Baylisascariasis is a human helminthic zoonosis caused by larval invasion of tissues and resulting in different presentations:

• Neural larva migrans
• Ocular larva migrans
• Visceral larva migrans

Raccoons (and dogs on rare occasions) are the definitive hosts for the nematode parasite Baylisascaris procyonis, also known as the raccoon roundworm. Ingestion of parasite eggs in fecal matter leads to development of adult worms within the small intestine of definitive hosts.

Humans as well as small mammals (eg, woodchucks, rabbits) and birds serve as accidental hosts upon ingestion of eggs from the environment. Although symptomatic or subclinical infections in humans are likely to occur, the incidence among the general population is unknown.

The primary risk factors for human baylisascariasis infection include coming into contact with raccoon "latrines" (communal foci / collection of fecal matter), pica or geophagia, being less than 4 years of age, having developmental delay, and being of the male sex.

Typically within 2-4 weeks after infection, larval migration to the brain and accompanying host inflammatory response can lead to a serious, life-threatening eosinophilic encephalitis (neural larva migrans), with or without associated visceral larva migrans. Patients typically present with lethargy, irritability, somnolence, and/or visual changes, paralysis, seizures, coma, and death.

Ocular larva migrans in the absence of other neurological findings has been reported; patients experience vision problems associated with inflammatory and degenerative retina/optic disc changes from larval migration.