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Baylisascariasis! The Tragic Parasitic Implications of Raccoons In Your Backyard

Body Horrors
By Rebecca Kreston
Mar 29, 2012 6:46 PMMay 21, 2019 6:03 PM


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This article was published as a guest post on the blog of the Parents of Kids with Infectious Diseases (PKIDs) nonprofit on April 2, 2012. It can be visited here in an edited, shorter form. You can find out more about this great organization and their public health mission here.

The re-wilding or “greening” of urban and suburban spaces has been an indefatigable, faddy trend in urban planning for the past two decades. Urbanites like accessible parks and community gardens and food forests and stately trees and along with our car-filled cities. Hell, we name our streets after trees – spruce, elm, oak, pine and so on. These are the things we do to justify our shoddy recycling habits and not giving due care to our carbon footprint. Sustainability is the new mantra, screen-printed on our reusable grocery totes. So it can be troubling when we see the repercussions when we bring nature into the neighborhood and blur the line between urban comforts and rural charms. One of those manifestations can be rodents, coyotes, foxes, opossums, and raccoons joining the ‘hood.

And it works! Coyotes frequently visit the outskirts of cities. Bears drop by homes abutting forests in Colorado state. Rodents raid community gardens. Raccoons hang out in Central Park in NYC

omnivore raccoon

The omnivore raccoon (Procyon lotor). Image: John Biehler. Click for source.

Let’s talk raccoons. The bandit-style masking covering their faces, their insatiable curiosity, and nimble human-like hands has popularized them as mischievous varmints. Though their nocturnal habits tend to keep them out of the sights of most Americans, they can be unseemly guests with their destructive tendencies. All that these small mammals need is a permanent water source, a safely enclosed setting for their den and access to food. Luckily they can find all these things and more when living in suburbia and cities. Human dwellings and activities serve as both a den reservoir and food resource; we are nothing but extraordinarily generous, unwitting hosts.

They can eviscerate lawns in their search for earthworms and grubs, steal your invaluable rubbish from trash bins, suck dry bird feeders, and shred the ventilation ducts, sheetrock and insulation in attics. So cute! Aside from ruining the hard work you put into creating the perfect English garden or shingling your roof or whatever household chore has been on your to-do list for years, they also are host to diseases that really will, absolutely, positively kill you. Like rabies. And baylisascariasis.

Baylisascariasis describes the human infection with the raccoon roundworm Baylisascaris procyonis. The parasite is endemic in raccoons, with infection rates ranging from 72% to 100% (1)(2). If you live in a place with sidewalks, there are most certainly infected raccoons living in close proximity to you; B. procyonis is the pathogen hiding in your backyard (3). One of the larger parasitic worms out there, this big guy inhabits the intestinal tract of raccoons and produces thousands of eggs that are shed in the feces.

Like humans, raccoons can be fastidious about their pooping habits. They make communal latrines that can be found on natural or artificially flat surfaces – at the bases of trees and in branch crotches, on woodpiles, along and on the top of fences, on roofs, attics, and sandboxes (2). This results in a condensing and localizing of the eggs in one specific spot, attracting foraging birds and small rodents to undigested seeds and the like, and successfully ensuring the continuation of the parasite’s life cycle as it infects and kills the intermediate host. The raccoon scavenges the dead, becomes infected with the larva and the parasite keeps marching on into life cycle infinity

The life cycle of the raccoon roundworm Baylisascaris procyonis. Image: DPDx CDC. Click for source.

Raccoons sharing your living space, whether in the attic above you or in a nearby tree or under the porch, means that there’s a raccoon latrine near you; thankfully, they’ve kindly kept all their crap in one place unlike some roommates I’ve had the pleasure of co-habitating with. A 2009 survey checking out an area of suburban Chicago close to a marsh and forest preserve found that 51% of lawns had a raccoon latrine (4). This kind of work suggests that there’s a good chance of fecal contamination in many of our backyards, spaces that typically serve as children’s play areas. The uniqueness of the latrine itself – piles of feces with undigested seeds, berries and bones – can also attract curious toddlers. Alternatively, if raccoons occupy chimneys, infective feces can settle within and around fireplaces, contaminating the home.

