvae, Baylisascaris procyonis, and possibly other Baylisascaris species (from skunks, badgers, and bears). Baylisascaris procyonis is the cause of an aggressive larva migrans in animals and it can cause of fatal or severe neurologic disease in children. Ocular baylisascariasis can be produced experimentally in monkeys and rodents. Baylisascaris procyonis is a cause of human ocular larva migrans, and the helminth is considered to be a pathogen in diffuse unilateral subacute neuroretinitis.Humans become infected by accidentally ingesting infective Baylisascaris procyonis eggs from soil or other articles contaminated with the feces of infected raccoons. Important sources of human infection include raccoon latrines in and around the domestic environment, and contamination from pet raccoons. To a limited extent, the parasite also matures in dogs. Ocular baylisascariasis is seen most often in older children and adults, who do not have concomitant visceral larva migrans or other signs of infection. However, based on the widespread occurrence of infected raccoons and raccoon fecal contamination, the risk of human exposure and infection is high. Ocular baylisascariasis is a zoonosis contracted by ingesting infective Baylisascaris eggs from environments contaminated with the feces of infected raccoons (and possibly skunks, badgers, and bears). Ocular baylisascariasis is caused by larval migration in the retina, choroid, vitreous, and associated structures. Larvae gain access to the eye via the retinal or choroidal vasculature, and rarely via the optic nerve. In acute or active infections, there is eosinophilic retinochoroiditis, vitritis, and/or endophthalmitis. In chronic infections, larvae become encapsulated in granulomatous masses in the retina, choroid, or other locations. Larval migration can produce extensive retinal damage and eosinophilic inflammation, leading to visual deficits and blindness. The condition is most often unilateral, but could be bilateral in heavy primary infections.Clinical disease would be 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 could organize into a white, retrolental mass with development of a cyclitic membrane, posterior synechias, and other abnormalities. In DUSN, 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, if traction bands to the optic disc have formed, there may be a subsequent retinal fold or retinal detachment. The retina is most frequently affected by larval migration and granulomas, followed by the choroid, vitreous, and other structures. Larval granulomas are most often in the posterior or peripheral retina.The initiating complaint is usually sudden or progressive visual loss and/or blurred vision, and the patient may note the worm moving in the visual field; leukocoria and/or strabismus may also be present. Various degrees of retinitis, choroiditis, and vitritis are present. Signs of DUSN are often noted, including disseminated gray-white retinal lesions, vitriitis, optic disc swelling or pallor, attenuation of retinal vessels, and focal derangements of the retinal pigment epithelium. Retinal lesions may include meandering gray-white or pigmented migration tracks, and occasionally a larva is visualized, which measures 1500-2000 µm long by 60-80 µm in diameter and moves in a sinuous manner. Larval granulomas in the retina appear as well-circumscribed, elevated white or gray masses. In cases of endophthalmitis, the vitreous would be hazy or turbid and a yellow-white mass may be seen in the peripheral retina; in advanced cases with retinal detachment, the pupillary opening would be obscured by a white, retrolental mass and cyclitic membrane.
Ocular baylisascariasis usually occurs as a distinct entity, without concomitant systemic manifestations, i.e., signs of visceral larva migrans 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 (high peripheral eosinophilia, hypergammaglobulinemia) or neural larva migrans; in three young children with heavy infections, unilateral ocular larva migrans accompanied severe CNS involvement. A 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 highly suggestive. An ELISA using larval excretory-secretory antigens, and an indirect immunofluorescence assay have been developed, but are not readily available. Since ocular larva migrans usually involves low infection levels, a negative ELISA would not exclude this diagnosis in a patient with compatible clinical signs and exposure history; in such cases, ocular fluids would likely have higher antibody titers than serum and should be positive when serum is negative. Aqueous humor or vitreous cytology would likely demonstrate eosinophils.