Rats and mice, of course, have long been associated with disease. Plague-infected fleas on rats spread Yersinia pestis, the bacterium responsible for the Black Death. We know today that other species can also transmit this pathogen—including much cuter prairie dogs in the southwestern United States. Mice have recently been implicated in an outbreak of hantavirus pulmonary syndrome in Yosemite National Park that has killed three and sickened at least 10 campers. Mice carry this virus without showing symptoms and spread it to humans via urine and feces.
Livestock and Pets
The domestication of livestock and the taming of animals for pets certainly marked a turning point in human history. Having these animals on hand to provide food and milk, as well as companionship and assistance with hunting, gave humans a more ready food supply and meant less time had to be spent gathering food. However, it also put us in regular contact with germs that these animals carried. Human measles virus infections may have evolved from a similar cattle virus, rinderpest. Cattle can also be a source of tuberculosis in humans, even today. Industrial livestock production means that it’s not just a farm family that may be sickened by pathogens from a pig or cow, but potentially hundreds or thousands who consume meat or other products from those animals. Foodborne illnesses are estimated to sicken 76 million people yearly in the United States and kill approximately 5,000. Economic costs from these food-borne illnesses alone are estimated at approximately $77 billion per year
Finally, our smaller domesticated friends can expose us to their own pathogens, including the parasite Toxoplasma gondii in cats (which is particularly dangerous to pregnant women), and they can also bring along unintended visitors and their pathogens into the home in the form of fleas and ticks. Even “pocket pets” such as hamsters and guinea pigs can bring along potentially deadly viruses and infect their owners.
So, Why Aren’t We All Dead?
With so many potentially deadly organisms lurking in the animal species we share the Earth with, the question becomes not if we’ll have another novel pandemic, but when. However, these events—the new influenzas, the SARS coronaviruses, the HIV outbreaks—are actually relatively rare. “Spillover” events—an individual becoming infected with a zoonotic pathogen—are common, but typically the infected host is a dead end. He or she doesn’t spread the germ to a second person, which is a necessary factor for an epidemic (which is a localized outbreak) or a pandemic (a worldwide infection) to occur. Going back to H5N1 influenza versus H1N1, that’s why the former has caused only sporadic outbreaks and the latter has become pandemic. H1N1 is readily transmissible between people, and H5N1 (so far) is not. This is also why there was so much concern earlier this year when a genetically-modified H5N1 was created in a laboratory setting. This modified virus was able to spread readily between ferrets, a common animal model for human influenza research. The work caused worry that such a virus may escape from a lab and spread in the wild—The Stand come to life.
This controversy also highlights the difficulty in studying potential zoonotic pathogens. Many of these organisms have adapted to their hosts and do not always cause symptoms in their “natural” species. As such, it’s difficult to anticipate which microbes will 1) make the species jump successfully; 2) cause illness in the new host species (for example, in humans); and 3) transmit efficiently among members of the new host species. Prediction right now is very foggy, though we’re beginning to better understand the diversity of organisms out there, and with that, hopefully gain understanding into why some spill over and others do not.
One final note—while we often consider humans the victims of such pandemic events, that’s not always the case. Zoonoses are a two-way street, and humans can also spread our own native microbes to other species. Recent studies have shown that humans have spread antibiotic-resistant strains of Staphylococcus aureus to many different species, including domestic chickens, pigs, and even chimpanzees and dolphins. We, too, are a walking biohazard.