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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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eFigure 325.1.
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Adult Baylisascaris worm in raccoon intestine.

(Source: Gavin PJ, Kazacos KR, Shulman ST. Baylisascariasis. Clin Microbiol Rev. 2005;18(4):703-718.)

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eFigure 325.2.
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Life cycle of Baylisascaris. VLM, visceral larva migrans; OLM, ocular larva migrans.

(Source: http://www.dpd.cdc.gov/dpdx/HTML/Baylisascariasis.htm. Accessed January 13, 2008.)

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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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eFigure 325.3.
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Baylisascaris egg.

(Source: Gavin PJ, Kazacos KR, Shulman ST. Baylisascariasis. Clin Microbiol Rev. 2005;18(4):703-718.)

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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 ...

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