In 1931, the helminth, or worm, was first isolated from raccoons at a zoo in New York City. It was first classified under the Ascaris genus, then given its own genus in 1968. Larva migrans, the signature consequence of a B. procyonis infection in intermediate hosts, had been recognized to occur in small animals well before any connection to human infections had been established. Only within the last 25 years has this changed; not only are humans now included in the group of intermediate hosts, but reported cases of baylisascariasis started to be reported. The first confirmed baylisascariasis infection was reported in 1984. Few actual infections have been reported; only twenty-five cases have been reported. Yet the danger remains as big to humans as to the smaller mammals.


Kingdom Animalia
Phylum Nemathelminthes
Class Nematoda
Order Ascaridida
Family Ascarididae
Genus Baylisascaris
Species procyonis



The nematode Baylisascaris procyonis, also known as raccoon roundworm, is found mostly in raccoons, its definitive host. The females can grow to be 24 cm long, while the males can grow up to 12 cm long. It causes baylisascariasis, also known as raccoon roundworm encephalitis, in non-definitive hosts, such as mice, rabbits, birds, and humans. While not ubiquitous among the North American raccoon population, large amounts of the raccoon population are infected with B. procyonis. It is suggested that over 90% of juvenile raccoons carry the helminth in some areas, while up to 70% of adult raccoons carry the helminth.

Female (left) and male (right) Baylisascaris procyonis.

Courtesy Roussere et al.


Gavin, Patrick, Kevin Kazacos, Stanford Shulman. Clin Microbiol Rev. 18(4)

Sorvillo, Frank, et al. Emerg Infect Dis. 2002 April; 8(4).

University of Pennsylvania School of Veterinary Medicine. http://cal.vet.upenn.edu/dxendopar/parasitepages/ascarids/b_procyonis.html