Baylisacaris is a parasitic disease caused by a roundworm (Baylisascaris procyonis). Infection is caused when the eggs of the parasite are ingested or meat from an infected animal is eaten. Raccoons are the natural definitive host for the parasite and do not experience disease from it. However, unlike many parasites, Baylisascaris can infect a number of intermediate host species and is known to infect at least fifty types of animals. However, each sub-species is most comfortable in a particular host and will affect a foreign animal differently.
The eggs of the parasite are expelled in the feces of the host, where they are eaten by other animals. The eggs can survive outside a host for several years. Once swallowed, the eggs hatch in the intestines and burrow through the intestinal wall. In raccoons, the parasite will merely reproduce in the intestines to complete its life cycle. However, in humans, the parasite will usually burrow through the intestinal wall into the bloodstream looking for a more hospitable environment and attack the host's organs and will often affect the central nervous system. In response, the immune system will try to wall off the infestation, but the parasite reacts by seeking out other organs such as the liver, eyes, spinal cord and brain. It can also infect the lungs and heart
In many cases, the larvae will die leaving little damage and no symptoms. However, if larvae reach the brain, damage can be severe and permanent.
The intestines can be physically de-wormed to remove the parasite. However, the larvae are much harder to treat. Albendazole has been shown to be effective agains the larvae in some cases, prior to the onset of neurological symptoms.