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Researchers working to unmask parasitic disease in raccoons

Raccoons have been the focus of a lot of attention of late, between their starring role in Planet Earth 2 and Toronto’s constant battle to keep them out of the garbage. Unfortunately, they are also in the news for less amusing reasons.

Photo of raccoon in treeThat’s because raccoons can spread diseases, such as rabies, to other animals and even to humans. Although the risk of rabies is well known, raccoons can be the source of other lesser known zoonoses (diseases spread from animals to humans) including a parasitic roundworm called Baylisascaris procyonis that can cause serious disease in humans and other animals.  

University of Guelph PhD student Shannon French is trying to better understand the parasite and factors that affect the spread of infection in raccoons.

According to a current estimate, 40 per cent of Ontario raccoons are infected with the parasite. It’s even more common elsewhere in North America; in Illinois, the infection rate is estimated to be as high as 86 per cent.

Raccoons, which are the host for the adult stage of the parasite, rarely show any symptoms of infection.  However, the larval stage of the raccoon roundworm, which can infect more than 150 species of birds and mammals, may cause disease and even death in these hosts. For example, just one larval worm is often lethal to small animals such as the Allegheny Woodrat, which has become extinct in some regions in-part because of this parasite.

Disease associated with the larval stage of the raccoon roundworm are caused by larval migration in the host. When the larvae travel between organs within an individual – human or animal – they cause inflammation and can damage the organs that they travel through, as they search for the right environment to reproduce. That can’t be found anywhere except in the intestines of raccoons, the definitive host of the raccoon roundworm. Still, they look for a spot to settle. For example, if the larvae are able to make their way to the retina, they can cause blindness.

“When they’re in the wrong animal, they get lost,” says French, who is completing her research in the Ontario Veterinary College’s Pathobiology Department.

Early diagnosis is crucial. When an animal has become infected with raccoon roundworm, it can take only a few of the parasites inside the animal’s body travelling into the brain to affect the animal neurologically. If the parasite goes undiagnosed in an individual, the neurological damage can become permanent.

Under ideal conditions (the right temperature and a moist environment), the parasite becomes infectious after about two weeks’ time. But it’s highly resilient, and can survive outdoors through harsh conditions for months. Raccoon roundworm is easily spread to other animals through ingestion of eggs from affected raccoon feces, found in large quantities at raccoon latrines, a spot designated by individual raccoons for urination and defecation.

In urban environments, humans and their pets are at risk of becoming infected if raccoons make a latrine near their homes, in places such as backyards and attics. Small children are especially at risk because of their penchant for putting dirt and foreign objects into their mouths – objects that might have been in contact with raccoon feces harbouring the eggs of raccoon roundworm.

Despite this, human cases are uncommon, with only approximately 50 known cases. A possible explanation for these low numbers is that cases are never diagnosed and therefore go unreported.

Raccoons have been introduced to Europe and Japan successfully, establishing wild populations. In Europe, the parasite is present in parts of the range of raccoons, and baylisascariasis (disease caused by the parasite) has been reported in two people. Curiously, in Japan the presence of raccoon roundworm has not been identified in any wild animals.

Researchers perform autopsies on raccoons that are submitted to the Canadian Wildlife Health Cooperative. They then determine the cause of death. If the parasite is present, they record the number of worms that are found in their intestines. By doing so, researchers can gather data on the incidence, as well as individual and environmental factors that could potentially affect the spread of raccoon roundworm.

The presence of zoonotic diseases in wildlife, like raccoons, emphasizes the importance of admiring these animals from a distance, and not encouraging them to visit your home. Through the understanding of diseases that predominantly affect wildlife, researchers aim to be able to take preventative measures that will benefit everyone by protecting other wildlife, ecosystems, pets, and humans.

French hopes to take her research further by exploring the roles of paratenic hosts (organisms that can carry parasites but do not provide a suitable environment for reproduction unless the paratenic host is consumed by a more appropriate host) and how they fit into the life cycle of parasites.

Dr. French’s graduate studies and research were funded by the OVC (Ontario Veterinary College) Fellowship program and the Canadian Wildlife Health Cooperative, respectively. Collaborators of the project include U of G Profs. Claire Jardine, David Pearl, Andrew Peregrine, and Dr. Doug Campbell, a wildlife expert from the Canadian Wildlife Health Cooperative.

Article by SPARK writer Karen Tran