Baylisascaris infection is caused by a roundworm found in raccoons. This roundworm can infect people as well as a variety of other animals, including dogs. Human infections are rare, but can be severe if the parasites invade the eye (ocular larva migrans), organs (visceral larva migrans) or the brain (neural larva migrans).
Epidemiology & Risk Factors
Raccoons are the primary, or definitive, host of Baylisascaris procyonis, a roundworm. Raccoons become infected with Baylisascaris in one of two ways:

• Young raccoons become infected by eating eggs during foraging, feeding, and grooming.
• Adult raccoons acquire the infection by eating rodents, rabbits, and birds infected with the larvae of Baylisascaris.

Infected raccoons have been found throughout the United States, mainly in the Midwest, Northeast, Middle Atlantic, and West Coast.
Raccoons are peridomestic animals, which means they live in or around areas where people live. Roundworm eggs are passed in the feces of infected raccoons. Raccoons defecate in communal sites, called latrines. Raccoon latrines are often found at bases of trees, unsealed attics, or on flat surfaces such as logs, tree stumps, rocks, decks, and rooftops. As more raccoons move into populated areas, the number and density of their latrines will increase.
While raccoons are the roundworm's primary host, other types of animals can become infected. Birds and small mammals, such as rodents and rabbits, are susceptible to the parasite. Unlike raccoons, these animals sometimes show signs of infection, such as muscle spasms, tremors, and progressive weakness; infection can lead to death. Predator animals, including dogs, may become infected by eating an animal that has been infected with Baylisascaris. In some dogs, Baylisascaris may develop to adult worms and pass eggs in the dogs' feces.
The worms develop to maturity in the raccoon intestine, where they produce millions of eggs that are passed in the feces. Eggs that are excreted by raccoons are not immediately infectious. These eggs must develop in the environment for 2 to 4 weeks, after which the eggs are able to cause infection. The eggs are resistant to most environmental conditions and with adequate moisture, can survive for years.
Humans become infected by ingesting embryonated (fertile) eggs. Anyone who is exposed to environments where raccoons frequent is potentially at risk. Young children or developmentally disabled persons are at highest risk for infection as they may be more likely to put contaminated fingers, soil, or objects into their mouths.
Hunters, trappers, taxidermists, and wildlife handlers may also be at increased risk if they have contact with raccoons or raccoon habitats.
Fewer than 25 cases of Baylisascaris disease have been documented in the United States. However, it is possible that some cases are incorrectly diagnosed as other infections or go undiagnosed. Cases that are diagnosed tend to be severe.
Biology
Causal Agent:
Human baylisascariasis is caused by larvae of Baylisascaris procyonis, an intestinal nematode of raccoons.
Baylisascaris procyonis completes its life cycle in raccoons, with humans acquiring the infection as accidental hosts (dogs serve as alternate definitive hosts, as they can harbor patent infection and shed eggs). Unembryonated eggs are shed in the environment , where they take 2-4 weeks to embryonate and become infective . Raccoons can be infected by ingesting embryonated eggs from the environment . Additionally, over 100 species of birds and mammals (especially rodents) can act as paratenic hosts for this parasite: eggs ingested by these hosts hatch and larvae penetrate the gut wall and migrate into various tissues where they encyst . The life cycle is completed when raccoons eat these hosts . The larvae develop into egg-laying adult worms in the small intestine and eggs are eliminated in raccoon feces. Humans become accidentally infected when they ingest infective eggs from the environment; typically this occurs in young children playing in the dirt . Migration of the larvae through a wide variety of tissues (liver, heart, lungs, brain, eyes) results in VLM and OLM syndromes, similar to toxocariasis . In contrast to Toxocara larvae, Baylisascaris larvae continue to grow during their time in the human host. Tissue damage and the signs and symptoms of baylisascariasis are often severe because of the size of Baylisascaris larvae, their tendency to wander widely, and the fact that they do not readily die. Diagnosis is usually made by serology, or by identifying larvae in biopsy or autopsy specimens.
Disease
People become infected when they accidentally ingest infective eggs in soil, water, or on objects that have been contaminated with raccoon feces. Baylisascaris infection is not spread from one person to another. When humans ingest these eggs, they hatch into larvae in the person's intestine and travel throughout the body, affecting the organs and muscles.
Depending on where the larvae migrate, Baylisascaris infection can affect the brain and spinal cord (neural larva migrans), the eye (ocular larva migrans), and/or other organs (visceral larva migrans).
Signs and symptoms depend on how many eggs are ingested and where in the body the larvae migrate (travel). Ingesting a few eggs may cause few or no symptoms, while ingesting large numbers of eggs may lead to serious symptoms. Symptoms of infection may take a week or so to develop and may include:

