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Baylisascariasis is a potentially serious form of larva migrans
that is caused by larvae of the raccoon ascarid, Baylisascaris
procyonis, a nematode found in 50% to 80% of
raccoons in North America. It also occurs in raccoons in other countries,
including Germany and Japan. B procyonis is a well-known
cause of larva migrans in animals and is usually associated with
central nervous system disease. Larvae of B procyonis have
produced fatal or severe central nervous system disease in over
100 species of mammals and birds. Other Baylisascaris species,
including B melis of badgers and B columnaris of
skunks, are also potential causes of human disease.1
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Adult female Baylisascaris organisms are 12 to
23 cm long and reside in the raccoon’s small intestine
(eFig. 325.1).2 Adult
males are about half as large. Adult females produce huge numbers
of eggs, with estimates as high as 179,000 eggs per worm per day.3 Infected
raccoons can thus shed millions of eggs per day, leading to heavy
environmental contamination. Eggs shed with the raccoon’s
feces become infective in 3 to 4 weeks. Young raccoons become infected
by ingesting infective eggs, whereas older raccoons become infected
by ingesting larvae in the tissues of intermediate hosts, including
rodents, rabbits, and birds (eFig. 325.2).
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Humans become infected with Baylisascaris by
accidentally ingesting infective eggs (eFig. 325.3) from
objects contaminated with the feces of wild or pet raccoons. In
the wild, raccoons defecate in communal sites, called latrines.
In rural areas, these sites can be found in tree stumps or limbs
or rock piles. However, in suburban and urban areas, they are often
found in attics, chimneys, flat roofs or patios, creating potentially
long-standing sources of infection. Baylisascaris eggs
in the soil can remain infective for years. Larvae emerge from ingested
eggs and migrate to many tissues, including lung, skeletal muscles,
eye, and brain. Approximately 5% to 7% of ingested
larvae enter the brain, where they can produce extensive damage
before they are walled off. Migrating larvae cause mechanical damage
and incite vigorous host inflammatory reactions, producing eosinophilic
granulomas in many tissues.
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The risk of human infection with Baylisascaris is
greatest in children younger than 4 years of age because of hygienic
habits and propensity for pica and geophagia. B procyonis can
produce visceral larva ...