Making Animals Sick

In 2007, the Editor of TableAus began a thread where readers asked questions or challenges for my philosophy’s response.

Reader’s Question:

“There are many examples of diseases that infected animals can pass onto humans, e.g. bird flu and Foot and Mouth. Also diseases where the animal is a vector for a disease, e.g. mosquitoes which passes on the disease to Humans. What about diseases/infections that animals and plants get from sick Humans? (Let’s ignore examples where humans simply spread infections between animals and between plants).”

First we need to distinguish between different kinds of diseases. Some disease organisms are specifically evolved to have multiple hosts, such as parasitic worms and malaria. These by their nature regularly pass between host species, humans included – but that is what they have evolved to do.

Other disease organisms are usually harmless, either free living or commensal, but will cause disease if given a good enough chance, e.g. via access through a wound or a depressed immune system. As these are opportunistic rather than fundamental pathogens, they generally can infect many different organisms, and therefore are good candidates for passing not only from animals to humans but back again. Indeed, cases of pets catching such opportunistic infections from their owners have been reported.

Still others simply have a broad host range. For example, ringworm can spread between animals and people in either direction, and one of the most feared animal diseases, rabies, infects mammals in general.

Finally, we have the highly specialised pathogens, restricted to one or a narrow range of species. These have generally coevolved with their host for a long time and the price they pay for their specific relationship with their normal host is being ill-equipped to infect other species. This, however, appears to be the chief source of “emerging diseases”, those new diseases (such as Ebola) that periodically strike at us from the animal world. Bird flu is a similar case, though the peculiar multiple chromosomes of the flu virus give them a bit of a genetic recombining edge for jumping between hosts.

In all cases except where a pathogen has specifically evolved to exploit multiple hosts, the chance of jumping between species is higher the more closely related they are. This is for the simple reason that pathogens, like all other forms of life, have specific requirements for survival and specific adaptations to meet them. The more distant two species are, the bigger the genetic change is likely to be required in order for a pathogen of one to successfully find a home in the other, and therefore the less likely it is to happen.
From the news, one might get the impression that this is a one-way street, but of course there is no biological magic about humans.

The biological fact is that pathogens can move from species to species, either as part of the pathogen’s normal adaptations, or as a rare mutation, so there is no reason why animals can’t catch diseases from us just as we can from them. But it is likely to be rarer and less reported. FIrstly, because we are so adaptable and numerous, we are more likely to be put into the receiving position. You see more farmers surrounded by flocks of ducks, than ducks surrounded by flocks of farmers, and it is more likely for a human to be bitten by a bat or a rabid dog than vice versa. And if a man gets infected by a new disease from a squirrel that gives him a chronic or violent infection, odds are the medical fraternity will take a close look and the papers will enjoy telling us about his sad case. But if a squirrel catches something from a man, odds are it would simply die in the forest or be eaten by a fox and nobody would be the wiser.

© 2008 Robin Craig: first published in TableAus.