I get a surprising number of emails from people concerned about Baylisascaris procyonis, the raccoon roundworm. Some are just a bit concerned while others are almost paralyzed by fear at the thought of this parasite lurking in the environment. Most of my responses are largely re-assuring people that the risk is very limited and there are some easy things that the average person can do to avoid infection with this parasite. However, it can cause serious disease, so we can’t dismiss it entirely.

A recent paper in Emerging Infectious Diseases (Lipton et al. 2022) highlights both sides of the story: that Baylisascaris infection is a potentially nasty disease, but it’s really rare in people and largely restricted to some extreme situations.

What is Baylisascaris procyonis and what is the concern?

  • This is the raccoon roundworm, an intestinal parasite that normally lives in the gut of raccoons (the “definitive host”). Dogs can also be definitive hosts, but this is very rare.
  • Roundworm eggs are passed in feces of definitive hosts. Then they become infectious after 2-4 weeks in the environment, and if a definitive host (like another raccoon) or a typical paratenic host (like a rodent) then ingests them, the life cycle continues.
  • However, when roundworm eggs are ingested by other species (accidental hosts), the parasite can do strange (and nasty) things, because it doesn’t follow its normal life cycle. In these cases, after the eggs hatch in the gut, the larva migrate through the body and keep migrating, potentially damaging various tissues depending on where they go (a condition known as larva migrans). If they go to the brain, the damage can cause serious neurological disease. When that individual is a small animal and is eaten by a definitive host, that also perpetuates the life cycle.
  • For people, the concern is migration of the parasite larvae through sensitive tissues (e.g. brain, eye).

The Lipton et al. (2022) report describes severe neural larva migrans in a 7 year old autistic child. The fact that the child had autism and developmental delay is a key part of the story, since this child was predisposed to eating abnormal things (pica), which can lead to ingestion of things like feces encountered in the environment.

The child started exhibiting periods of abnormal neurological signs including impaired movement, lethargy and difficulty responding to commands. If you want more clinical and diagnosis details, they’re in the paper, but the child was ultimately diagnosed with eosinophilic encephalitis. Eosinophilic disease often points towards a parasitic cause, and treatment with albendazole was started. The child was reported to have played in a sandbox that had feces in it, and had been around farm animals, leading to concerns about a zoonotic infection. A blood test detected antibodies against B. procyonis, rounding out that suspicion.

After about 12 days in hospital, the child was discharged, with ongoing but improving symptoms reported a month later. This is actually a really positive outcome for neural larva migrans. Severe infections resulting in death or severe long term impairment are common. Whether the milder disease was because of a lower level of parasite ingestion, few larvae making it the brain, early treatment or some other factor isn’t known, but it was good to hear the child was recovering. Only time will tell if he fully recovers.

Upon further investigation, a raccoon latrine (a place raccoons defecate and where there can be a large amount of accumulated feces) was found at the patient’s house. The parents also recalled seeing the child put something from the ground in his mouth in the area near the latrine. Some raccoon fecal samples were subsequently collected from the area and the parasite was identified.

How common is B. procyonis in raccoons?

  • Very. The majority of raccoons in some (maybe all) regions are infected. It’s reasonable to assume that any raccoon is infected and that any raccoon feces potentially contain B. procyonis eggs.

How common are B. procyonis infections in people?

  • Really rare. The total number of reported cases in people in the dozens. However, we have to assume that there are other serious cases that don’t get reported, as well as likely a larger number of mild infections that never get identified. Still, considering it’s pretty much ubiquitous in raccoons and that we have close indirect contact through shared environments, it’s a very rare disease.

How do people get infected with B. procyonis?

  • This is a don’t eat poop” disease. Actually, it’s a “don’t eat raccoon poop that’s been sitting outside for more than 2 weeks” disease. Exposure to fresh raccoon feces poses no risk since it takes time for the parasite to become infective after it’s been passed in feces.

Does it take a lot of exposure to get infected with B. procyonis?

  • Presumably yes, it takes a lot of exposure to develop a clinical infection that causes signs of disease. Since there are lots of people who work a lot with raccoons (e.g. raccoon rehabbers) and in areas where raccoon defecate, but disease in people is really rare, that speaks to the limited risk, even with exposure to highly contaminated areas. It’s quite likely that there’s a much larger number of people that get exposed, mount an immune response and never get sick. A study from a few years ago reported antibodies against B. procyonis in about 7% of people tested in Santa Barbara, California. However, those were healthy people, so even if they were infected, it’s not a big deal if it doesn’t cause disease.
  • The main risk of disease, as seen the case reported above, is likely when a large volume of raccoon poop is ingested. That’s typically a concern in people with developmental disorders that lead them to eat atypical things. Avoiding that type of exposure is the key.

What can the average person do to avoid B. procyonis?

What about someone living with a person prone to pica?

  • It takes diligence to be on the lookout for lots of things that can pose problems for someone with pica. Caretakers need to be very diligent about looking for feces in the environment, picking up feces so that eggs cannot become infective, discouraging raccoons from setting up residence in structures (e.g. sheds, attic), looking for raccoon latrines and (if identified) both cleaning them up properly and discouraging raccoons from coming back.

What if someone is known to have ingested raccoon feces?

  • I get called about these situations periodically, and it leads to a decision by the physician whether preventative treatment is indicated. I’ve known parents to have pulled feces out of kids’ mouths (to reduce exposure and have something to test). Ultimately, it comes down to whether the physician thinks the exposure risk was high, and if so, treatment is often provided. More information about B. procyonis for health professionals is available on the CDC website.

So, the messaging about this parasite in people is often a challenge:

  • It’s a parasite that can cause fatal or debilitating disease, but it rarely does so.
  • Eggs can be present in many environments, but the risk is probably mainly in high risk sites like raccoon latrines.
  • We don’t know the infective dose, but for disease to occur in people it’s probably quite high.
  • Any pile of raccoon feces can be contaminated, but fresh (less than 2-4 week) feces pose no risk.
  • Common sense practices to keep raccoons away and keep feces out of peoples’ mouths are the key. It’s nothing fancy but some basic hygiene can go a long way.
  • Awareness is good, but this isn’t a disease to keep you up at night worrying.

Image from https://www.cdc.gov/parasites/baylisascaris/biology.html