• Nausea
• Tiredness
• Liver enlargement
• Loss of coordination
• Lack of attention to people and surroundings
• Loss of muscle control
• Blindness
• Coma

Diagnosis
If you suspect you have been infected, consult your health care provider immediately. Be sure to tell your health care provider if you have recently been exposed to raccoons or their feces.
Diagnosis is difficult because symptoms depend on the number of infecting larvae and location in the body. Ocular larva migrans, when the larvae migrate to the eye, can cause sensitivity to light, inflammation of the eye, and blindness. Symptoms of visceral larva migrans, when the larvae travel to organs, depend on which organs are affected. For example, an invasion of the liver may cause hepatomegaly (inflammation and enlargement of the liver), while an invasion of the lung may cause pulmonary symptoms such as cough or chest pain. Larvae rarely end up in the nervous system but the most severe cases are neural larva migrans, when the larvae migrate into the brain and cause it to swell (encephalitis). There is no commercially available test for Baylisascaris infection. A health care provider may test blood, cerebrospinal fluid (CSF), and tissue to determine if an individual is infected. Eye examinations may reveal a migrating larva or lesions and are often the most significant clue to infection with Baylisascaris.
Diagnosis often is made by ruling out other infections that cause similar symptoms. Information on diagnosis and testing can be obtained through your local or state health department.
Treatment
No drugs have been shown to be totally effective for the treatment of Baylisascaris infection.  Albendazole, a broad spectrum anthelmintic, has been recommended for specific cases. 
Early treatment might reduce serious damage caused by the infection. Should you suspect you may have ingested raccoon feces, seek immediate medical attention.
Prevention & Control
Baylisascaris infection can be prevented by avoiding contact with raccoons and their feces. Washing your hands after working or playing outdoors is good practice for preventing a number of diseases.
Do not keep, feed, or adopt wild animals, including raccoons, as pets. Infection rarely causes symptoms in raccoons, so you cannot tell if a raccoon is infected by observing its behavior. Roundworm eggs passed in the feces of infected raccoons are not visible to the naked eye. Eggs can only be seen using a microscope.
You may discourage raccoons from living in and around your home or parks by taking these steps:

• prevent access to food
• keep trash containers tightly closed
• close off access to attics and basements
• keep sandboxes covered when not in use (raccoons may use sandboxes as a latrine)
• remove fish ponds -- they eat the fish and drink the water
• eliminate water sources
• remove bird feeders
• clear brush so raccoons are not likely to make a den on your property

Stay away from areas and materials that might be contaminated by raccoon feces. Raccoons typically defecate at the base of or in raised forks of trees, or on raised horizontal surfaces such as fallen logs, stumps, or large rocks. Raccoon feces also can be found on woodpiles, decks, rooftops, and in attics, garages, and haylofts. Feces usually are dark and tubular, have a pungent odor (usually worse than dog or cat feces), and often contain undigested seeds or other food items.
If you have found a raccoon latrine near your home, cleaning the area may prevent possible infection.  Newly deposited eggs take at least 2-4 weeks to become infective. Prompt removal and destruction of raccoon feces will reduce risk for exposure and possible infection.