How is that this random raccoon parasite can make it onto a child’s fingertips, aside from the obvious hand-to-mouth behavioral sequence most parents are familiar with? There are three major reasons. One is that the raccoon can defecate prodigious amounts of B. procyonis eggs, on the order of a million a day. The other is that the parasite’s egg is unbelievably hardy and damn near resistant to nearly all of our arsenals against eradicating grime, schmutz and filth. The egg has four shell layers and is resistant to high temperatures, strong acids and bases, oxidants and reductants, and protein-disrupting agents (2). Guys, that’s pretty much all we got in terms of dealing with dirty surfaces and buggies, and it’s resistant to all of them except for applying direct flames to the egg. And how often do you find yourself planning to incinerate your backyard, aside from Fourth of July celebrations? Lastly, the eggs are remarkably sticky, gluing themselves to available surfaces which may include toys littered in the backyard

The large female (left) and male (right) adults of the Baylisascaris procyonis roundworm compared to a penny. Source: Roussere et al. See References for details.

Once hatched from the egg, the larva has a cruel propensity for the cranial region of their intermediate hosts and, as such, it’s carousing in the head and thoracic region has grim neurological implications. In fact, the parasite is the most common cause of larva migrans – larval worm migration in bodily tissues – in animals and can produce severe neurological larva migrans (NLM) in over 100 species of birds and animals (1). This can be a nasty worm and has heartily evolved and adapted to get into the raccoon anyway that it can.

That these alarmingly large adult worms produce sizable larva bodes also poorly for us humans. The immensity of these worms can cause significant tissue trauma, especially in young children with their small bodies. The most common diagnosis from infection is eosinophilic meningoencephalitis, in which the brain and spinal cord and the meninges membranes surrounding the two becomes enormously inflamed due to the larva activating a type of white blood cell known as an eosinophil.

Clinical symptoms can develop two to four weeks after initial infection (5). Usually children have been lethargic, feverish and a little “off” for a few weeks before being rushed to the emergency room with seizures, unsteady gait or abnormally regressive behavior. There is no commercial test to confirm antibodies to the infection, making it difficult to have an accurate diagnosis (6). Meningoencephalitis is an affliction that can be tough to track down the originating cause; there are numerous other larval worms to cross of the list before making a diagnosis (including Toxocara canisAscaris lumbricoides, as well as species of AngiostrongylusAncylostomaTaenia and Echinococcus). The infection is not well known to many clinicians, outside of the infectious disease field, which can complicate proper treatment of the symptoms (6). In the emergency room, the most pressing need is to control the damaging inflammation, not figure out which worm is the culprit.

Children are more likely to suffer devastating effects of this reaction than that of adults due to their relatively smaller brains (2). Infection outcomes have been statistically grim – many of these young patients either suffer permanent neurological and ocular defects or death. A microbiology paper from 2005 puts it bluntly, “To date, all survivors have been left in a persistent vegetative state or with severe residual deficits”(1).

And the victims are mostly very young children, typically boys. Infants and toddlers are predisposed to learn about their world by oral contact and commonly engage in pica and geophagia (eating of dirt). These types of behaviors can also be demonstrated by older children with developmental disabilities, another important subset of patients that have become infected with the parasite (3). Overall, important risk factors for baylisascariasis infection include exposure to raccoon feces, pica or geophagia, age under 4 years, male gender, and intellectual development delay.

Thankfully, the number of cases is low but it’s hard to discern whether that’s attributable to an actually low incidence of disease or a lack of reporting. Many parents decline autopsies, the only true way to definitively identify a case of baylisascarisasis. I tracked down 22 cases that have been reported in the literature (1)(3)(6)(7)(8)(9). Subclinical infection may also occur – a Chicago study found that 8% of children showed antibodies to the parasite though none had ever shown symptoms of disease (10); baylisascariasis may be much more prevalent than we think.

A foreclosed home in Detroit taken over by nature. Photo is part of a series of "feral homes" in the city taken by photographer JD Griffioen. These properties compromise our ability to separate our urban landscapes from nature and invite wildlife into our neighborhoods. Image: James D. Griffioen.

