Baylisascariasis infection in humans is rare, though extremely dangerous due to the ability of the parasite’s larvae to migrate into brain tissue and cause damage. Concern for human infection has been increasing over the years due to urbanization of rural areas resulting in the increase in proximity and potential human interaction with raccoons.
In North America, B. procyonis infection rates in raccoons are very high, being found in around 70% of adult raccoons and 90% of juvenile raccoons. Transmission occurs similarly to other roundworm species, through the fecal-oral route. Eggs are produced by the worm while in the intestine, and the released eggs will mature to an infective state externally in the soil. When an infected egg is ingested, the larvae will hatch and enter the intestine. Transmission of B. procyonis may also occur through the ingestion of larvae found in infected tissue.
The potential for human infection was noted by Paul C. Beaver in 1969 who studied infected mice, and the first case was reported 15 years later. Human infection with B. procyonis has been relatively rare, with about 13 cases reported since 1980. However, disease caused by this parasite can be extremely dangerous, causing death or severe symptoms. Reported disease has primarily afflicted children and almost all cases were a result of the ingestion of contaminated soil or feces. Out of the 13 cases, 5 were fatal and the remaining victims were left with severe neurological damage. Even with treatment, prognosis is poor. The common antihelmintic medicines are able to treat adult worms living in the intestines, but are less potent against migrating larvae. Animal studies have shown that treatment is more effective before the larvae have reached the brain; however, migration to the brain was shown to occur only 3 days after ingestion, leaving a very small window of opportunity.
It is possible that human infection is more common than diagnosed and most cases do not reach a clinical stage.
