Baylisascaris Infection

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Baylisascaris Infection


Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues



Baylisascaris (Bay-liss-AS-kuh-ris) is an intestinal infection caused by the Baylisascaris procyonis larvae of roundworms that infect raccoons, their primary host. According to the Centers for Disease Control and Prevention (CDC), the infection has also been diagnosed as secondarily infecting over ninety animal species, both domesticated and wild, including birds, mice, rabbits, and humans. Initially, raccoon roundworms grow within the intestine of raccoons. Millions of microscopic-sized eggs produced by the mature worms are passed with the raccoon's feces. The raccoon is not generally affected by being infested with the worms. However, infestation inside humans can cause serious illness or death.

Generally, two to four weeks after their fecal release into the environment, the eggs are considered to be infectious to animals and humans. The group of large raccoon roundworms is a very robust species, able to survive environments such as harsh cold and hot temperatures. With enough moisture, Baylisascaris procyonis can survive for years.

Disease History, Characteristics, and Transmission

According to the CDC, the first U.S. fatal human case was reported in 1984 in a ten-month-old infant living in rural Pennsylvania.

Common characteristics of adult Baylisascaris worms include a length from 5–8 inches (13–20 centimeters) and a width of about 0.5 inch (1.3 centimeter). Colored whitish-tan, they have a cylindrically shaped body that narrows at both ends.

Human transmission occurs when infective eggs are eaten within water, soil, or on objects contaminated with raccoon feces. Upon ingestion, eggs hatch into larvae within the intestines. They travel throughout the body, often affecting the muscles and organs, especially the brain.

Symptoms generally take from two to three weeks to appear, up to a maximum of two months. Common symptoms include skin irritations, nausea, tiredness, lack of muscle control, and inability to focus. More severe symptoms include brain and eye damage, liver enlargement, loss of muscle control, blindness, and coma. Ultimately, death can occur.

Symptom severity depends on the number of eggs ingested and where the larvae spread. Estimates show that around a few thousand eggs are needed to cause infection. A few eggs cause little or no symptoms, while large numbers can cause serious problems.

Raccoons are infected in two ways. In the direct cycle, raccoons, especially young ones, ingest the Baylisascaris eggs while feeding and grooming. In the indirect cycle, eggs are eaten by intermediate hosts such as armadillos, birds, chipmunks, dogs, mice, rabbits, and squirrels. Within the host, the eggs hatch, and the larvae travel into the intestines, liver, and lungs and, later, into the head, neck, or chest. Adult raccoons then eat the intermediate host, and the Baylisascaris larvae are released and sent to the intestine to mature.

There are usually no outward symptoms visible when raccoons are infected. Symptoms can be observed, however, when intermediate hosts are infected. When in their brains, larvae can cause behavioral changes, destroy the brain, or kill the host. Early symptoms include awkwardness in walking and climbing, sight problems, and a tilting head. Later symptoms includes loss of fear of humans; activities of rolling on the ground, laying on its side, and feet paddling; and finally, a comatose state and death.

Scope and Distribution

Raccoons are commonly found throughout the United States; however the occurrence of Baylisascaris infection is most prevalent in the Midwestern, Northeastern, Middle Atlantic, and West Coast states. Specifically, according to the CDC Division of Parasitic Diseases, the states with the most Baylisascaris infections are California, Illinois, Michigan, Minnesota, New York, Oregon, and Pennsylvania.


HELMINTH: A representative of various phyla of worm-like animals.

HOST: Organism that serves as the habitat for a parasite, or possibly for a symbiont. A host may provide nutrition to the parasite or symbiont, or simply a place in which to live.

INTERMEDIATE HOST: An organism infected by a parasite while the parasite is in a developmental form, not sexually mature.

ZOONOSES: Zoonoses are diseases of microbiological origin that can be transmitted from animals to people. The causes of the diseases can be bacteria, viruses, parasites, and fungi.

