Though rare, human infections with Baylisascaris procyonis can be devastating. Usually seen in small children, the infection can leave sufferers with profound brain damage.
There are few reports of the disease in the medical literature. In fact, the two newly described Canadian cases are believed to be only the second and third reported from this country.
But people who study the pathogen are worried that the surging population of raccoons in many North American cities could lead to a rise in the number of these cases.
"One of the things we have talked about is because we have an increase in the raccoon population, that there is a potential that we are going to see an increase in Baylisascaris cases," said Dr. Shira Shafir, an assistant professor of epidemiology at UCLA School of Public Health in Los Angeles.
"And so we have been trying to call attention to the fact that (health-care) providers, particularly pediatricians, should be aware. Because the earlier it can be diagnosed and the earlier appropriate treatment can be given, we know we can minimize the severity."
Infection occurs when the parasites eggs — found in raccoon feces — are ingested.
The eggs hatch in the gut; the larvae then make their way via the bloodstream to the brain. Their movement within the brain — and the inflammation their presence there triggers — is highly destructive to brain tissue.
Deworming drugs exist and are effective if delivered very soon after ingestion. But because infections typically occur in very young children and cognitively impaired older children or adults, often parents or caregivers are unaware the exposure has taken place until the infection is too far along to prevent brain damage.
Reported cases are typically seen in children who are still in the phase where they put everything in their mouths. A sandbox could be contaminated with raccoon feces as could be clumps of dirt. Infections are also occasionally seen in adults with cognitive impairment or who suffer from pica, a condition where a person has a compulsion to eat dirt or other non-food items.
The two Canadian cases being reported in the February issue of the journal Emerging Infectious Diseases are not recent. One occurred in 2008 in Ontario and the other in 2007 or 2008 in British Columbia.
The Ontario case was a previously healthy 14-month old boy from Hamilton. Nine months after his initial hospitalization, he was reported to have profound neurological impairment, was legally blind in both eyes and suffered from epilepsy.
The doctors who treated and investigated the case, from McMaster University, reported it because no source of the infection was determined. They concluded the toddler may have been exposed to roundworm eggs brought into his home by the family dog, either because the dog became infected and later excreted the parasite's eggs or because it carried some of the microscopic-sized eggs on its coat.
His parents had seen raccoons in their backyard, but the child had no access to the yard. Raccoon feces on the property tested positive for the roundworm eggs. The authors can only speculate the dog might have played a role in bringing the eggs into the house; there was no proof.
The British Columbia case was discovered when doctors in Vancouver performed a autopsy of the brain of a woman who had died at age 73. She had Alzheimer's disease, which was why the brain autopsy was performed.
Neuropathologist Dr. Ian Mackenzie of Vancouver General Hospital said when the woman's brain tissue was examined, several Baylisascaris larvae were discovered.
"This is something that, if you could take it out of the tissue and lay it down on a piece of paper your eye could see it," he said.
The woman had lived in the B.C. interior; Mackenzie would not specify where. It was not known how she contract the parasite.
The report suggests she showed no signs of infection, though her case did not appear to be advanced. There is a possibility the Alzheimer's disease masked symptoms, though Mackenzie said it's also conceivable the roundworms might do less damage in an adult brain than in the still-forming brain of a young child.
Mackenzie said he and his co-authors reported the case because they wanted to draw attention to the fact some infections may be flying under the radar.
"That's the point of this case report, is that it suggests that there may be greater exposure in a broader cross section of the human population that is simply not recognized, because it doesn't lead to significant neurological disease or it's not looked for or it's missed," he said.
That underscores a problem with this pathogen; no one is really clear if the few cases seen represent all the infections or whether some people become infected but don't develop the severe infections that bring cases to the attention of the medical community.
Shafir noted the infection is not a reportable disease so even if doctors see a case, it may not become more broadly known.
"If we don't know how common the disease is, that kind of dictates how much energy, time and money (is spent studying it)," said Scott Weese, a microbiologist from the Ontario Veterinary College in Guelph. Weese specializes in the germs that pass between animals and people.
He suggested the best protection is the most obvious — avoid raccoon feces.
On the plus side, the animals are creatures of habit and tend to defecate in the same place repeatedly. But raccoons that are infected — and many are — can shed millions of eggs in their feces daily.
And those eggs are pretty near impossible to kill, said Shafir, who published a journal article last summer outlining how resilient Baylisascaris eggs actually are.
The eggs can withstand repeated freezing and thawing. They can withstand desiccation (drying out) for up to six months and still remain viable. Bleach doesn't kill them. It takes temperatures over 62 Celsius to destroy them.
"One of the things that we recommend if they are contaminating non-porous surfaces (tiles, rocks) is the use of an acetylene torch," Shafir said.