Explainer: Animals’ role in human disease

Nearly 75 percent of new, or emerging, infectious diseases in people were first spread by animals. Indeed, half of all germs known to cause human disease come from other animals. Some sources were birds, bats and other types of wildlife. Livestock and pet animals have spread many other diseases. Scientists refer to the infections that people pick up from animals as being zoonotic (ZOO-oh-NOT-ik).

The germs and other infectious agents that cause these diseases are known as pathogens. Most are microbes such as viruses or bacteria; others include fungi — even teeny-tiny worms and ticks.

In zoonotic diseases, animals serve as a pathogen’s host. Over time, some long-term hosts no longer become sickened. When a virus commonly lives inside an animal without harming it, that host is now called a reservoir. For instance, birds — especially ducks — have evolved into a natural reservoir for flu viruses.

Pathogens move among hosts continuously, explains Jonathan Epstein. A veterinary epidemiologist, he’s a scientist who studies the spread of disease in animals. (He works at EcoHealth Alliance in New York City.) Many pathogens will encounter a human host. It that person’s immune system had never yet encountered the microbe, it will have built up no immunity to fight the germ. That lucky pathogen can now survive and spread to others.

Understanding how pathogens spread between species can help scientists not only combat current disease outbreaks, but also prevent or lessen future ones.

For instance, Epstein specializes in viruses whose reservoir is bats. He has been on the trail of numerous viruses that have spilled over into people from these mammals. Among them: Nipah.

This viral disease started in Southeast Asia during the late 1990s. Workers at a massive pig farm began noticing troubling symptoms. Their pigs came down with a loud, barking cough and behaved strangely. They twitched and developed muscle spasms. Some pigs died. Tragically, farm workers also started getting sick. In severe cases, people entered a coma and died.

No virus can survive long outside a living organism. So Epstein teamed up with other experts to hunt the reservoir animal that had allowed Nipah to enter pigs.

It turned out to be a bat species. It normally stays away from people, living in the nearby rainforest. But when farmers planted an orchard of mango trees close to their pigpens, bats came by to dine on the juicy fruit. Those bats shed germy saliva, urine and feces onto the pigpens below them.

From 1998 to 1999, Nipah sickened more than 250 people. More than four out of every 10 of these people died. One million pigs were killed and disposed of to stop the disease’s spread.

It is important not to blame wildlife for diseases, says Kristine Smith, a wildlife veterinarian who works for EcoHealth Alliance. Instead, she argues, people must become aware of the risks of being in close proximity to animals and adjust their behavior accordingly.