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Baylisascariasis is an important zoonotic disease caused by the raccoon roundworm Baylisascaris procyonis. Baylisascariasis is a potentially severe form of larva migrans in humans that can produce fatal or severe central nervous system disease.


Humans are a paratenic (intermediate) host, meaning that infection by larval stages of the parasite can occur but the parasite does not complete its lifecycle in the host. Raccoons infected with the roundworms shed the eggs in their feces. Adult Baylisascaris organisms reside in the raccoon small intestine, and adult Baylisascaris females produce a huge number of eggs with estimates as high as greater than 100,000 eggs per worm per day. Adult raccoons may shed millions of eggs per day, which leads to heavy environmental contamination. Humans can become infected when they accidentally ingest infected eggs from objects contaminated with the feces of wild or pet raccoons. People at risk of baylisascariasis infection include young children and developmentally disabled persons who are more likely to place objects with contaminated dirt or animal waste in their mouth. In addition, hunters, taxidermists, and wildlife conservationists who come in contact with animals and their habitats are also at risk (Fig. 320-1).

Figure 320-1

Life cycle of Baylisascaris procyonis. OLM, ocular larva migrans; VLM, visceral larva migrans. (Reproduced with permission from Centers for Disease Control [CDC].)

Infected eggs can be found in tree stumps, decaying trees, or rock piles. In more urban areas, they are often found in attics, chimneys and flat roofs, or patios. The eggs in the soil remain infectious for years. Once ingested, the larvae emerge and migrate to different parts of the body including lung, skeletal muscle, eye, and brain. Approximately 5% to 7% of ingested larvae may enter the brain, where they produce extensive damage before they are enclosed within cystic lesions. The migrating larvae may also cause mechanical damage while they travel throughout the tissue and trigger intense host inflammatory reaction producing eosinophilic granulomas in many organs. The infecting dose of larvae may be related to the disease presentation; when large numbers of embryonated eggs are ingested, larvae may be more likely to penetrate the central nervous system, causing neural larva migrans (NLM) (Fig. 320-2). Death or permanent disability is a common outcome of NLM due to Baylisascaris.

Figure 320-2

Embryonated eggs of B procyonis, showing the developing larva inside. (Reproduced with permission from Centers for Disease Control [CDC]. Photo contributor: Dr. Cheryl Davis, Western Kentucky University, KY.)

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