Diseases We Share With Animals

What is a zoonosis? My dictionary says a zoonosis is “any infection or infestation that can be transmitted to humans from lower vertebrates under natural conditions” (Gage Canadian Dictionary). The MedlinePlus Medical Dictionary agrees: “a disease communicable from animals to humans under natural conditions."

This definition has always seemed so vague to me as to be virtually useless. Though there are some pathogens that only infect humans and can only be transmitted from human to human, they must be in the distinct minority. I’ve tended to think of a zoonosis as a disease of animals that can incidentally be transmitted to humans, thus excluding diseases that are common in humans. That others have made this distinction also is illustrated by statements such as this: “it is not a true zoonosis… it is endemic in humans, rather than periodically penetrating human populations from an animal reservoir…” (Mark Wheelis, Principles of Modern Microbiology, 2008).

Consider the beef tapeworm, Taenia saginata. The adult, sexually reproducing stage of this parasite – the tapeworm we are familiar with – lives in the intestines of humans, but we acquire it by eating the larval form in rare beef. We cannot get a beef tapeworm directly from another infected person, but is this a disease of animals? Since we have the adult worm, isn’t this more a parasite of humans passed to animals? (This would be an anthroponosis.)

[caption id="attachment_268" align="alignleft" width="300" caption="Cows get Taenia saginata from people"][/caption]

Giardia lamblia, agent of ‘beaver fever’ is another example. Sure, beavers carry it and pass it to people, but who had Giardia first, people or beavers?

If we go with Wheelis’ implied definition - a disease not endemic in humans, but which is transmitted to humans from animals periodically - we are left with many fewer pathogens. This would include parasites like Baylisascaris procyonis, an intestinal roundworm of raccoons that can be fatal in humans. These are things we only come in contact with rarely, by virtue of our lifestyles and cultural separation from nature. Presumably if we still lived the hunter-gatherer life, and ate more of our food raw, we’d be naturally exposed to these things sporadically just as other animals are.

But wait. Doesn’t that create a contradiction? A zoonosis is supposed to be passed from animals to humans under natural conditions; but, doesn’t that mean a zoonosis is only a zoonosis because we live our lives in unnatural conditions?

Are Raccoons Cute? The Trouble With Raccoons

I have stood on my front deck on a late summer evening and watched a raccoon cross the street not 50 feet from my front door. I’ve seen young raccoons with their butts sticking out of my bird feeder, and I’ve seen their indented trails in the snow, where they venture out of their dens on warmer winter nights.

[caption id="attachment_263" align="alignleft" width="300" caption="Urban raccoon, Christopher Michaud, Creative Commons 3.0"][/caption]

Though no one who’s ever heard raccoons brawling at night would mistake them for cuddly friends, there’s something charming about their striped faces, their round furry physique, their dexterous paws.  I don’t let it fool me. These wild animals are becoming common in urban and suburban environments because we are feeding them. Where raccoons are living and eating, they are also leaving their droppings – in latrines. And where there are raccoon latrines, there will likely be Baylisascaris eggs (intestinal roundworm), and these can be deadly to people.

Raccoons, like people, don’t tend to spread their droppings at random all over the neighborhood: they establish latrines on horizontal surfaces such as fences tops, wood piles, roofs, branches. They return to the same place again and again, creating areas that are heavily contaminated with their feces, and which may contain millions of Baylisascaris eggs. Swallow those eggs by accident or chance, and you could be in serious trouble.

The eggs hatch after being swallowed, releasing larvae that migrate through the tissues and typically invade the head and brain, where they can do terrible damage. Children, and the mentally challenged are at highest risk because these people are more likely to put contaminated fingers in their mouths.

Raccoons are cute, but they should be cute in the wild, not in human communities. Don’t encourage raccoons – don’t feed them, keep them out of buildings, block access under decks, clean up latrines and remove contaminated soil or wood. Always consult a knowledgeable source about how to do this safely and effectively.