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Raccoon Roundworm (Baylisascaris)

Raccoon roundworm from raccoon intestines


The raccoon roundworm (Baylisascaris procyonis) is the common large roundworm or ascarid found in the small intestinal tract of raccoons. Adult worms measure 15 to 20 cm in length and 1 cm in width. They are tan-white in color, cylindrical and taper at both ends.


B. procyonis has been reported from numerous states, but probably occurs wherever the raccoon inhabits. Prevalence of infections ranges from 3.4% to nearly 100% of all raccoons sampled. In Michigan, the parasite has commonly been seen in raccoons statewide.

There are several intermediate hosts that have been diagnosed as having a Baylisascaris infection: mice, squirrels, rabbits, birds, woodchucks, and chipmunks.

Transmission and Development

Transmission of B. procyonis can occur either directly or via an intermediate host. Naturally infected raccoons shed eggs (millions daily) in their feces and, under adequate temperature and moist conditions, a larvae will develop within the egg (embryonated) and can be infective (2nd stage larva) in 11-14 days. Raccoons, especially young ones, become infected directly by accidental ingestion of these eggs. This may occur via the mother's egg-contaminated body or from the local environment of the den (soil or vegetation). When an intermediate host is involved, embryonated eggs are ingested, the eggs hatch, the larvae penetrate the intestines and migrate through the liver and the lungs. The larvae then enter the pulmonary veins, pass into the left side of the heart and are distributed throughout the body especially the head, neck and/or thoracic areas. The larvae become encysted in small, fibrous nodules in the affected tissue. If the intermediate host is eaten by a raccoon, the encysted larvae are released and migrate to the small intestine where they develop into the adult stage.

In humans, embryonated eggs are ingested, the eggs hatch, and the larvae undergo random migration along the skin and through the viscera. The parasite commonly migrates to the eyes and the brain.

Clinical and Pathological Signs

In raccoons usually there are no clinical or pathological signs observed. In heavy infections, intestinal obstructions or a rupture of the intestinal tract may occur, due to the large number of parasites present.

The animals usually seen with clinical and pathological signs caused by Baylisascaris are the intermediate hosts (mice, woodchucks, squirrels, rabbits, and birds). Migration of large numbers of larvae may cause liver and lung damage. Usually changes in behavior are seen due to central nervous system disorders. This is the result of damage caused to the brain and spinal cord by the larvae. Larvae may also cause eye disorders by migrating through the ocular tissues. If small numbers of larvae are involved in the migration, there may be no clinical signs observed.

Fatal or severe central nervous system disorders have been documented for mice, gray and fox squirrels, ground squirrels, woodchucks, nutria, beavers, domestic quail, partridges, pigeons, exotic turkeys, emus, captive cockatiels, captive prairie dogs, foxes, armadillos and porcupines.

In intermediate hosts unusual behavior is generally observed. The affected animal will initially exhibit a head tilt and an inability to walk and/or climb properly. As the clinical illness progresses the animal may lose its fear of humans, circle, roll on the ground, fall over, lay on its side, and paddle its feet, become totally recumbent, comatose, and finally die.

In humans, pathological lesions observed consist of skin irritations (cutaneous larval migrans) and eye and brain tissue damage (visceral larval migrans) due to the random migration of the larvae. The affected individuals may experience nausea, a lethargic feeling, incoordination, and loss of eyesight.


In raccoons, infection with Baylisascaris can be confirmed by recovering and identifying the adult worms (postmortem examination) or by fecal flotation (live animal) to identify characteristic ascarid eggs in the feces. Occasionally sub-adult worms are passed in the feces or vomitus.

In intermediate hosts, the disease can be diagnosed after a postmortem examination. Larvae and associated lesions in the brain, eyes, and other tissues can be observed on histopathological examination. A Baylisascaris infection is often suspected by the history and clinical signs observed.

In humans, based on the size of the larvae in the skin or eye lesions, cases of Baylisascaris infection can be determined. In the event of the death of the human, larvae can be detected in microscopic sections of the brain, heart, lungs, eyes, and other affected tissues.

Treatment and Control

Raccoons can be successfully treated with several anthelmintics to kill the adult worms. Effective drugs are piperazine, fenbendazole, pyrantel pamoate, levamisole and organophosphates such as dichlorvos.

There are currently no drugs that can effectively kill the migrating larvae in the body. Laser surgery has been successfully performed to kill larvae present in the retina of the eye, but the damage caused by the migrating larvae is irreversible. Treatment with steroids in intermediate hosts is mainly supportive and is designed to decrease the inflammatory reaction.

Controlling infections of this parasite requires minimizing contact with areas inhabited by raccoons. Fecal contamination of an area can result in millions of eggs being deposited and therefore available for infection. These eggs are extremely resistant to environmental conditions, being able to survive for several years. Any area contaminated with raccoon feces should be cleaned and the feces, as well as any contaminated feed, straw, or hay, burned. Children and pets should be kept away from these contaminated areas until a thorough cleaning has occurred.


B. procyonis is of public health significance because it can infect humans, causing skin irritations and eye and brain damage due to the random migration of the larvae. There have been a small number of human fatalities involving young children and these fatalities were the result of the child ingesting a large number of eggs.

There are other ascarids (Toxocara canis and Toxocara cati, the dog and cat roundworm, respectively) that can cause similar skin, ocular and nervous system problems. Transmission of these parasites is more likely than with Baylisascaris due to close human-pet association, but the dog and cat roundworms are less pathogenic.

Some wildlife species, as was stated above, are susceptible to Baylisascaris. From the individual animal standpoint, the parasite may be significant, but from the total population perspective, the parasite has a minimal impact.

Because of the possibility of infection with Baylisascaris by raccoons, people are advised not to raise raccoons as pets. If rehabilitators are raising raccoons, they should take precautions (wear rubber gloves and practice good personal hygiene) when handling raccoon feces, and dispose of the feces quickly (it takes 30 days for the eggs to embryonate) and properly (burning). Raccoons should be wormed with piperazine. Initially the animal should be wormed 3 times at 2-week intervals and then every 6 months thereafter.

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For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.