Baylisascaris procyonis is the common intestinal roundworm of raccoons. It is found wherever raccoons live and in some localities it infects nearly 100% of raccoons. It is important because of an increasing number of devastating infections in humans, mostly in young children.
Raccoon Roundworm Life Cycle
Baylisascaris has a life cycle similar to Ascaris lumbricoides, the large intestinal roundworm of humans, but there are differences:
- The adult male and female worms live in the raccoons' large intestine. They mate and females produce millions of eggs a day, which are passed in feces.
- The eggs mature and are infective in two to four weeks, longer in cold temperatures.
- Animals, including humans, accidentally swallow the eggs.
- In raccoons, swallowed eggs hatch in the intestine; freed larvae mature in the intestinal tissues and return to lie free in the intestine, restarting the cycle.
- In other hosts, larvae migrate through the tissues—a condition known as visceral larva migrans—often invading eyes and the brain, where they do serious damage.
- Sick animals often fall prey to predators. When they are eaten by hunting raccoons, the larvae ingested with the tissues mature to adults in the raccoon’s intestine, starting the cycle over again.
It’s tempting to view visceral larva migrans as an accidental parasitic infection—a parasite that simply ‘gets lost’ in the wrong host—but the wandering larvae and the resulting illness actually make it more likely that the parasite will get back to its natural host, the raccoon. This is an example of a subtle type of control that many parasites exercise over their hosts.
In human infection, it’s highly unlikely that the illness will result in the victim being eaten by raccoons, so it’s a dead end for the parasite. Unfortunately, the infection is often fatal for the human as well, and those who recover commonly have lasting brain damage.
Environmental Factors Favoring Raccoon Roundworm
Factors that favor the transmission of Baylisascaris include a warm moist climate, raccoon toilet habits, and high raccoon populations:
- Warm moist soil provides ideal conditions for the development of roundworm eggs to the infective stage. Even where temperatures are cold, eggs remain viable for years in soil, and once an area is contaminated, clean-up is difficult.
- Raccoons habitually create latrines where every passing raccoon defecates. This creates places where eggs are concentrated, making it more likely that subsequent raccoons and other animals will come in contact with the parasite. Eggs stick to paws, hands, and fur, and are carried away, possibly to be licked off later.
- Raccoons are not highly territorial; wherever populations are high, they are more likely to come in contact with each other, and with each other’s feces, and pass their parasites around. Unfortunately, the densest raccoon populations are found in urban and suburban human communities where predators are scarce, food is plentiful, and raccoons are known to hunt and forage in groups. This probably accounts for increasing human cases.
Many people don’t realize that having raccoons living nearby can be a risk to health. Increased public awareness of Baylisascaris procyonis will prevent infections and save lives.
Sources:
Diagnostic Medical Parasitology 3rd ed. Garcia, Lynn S. and David A. Bruckner. Washington: ASM Press, 1997.
Foundations of Parasitology 6th ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
“Raccoon.” Hinterland Who’s Who.
“Raccoon Roundworm Infection (Baylisascaris): a Zoonosis of Pediatric and Public Health Concern.” Murray, William J. In: Scheld, W. Michael, Barbara E. Murray, and James M. Hughes, eds. Emerging Infections 6. Washington D.C.: ASM Press, 2000.
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