Ebola, SARS, avian flu. All of these are horrifying diseases. They have all attracted quite a bit of media attention lately. And, luckily, they seem to be diseases that are more of a concern to people somewhere else, not here.
But these three diseases have something else in common: they are shared both by humans and by animals. These types of diseases, the ones that can infect both humans and animals, are called zoonotic diseases.
The animals referred to by the term zoonotic include both wild animals and domesticated animals. One study showed that 62 percent of the organisms (we’re talking mostly viruses and bacteria) that cause disease in humans also infect animals, meaning that most of the communicable diseases we humans catch are zoonotic.
Though there are hundreds of zoonotic diseases, I am only going to focus on the diseases found in our area that people primarily catch from wildlife, leaving the more-complicated domestic animal scene for another day.
According to Patsy Tassler, an epidemiologist with the Vermont Department of Health, and Jason Stull, the New Hampshire public health veterinarian, diseases in this category include rabies, hantavirus, Lyme diseases, giardia, leptospirosis, West Nile, and, in New Hampshire, Eastern equine encephalitis.
There may be others. Some are obscure, and separating the diseases that mostly come from pets or livestock from the diseases that mostly come from wildlife can be difficult.
Rabies is the most familiar of these diseases. It’s hard not to think of dogs when you think of rabies, but almost all of the fatal cases of rabies in humans in the United States for the past 15 years have been from the strain of rabies found in bats.
Raccoons are considered a major carrier of rabies, but the disease is also found in foxes and skunks. Almost any mammal can become infected by rabies, and infected animals can pass on the disease through their saliva.
Hantavirus and Lyme disease, two more or of our local zoonotics, would seem on the surface to have nothing in common with each other. Hantavirus is a rare disease, most typically found in the desert southwestern United States. Lyme disease is associated with ticks and the oak forests of the Northeast.
But both of these are diseases carried by wild mice. In the case of hantavirus, humans catch the disease when they breathe in the virus from mouse droppings or urine. In the case of Lyme disease, a tick bite carries the disease between mouse and human. Human cases of both of these diseases have been reported in our area.
Leptospirosis is transferred to humans who drink water that has been contaminated by the urine of infected animals. For giardia, or “beaver fever,” it’s feces. The beaver gets the blame because of its propensity for pooping in the water. But either of these water-borne diseases can be spread by a wide range of animals, including domesticated animals and
pets.
West Nile and eastern equine encephalitis are both found primarily in birds, but are certainly found in other animals – including, yes, horses. Infections typically spread from bird to human through mosquito bites. The first human cases of eastern equine encephalitis in humans were recorded in New Hampshire in 2004; Vermont has yet to record a human case.
Rabies and giardia have been around New England for several decades. But some of the other wildlife-based zoonotic diseases seem relatively new to Vermont and New Hampshire. Are the number of wildlife-based zoonotic diseases increasing, or are we just more aware of them?
“That’s the million dollar question,” says Stull. “No one knows.”
Certainly, the chance for wildlife-based zoonotic diseases to spread increases when suburbs are built in areas once only inhabited by wildlife, Stull and Tassler agree.
Tassler gives Lyme disease as an example, as houses are built in deer habitat, and as people interact more with deer (which also carry the disease) and rodents, more cases of the disease are reported. She notes that the disease, once only known in the coastal regions of Connecticut and New York, is now found even in northern Vermont.
Tassler also sees the globalization of human commerce and travel as an underlying issue. Diseases that start elsewhere can wind up here quickly.
On the other hand, some of these diseases may have been infecting humans in our area all along. With increased awareness, and our ever-increasing ability to test for certain diseases, the number of reported cases of any one disease are likely to increase, although the actual number of cases may remain constant, Stull and Tassler say.
Prevention is a matter of staying informed of what diseases are out there and taking precautions, such as using insect repellent and long clothing to avoid bites from mosquitoes and ticks, purifying drinking water, and not handling wild animals.