Raccoon roundworm: Not just a raccoon parasite

by Sara Weinstein, graduate student , UC Santa Barbara Ecology, Evolution, and Marine Biology

microscope image of the raccoon roundworm
The raccoon roundworm (Baylisascaris procyonis). Image credit: Sara Weinstein

Raccoons host a variety of parasites and one parasite of particular concern is the raccoon roundworm. This large nematode can cause severe disease in humans and other wildlife.

Raccoon roundworm is linked to rodent declines and has the potential to significantly impact ecosystems, yet relatively little is known about its ecology. We are studying this parasite in the mammal community at Coal Oil Point Reserve, focusing on the adult worm population in raccoons, juvenile worms in rodents, and infectious eggs in the environment.

The raccoon roundworm infects 80% of raccoons at the reserve. Worm loads are higher in younger individuals and heavily infected animals release more parasite eggs in their feces. Raccoons use communal defecation sites called latrines. Other animals, such as rodents, are exposed to infectious parasite eggs when they forage in raccoon feces.

diagram of raccoon roundworm life cycle
The life cycle of the raccoon roundworm. The parasite typically cycles between raccoon and rodent hosts but occasionally infects people. Image credit: Sara Weinstein

We are mapping the distribution and density of raccoon latrines at the reserve. By using motion activated wildlife cameras placed at the latrines, we hope to understand how other animals interact with these contaminated sites and determine their risk of exposure.

Mice are frequent visitors of latrines and we have found larval parasites in three of the most abundant rodents in the reserve: the harvest mouse, the deer mouse, and the black rat. The black rat, an invasive species, is heavily infected and some individuals carry over 500 larval worms. The native harvest mice are rarely infected while 45% of the deer mice are infected.

In over 50% of infected deer mice, larvae have migrated to the brain. A parasite in the brain can impair the daily routine of the host, making it an easy target for hunting raccoons and increasing the potential to amplify disease transmission.

Humans are also susceptible to raccoon roundworm infection. Although diagnosed cases are rare, Southern California has a large, heavily infected raccoon population and our exposure risk is high.

Human infection rate is currently unknown, but we have just launched the first ever population survey for human raccoon roundworm infection and are now recruiting volunteers from Santa Barbara County.