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Baylisascaris Procyonis


Raccoon roundworms (Baylisascaris procyonis) are common large parasitic worms known as ascarids that inhabit the small intestines of raccoons. The adult worms are white to tan in color and range from about 10 to 20 cm (4-8 in) in length.


Baylisascaris can cause disease in humans when infective eggs are ingested and the larvae, the immature form between the egg and the adult, migrate throughout the body. This disease, also known as Larval Migrans (LM), can result in skin irritation, vision problems, or neurological disease that can be fatal. This disease is more likely to occur in children who may put soil or animal waste in their mouths. A great tragedy of LM in children occurs when an eye is removed because a larva has entered the eye and is mistaken for a cancerous condition. Hunters, trappers, wildlife rehabilitators, and other people who may handle raccoons or spend time in raccoon habitats are also at risk. To avoid contracting Baylisascaris (as well as other diseases), people should avoid contact with raccoons or raccoon feces, and should wash their hands after working or playing outdoors. Hunters, trappers, and others who must handle raccoons should wear gloves while doing so, and should wash their hands afterwards.

Species Affected

Many species of mammals (including humans) and birds can become infected with Baylisascaris larva; small vertebrates including mice, rats, squirrels, rabbits, and birds are commonly infected with the larval form of the raccoon roundworm. Raccoons are the primary host for this parasite, so adult worms typically only develop within raccoons and closely related species such as ringtails, kinkajous, and coatimundis; however, adult raccoon roundworms are occasionally found within the small intestines of domestic dogs. Fatal or severe central nervous system disease due to Baylisascaris has been observed in several species including woodchucks, nutria, beaver, mice, squirrels, foxes, armadillos, porcupines, pigeons, and turkeys.


Raccoon roundworm is found in the United States, Europe, and Japan. There is some evidence that it may also occur in South America. This parasite is known to infect raccoons in the mid-Atlantic, northeastern, and Midwestern United States as well as in California and Texas. Raccoon roundworms are most common in the mountains of Virginia, Kentucky, and West Virginia. Roundworm infections have been found in raccoons in several Pennsylvania counties. The current estimate of the average prevalence of Baylisascaris infections in Pennsylvania raccoons is about 38%, although the study is ongoing. Some counties in PA seem to have 0% infection while other counties may have as high as 80% infection rates.

Baylisascaris procyonis distribution  


Baylisascaris eggs are shed in the feces of infected raccoons. Millions of eggs may be shed by an infected animal each day. The eggs are extremely resilient and can survive in the environment for several years. Once the eggs are excreted in feces, larvae will develop inside of the eggs within 11 to 14 days under adequate environmental (temperature and humidity) conditions. These eggs are then known as "embryonated", and can infect a new host. Raccoons can become infected by ingesting embryonated eggs which will develop into adult worms within the small intestine. Young raccoons may ingest infective eggs that are attached to the mother's teat or fur. If another animal, such as a rodent or bird, ingests the embryonated eggs, the larvae will wander in the animal's body (see previous reference to Larval Migrans or LM) and can encyst (create and live in a protective structure) within tissue, but will not develop into adult worms. These animals are known as intermediate hosts because raccoons can become infected by ingesting animals containing encysted larvae.

Like these other animal species, humans can experience larval migrans following ingestion of infective Baylisascaris eggs.

Clinical Signs

Clinical signs are rarely observed in raccoons infected with Baylisascaris. However, intestinal obstructions and subsequent ruptures may occur in raccoons infected with large numbers of roundworms.

Small mammals and birds acting as intermediate hosts may show clinical signs due to tissue damage caused by migrating larvae. Infected animals may exhibit unusual behavior and even fatal neurological disease when larvae migrate through the brain and spinal cord. These animals may initially develop a head tilt and have difficulty walking, which may progress to a loss of fear, circling, recumbency, coma, and death. Lung and liver damage may occur as a result of larval migration. Eye disorders may also be observed. Animals infected with small numbers of migrating larvae may not show any clinical signs.


In raccoons, Baylisascaris may be detected either by observation of the eggs in feces using fecal floatation methods, or by observation of adult worms in the small intestine upon necropsy. In intermediate hosts, larvae may be observed microscopically in tissues during post-mortem examination.


Raccoons and domestic animals can be treated successfully with anthelmintics (de-wormers). There are currently no medications that reliably kill migrating larvae, so treatment of intermediate hosts involves supportive care if necessary.

Management/Prevention Management of raccoon roundworm should focus on prevention of transmission to humans and pet animals. It is unlikely that this disease will be eliminated because the parasite commonly infects wild raccoons without causing clinical illness, and the eggs are highly resistant in the environment. Although the use of baits containing anthelmintics is being explored in hopes of decreasing the prevalence of Baylisascaris infection in wild raccoons. Areas contaminated with raccoon feces should be cleaned thoroughly and the feces should be burned. Wildlife rehabilitators working with raccoons should remove and burn feces promptly before the eggs have time to embryonate. Rehabilitators may also consider treating raccoons with anthelmintics.

Suggested Reading

Centers for Disease Control and Prevention (CDC). 2011. Parasites– Baylisascaris infection.

Kazacos, K. R. 2001. Baylisascaris procyonis and related species. Pages 301-340 in W. M. Samuel, M. J. Pybus, and A. A. Kocan, editors. Parasitic diseases of wild mammals. Iowa State University Press, Ames, USA.

Michigan Department of Natural Resources. Wildlife Disease. Raccoon Roundworm (Baylisascaris).,1607,7-153-10370_12150_12220-27261--,00.html.

Page, K., J. C. Beasley, Z. H. Olson, T. J. Smyser, M. Downey, K. F. Kellner, S. E. McCord, T. S. Egan, II, and O. E. Rhodes, Jr. 2011. Reducing Baylisascaris procyonis roundworm larvae in raccoon latrines. Emerging Infectious Diseases 17: 90-93.