Baylisascaris procyonis is a roundworm nematode

Nematode

The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

, found ubiquitously in raccoon

Raccoon

Procyon is a genus of nocturnal mammals, comprising three species commonly known as raccoons, in the family Procyonidae. The most familiar species, the common raccoon , is often known simply as "the" raccoon, as the two other raccoon species in the genus are native only to the tropics and are...

s, its larvae migrating in the intermediate host

Intermediate host

A secondary host or intermediate host is a host that harbors the parasite only for a short transition period, during which some developmental stage is completed. For trypanosomes, the cause of sleeping sickness, humans are the primary host, while the tsetse fly is the secondary host...

s causing visceral larva migrans (VLM). Baylisascariasis as the zoonotic infection of 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.

Transmission

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.

Epidemiology

B. procyonis is found abundantly in its definitive host, the raccoon

Raccoon

Procyon is a genus of nocturnal mammals, comprising three species commonly known as raccoons, in the family Procyonidae. The most familiar species, the common raccoon , is often known simply as "the" raccoon, as the two other raccoon species in the genus are native only to the tropics and are...

. The parasite has been found to have the ability to infect more than 90 kinds of wild and domestic animals. Many of these animals act as intermediate hosts and the infection results in the penetration of the gut wall by the larvae and subsequent invasion of tissue, resulting in severe disease. In animals, it is the most common cause of larva migrans

Larva migrans

Larva migrans can refer to:* Cutaneous larva migrans, a skin disease in humans, caused by the larvae of various nematode parasites* Visceral larva migrans, a condition in children caused by the migratory larvae of nematodes...

. The raccoon plays an important role in the life cycle of the disease. Raccoons are solitary but will frequently defecate in communal areas known as latrines. These latrines are an abundant source of B. procyonis eggs, which can remain viable for years. Raccoons therefore are important in maintaining the parasite, providing a source of infection for humans and other animals.

Life cycle

An adult worm lives and reproduces in the intestine of its definitive host, the raccoon. The female worm can produce between 115,000-179,000 eggs per day. Eggs are excreted along with feces, and become infective in the soil after 2–4 weeks. If ingested by another raccoon, the life cycle repeats. However, if these eggs are ingested by an intermediate host(small mammals, birds) the larvae of B. procyonis will penetrate the gut wall of the host and migrate into tissues. Larvae tend to migrate to the brain, cause damage, and affect the behaviour of the intermediate host making it an easier prey for raccoons. Reproduction does not occur in these parasitic hosts, however, if a raccoon preys on an infected intermediate host, the encysted larvae can become adults in the raccoon and the cycle resumes.

Infection in Humans

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 of 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 anti-helminthic medicines are able to treat adult worms living in the intestines, but are poor against migrating larvae. Animal studies have shown that treatment is more favorable before the larvae have migrated to 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.

Bioterrorism Threat

B. procyonis has become a concern for its potential use as an agent of bioterrorism

Bioterrorism

Bioterrorism is terrorism involving the intentional release or dissemination of biological agents. These agents are bacteria, viruses, or toxins, and may be in a naturally occurring or a human-modified form. For the use of this method in warfare, see biological warfare.-Definition:According to the...

. The fact that this parasite's eggs are easy to acquire, able to live for years, extremely resistant to many disinfectants and heat, and cause serious infections in humans with poor treatment options could make it a dangerous weapon.