Treatment and Prevention

Baylisascaris can infect humans when contacting animal feces and when hands are not properly washed. Careful decontamination procedures after contact can aid in prevention. Droppings—dark, tubular, and with a strong odor—may contain infectious larvae even after months. Larvae can survive severe heat and cold environments and harsh chemicals.

To disinfect contaminated areas, feces should be immediately buried, burned, or isolated at a landfill. Wearing gloves, a facemask, and protective clothing can prevent further contamination. Known infected surfaces should be cleaned with high temperatures, such as with boiling water, or treated with strong disinfectants. Feeding and interacting with stray raccoons should also be avoided.

It is difficult to diagnose Baylisascaris infection. Medical professionals often first eliminate other infections with similar symptoms. There are no known treatments to lessen the illness.

Currently, no definitive commercial serologic (blood) test exists to diagnose the infection. Drugs and vaccines are not available to effectively kill larvae. Laser surgery has been successful in killing larvae within the eyes. However, damage already present is likely permanent.


The Centers for Disease Control and Prevention (CDC) recommends that to reduce risks of Baylisascaris infection that people should “avoid direct contact with raccoons—especially their feces. Do not keep, feed, or adopt raccoons as pets. Raccoons are wild animals.”

The CDC further advises people to discourage raccoons from living in and around homes and parks by preventative measure that include:

  • Keeping sand boxes covered at all times (sand boxes can become wild animal latrines).
  • Keeping trash containers tightly closed.
  • Clearing brush so raccoons are not likely to make a den on your property.

SOURCE: Centers for Disease Control and Prevention (CDC)

Impacts and Issues

Baylisascaris infection is an emerging helminthic zoonosis (increasingly seen worm infection acquired from animals), and therefore, a growing public health concern. Due to time spent outdoors, it is becoming an increasing cause of severe human disease. In addition, due to increased encroachment of humans into raccoon habitat, raccoons have increasingly more contact with humans, which exacerbates the problem.

People most likely to become infected include children and persons who spend more time outdoors than other groups and are more likely to swallow infected substances. Hikers, taxidermists, veterinarians, trappers, wildlife handlers, and other similar groups who spend large amounts of time outdoors near raccoons and their habitats are also at increased risk.

According to the CDC, exposure and infection rates are in humans likely much larger than is medically reported. Rates of infected raccoons have been widely found in the United States to be up to 70% in adults and 90% in juveniles. Many veterinarians advise against keeping raccoons as pets, due to the widespread high rate of infection of the roundworm in raccoons. One adult female worm can produce hundreds of thousands of eggs daily, and an infected raccoon can daily deposit as many as 45 million eggs.

Raccoons are among the most numerous wild animals in the U.S. Their close proximity to humans makes Baylisascaris infection a potentially major infectious disease. However, to date, the prevalence of infection in the U.S. population is not known and its identity as a growing public health problem is under-recognized.

Due to several factors, including the low infective dose, numerous availability of the host (raccoons), and lack of a definitive, effective treatment in human infection, Baylisascaris procyonis is also considered a potential agent of bioterrorism.

See AlsoHandwashing; Host and Vector; Public Health and Infectious Disease; Roundworm (Ascariasis) Infection.



Samuel, William M., et al., editors. Parasitic Diseases of Wild Mammals. Ames, IA: Iowa State University Press, 2001.

Scheld, W. Michael, et al. Emerging Infections. Washington, D.C.: ASM, 2006.


Gompper, Matthew E. and Amber N. Wright. “Altered Prevalence of Raccoon Roundworm (Baylisascaris procyonis) Owing to Manipulated Contact Rates of Hosts.” Journal of Zoology. (2005),266:215–219.

Sorvillo, Frank, et al. “Baylisascaris procyonis: An Emerging Helminthic Zoonosis.” Emerging Infectious Diseases. (April 2002), 8, 4: 355–359.

Web Sites

Centers for Disease Control and Prevention. “Baylisascaris.” <> (accessed March 4, 2007).

Centers for Disease Control and Prevention. “Baylisascaris Infection.” September 23, 2004 <> (accessed March 4, 2007).