A foreclosed home in Detroit taken over by nature. Photo is part of a series of “feral homes” in the city taken by photographer JD Griffioen. These properties compromise our ability to separate our urban landscapes from nature and invite wildlife into our neighborhoods. Image: James D. Griffioen.

Importantly, the numbers of reportable cases has been steadily rising in the past decade for unknown reasons, though I suspect that the greening of our surroundings and the increase in diagnosed autism cases, a medical situation that can predispose children to pica habits, may have something to do with it.  Most recently, adult B. procyonis worms have been detected in pet dogs (2). This worrisome finding suggests that cases could also rise further. There are few wild animals that live so freely along humans that are cable of transmitting such a ferociously nasty and fatal disease. Parents and communities should be made aware of the dangers of having such animals close by and should go to efforts to locate and remove any raccoon latrines within or near the home.

Bringing nature to the neighborhood isn’t always a deliberate process, like revitalizing our neighborhoods by putting in a park or planting some trees. Industrial decay, environmental catastrophes and housing foreclosures can transform our urban landscape into a more inviting setting for nature to recolonize our space. Bobcats have been found living in foreclosed homes, neighborhoods in New Orleans are literally turning into marshland, and feral animals prowl through Detroit. One of the benefits of living in a city, away from nature, is that the only animals we have to deal with are ourselves.


I stumbled upon one of the most fascinating fact-sheets I’ve ever encountered. It’s on raccoons. A quote: “They are constantly feeling around with their paws. Every glittering stone, every unusual object, every smelly tidbit catches their eye or nose and they will stop to inspect these objects with intense scrutiny.”

Just how does one clean a raccoon latrine?

squirrel infected with the parasite exhibiting ataxia, loss of bodily movement control, a symptom of neurological larva migrans.

Curious about urban greening, re-wilding and planning initiatives bringing nature into the ‘hood? HereHereHereHereHere.


(1) Gavin PJ, Kazacos KR, and Shulman ST. (2005) Baylisascariasis. Clin Micro Rev. 18(4): 703-18 (2) Gilbert CE (Date unknown) Concern with Communicable (Infectious) Diseases of Raccoons. Epidemiology and Toxicology Institute, LLC.  Accessed online March 27, 2012. (3) Hajek J et al. (2009) A child with raccoon roundworm meningoencephalitis: A pathogen emerging in your own backyard? Can J Infect Dis Med Microbiol20(4): e177 – e180 (4) Page K et al (2009) Letter: Backyard Raccoon Latrines and Risk for Baylisascaris procyonis Transmission to Humans.  Emerg Infect Dis. 15(9):1530-1 (5) Park SY et al (2000) Raccoon Roundworm (Baylisascaris procyonis) Encephalitis: Case Report and Field Investigation Pediatrics106(4): E56 (6) Frank Sorvillo et al. (2002) Baylisascaris procyonis: An Emerging Helminthic Zoonosis. Emerg Infect Dis. 8(4): 355-9. (7) Moertel CL et al. (2001) Eosinophil-Associated Inflammation and Elaboration of Eosinophil-Derived Proteins in 2 Children With Raccoon Roundworm (Baylisascaris procyonis) Encephalitis. Pediatrics. 108(5):E93. (8) Haider S et al. (2012) Possible pet-associated baylisascariasis in child, Canada [letter]. Emerg Infect Dis18(2): 347-9. Accessible online. (9) Kelly TG et al. (2012) Spinal cord involvement in a child with raccoon roundworm (Baylisascaris procyonis) meningoencephalitis. Pediatr Radiol42(3):369-73. Epub 2011 Jun 1. (10) Pai PJ et al. (2007) Full Recovery from Baylisascaris procyonisEosinophilic Meningitis. Emerg Infect Dis13(6): 928-30

(Images 2 and 4) Roussere et al (2003) Raccoon Roundworm Eggs near Homes and Risk for Larva Migrans Disease, California Communities. Emerg Infect Dis. 9(12): 1516-22. Accessible here.

Roussere GP, Murray WJ, Raudenbush CB, Kutilek MJ, Levee DJ, & Kazacos KR (2003). Raccoon roundworm eggs near homes and risk for larva migrans disease, California communities. Emerging infectious diseases, 9 (12), 1516-22 PMID: 14720389

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