|Baylisascaris procyonis larvae|
Baylisascaris eggs are passed in faeces and become active within a month. They can remain viable in the environment for years, withstanding heat and cold. According to University of California, Davis, and the Santa Barbara County Public Health Department, animals become infested either by:
- Swallowing the eggs,
- Eating another animal infested with Baylisascaris. or
- Contact with faeces of infested animal, skin absorbed.
After an animal swallows the eggs, the microscopic larvae hatch in the intestine and invade the intestinal wall. If they are in their definitive host they develop for several weeks, then enter the intestinal lumen, mature, mate, and produce eggs, which are carried out in the fecal stream. If the larvae are in a paratenic host, they break into the bloodstream and enter various organs, particularly the central nervous system. A great deal of damage occurs wherever the larva try to make a home. In response to the attack, the body attempts to destroy it by walling it off or killing it. The larva moves rapidly to escape, seeking out the liver, eyes, spinal cord or brain. Occasionally they can be found in the heart, lungs, and other organs. Eventually the larva dies and is reabsorbed by the body. In very small species such as mice, it might take only one or two larvae in the brain to be fatal. If the larva does not cause significant damage in vital organs, then the victim will show no signs of disease. On the other hand, if it causes behavioral changes by destroying parts of the brain, the host becomes easier prey, bringing the larva into the intestine of a new host.
Clinical signs in humans
- Skin irritations from larvae migrating within the skin.
- Respiratory discomfort, liver enlargement, and fever due to reaction to larvae migration.
- Eye and brain tissue damage due to the random migration of the larvae.
- Nausea, a lethargic feeling, incoordination and loss of eyesight.
- Severe neurological signs including imbalance, circling and abnormal behavior, caused by extensive tissue damage due to larval migration through the brain, eventually seizures and coma.
While worming can rid the intestine of adult Baylisascaris, no treatment has been shown to alleviate illness caused by migrating larvae. Despite lack of larvicidal effects, albendazole (20–40 mg/kg/d for 1–4 weeks) has been used to treat many cases. 
Each Baylisascaris species has a host species that it uses to reproduce. The eggs appear in the host species' feces. They can then be ingested by, and infest, a variety of other animals (including humans) that serve as paratenic hosts.
Baylisascaris species include:
- Baylisascaris procyonis (of raccoons)
- Baylisascaris melis (of European badgers)
- Baylisascaris transfuga (of bears)
- Baylisascaris columnaris (of skunks) and American badgers)
- Baylisascaris devosi (of fishers) and martens)
- Baylisascaris laevis (of marmots)
Baylisascaris procyonis, the raccoon parasite, is related to the canine roundworm Toxocara canis. It is found in the intestines of raccoons in North America, Japan and Germany. It infests 68 to 82% of some raccoon populations, according to the House Rabbit Society. This parasite can be extremely harmful or deadly to humans.
Skunks carry Baylisascaris columnaris, a similar species to B. procyonis. Many pet skunks have died from this parasite. According to several skunk experts and Information on Parasites in Skunks by Matt Bolek, Diagnostic Parasitologist, many baby skunks from skunk farms have B. columnaris present in their bodies. The exact proportion of new skunks that are infested is unknown. Since the worms are often at too early a stage in development to begin shedding eggs into the feces, a fecal test may not detect the parasite, and the pet should be pre-emptively treated with wormers (See Pet skunk).
Baylisascaris columnaris is not as prevalent as B. procyonis.
Careful decontamination procedures need to be performed after contact with animal feces. Baylisascaris eggs can enter the digestive tract of a person who, for instance, removes dung from his property and then eats without thoroughly washing his hands.
Baylisascaris are highly resistant to decontamination procedures because of their dense cell walls and sticky surface. They can survive hot or freezing weather and certain chemicals, remaining viable for several years. Rats are a known vector, and rat droppings may deposit the eggs into the carpets and interiors of homes.
Bleach can prevent the eggs from sticking, but will not ensure destruction. According to Parasitism in Companion Animals by Olympic Veterinary Hospital, hand washing is an important countermeasure against ingestion, and decontamination of other surfaces is accomplished by thoroughly flaming with a propane torch or treating with lye. According to Bolek, other forms of high heat such as boiling water or steam will accomplish the same result. Children are more likely to be infected than adults because of their tendency towards pica, particularly geophagy (eating dirt).
- University of California, Davis, and the Santa Barbara County Public Health Department
- Gavin PJ, Kazacos KR, Shulman ST (2005). "Baylisascariasis". Clin. Microbiol. Rev. 18 (4): 703–18. doi:10.1128/CMR.18.4.703-718.2005. PMC 1265913. PMID 16223954.
- "Dominguez," Samuel R, Levin Myron J, "Chapter 41. Infections: Parasitic & Mycotic" (Chapter). Hay WW, Levin MJ, Sondheimer JM, Deterding RR: CURRENT Diagnosis & Treatment: Pediatrics, 20e: http://www.accessmedicine.com/content.aspx?aID=6591748.
- Yeitz JL, Gillin CM, Bildfell RJ, Debess EE (January 2009). "Prevalence of Baylisascaris procyonis in raccoons (Procyon lotor) in Portland, Oregon, USA". J. Wildl. Dis. 45 (1): 14–8. PMID 19204331.
- Baylisascaris Procyonis Article
- "www.olympicvet.com". Retrieved 2009-05-05.
- Old Rabbit Paralysis Part III: Baylisascaris Procyonis House Rabbit Society
- Baylisascaris procyonis in Dogs, D. D. Bowman, Department of Microbiology & Immunology, College of Veterinary Medicine, Cornell University, Mar. 11, 2000.
- Information on Parasites in Skunks by Matt Bolek, Diagnostic Parasitologist (link via InternetArchive, as original page no longer valid).
- University of California, Davis, and the Santa Barbara County Public Health Department.
- Parasitism in Companion Animals by Olympic Veterinary Hospital.
- Raccoon Roundworm by University of Northern British Columbia.
- Deadly Dung, University of Wisconsin Board of Regents.
- Baylisascaris procyonis: An Emerging Helminthic Zoonosis, Centers for Disease Control.
- Baylisascaris at the US National Library of Medicine Medical Subject Headings (MeSH)