skip navigation
Vol. LVIII, No. 9
May 5, 2006
cover

previous story

next story
Here the Wild Things Are
Your Back Yard: Close Enough for Zoonoses

On the front page...

What does Flopsy Bunny have in common with a Gambian giant pouched rat?

Both — without even trying — can make you sick as a dog.

Zoonoses (zo-uh-NO-seez) are communicable diseases that move from animals to humans — sometimes directly, and sometimes via an intermediate vector such as the flea.

Continued...

The zoonotic bestiary includes many vectors, including pig, bat, oyster and tick. The most urgent instance involves wild swans, ducks and geese: the H5N1 influenza virus, or "bird flu," has now spread to birds on the African continent. Then there are fad pets (say, Paris Hilton's kinkajou, whose natural habitat isn't the nightclub).

 
Harvard Medical School’s Dr. Arnold Weinberg showed how zoonoses can be triggered by anthropogenic change.  

Zoonoses, known and described since antiquity, are now being battled in a global context by (among others) the folks in conservation medicine. Conservation medicine, an emerging specialty, integrates three different fields: animal health, human health and ecosystem health. It works to improve all three by using interdisciplinary teams of veterinarians, physicians, ecologists, public health and conservation professionals.

Conservation medicine has arisen in response to unprecedented levels of disease in many species — including humans — and it addresses the environmental causes of health problems.

In his Great Teachers Lecture "Zoonoses: Where the Wild Things Are" on Mar. 8, Dr. Arnold Weinberg, professor of medicine at Harvard Medical School, showed how zoonoses can be triggered and exacerbated by anthropogenic change — war, chemical pollution, global trade, wilderness encroachment and the warming of the earth's oceans.

To illustrate how animals suffer disease within their own kingdom, Weinberg showed a Gary Larson ("The Far Side") cartoon: A wolf pack is being exhorted by its leader, who's scoped out a farmyard enclosure of pigs. "I say we do it!" says alpha wolf. "And trichinosis be damned!"

Trichinosis is a disease caused by eating undercooked pork containing parasitic worms.

"Animals have their own appetites," Weinberg said. "We are part of a large family that is very diverse." As in any family, the relationships are complex.

In evidence, he showed a slide of imported discarded tires. Mountains of them piled up in Houston. These lent the Asian tiger mosquito space to breed, hitchhike and eventually cause an outbreak of Eastern equine encephalitis.

The zoonotic scale is huge and its dissemination various. "Of 1,400 known pathogens," Weinberg said, "around 800 originate in non-human sources, and 200 are competent enough to be involved in human disease." Disease can spread on three levels — wild, domestic and human. In the wild, disease can loop from deer to tick and back. It can shift from wild to domestic — meadow mice to homes. And it can go from domestic to human, in the classic example of rabies transmission — dog bites man.

Distribution depends on several factors: arthropod vectors and reservoirs; animal and aquatic hosts; migration patterns; global trade; and climatic conditions such as global warming. Weinberg explained: "Water temperature must be over 20 degrees Centigrade [>70 degrees Fahrenheit] for Vibrio parahaemolyticus to reproduce and get into bivalves," he said. "Diseases that used to be found only in warmer waters are now found in the North Sea and the North Atlantic." Cape Cod's oysters are therefore no longer free from this family of bacteria, which causes gastrointestinal disease, skin infections and septicemia.

Other factors include human activities: importing untreated animal hides and skins; travel (especially of the adventurous sort); exotic pets; and bioterror. Mechanisms of transmission include contact, ingestion, inhalation, arthropod vectors (like tick bites) and animal bites.

Weinberg described how he once drank water from a clear-running brook "which had already been used by non-toilet-trained beavers." Giardia (a nasty bug that hits the GI tract) left him reeling.

He explained how, in 1978, Florida raccoons imported to a West Virginia hunting camp caused a major rabies epidemic in raccoons up the east coast, extending to Ohio.

Drawing on his clinical and teaching experience at Massachusetts General Hospital, Weinberg presented several cases in the art of diagnosis. In one, a 55-year-old man discovered a dead rabbit in the road. Its body was still warm, and he brought it home for stew. "The wife served as a control in this case," said Weinberg. "She said to her husband, 'Are you crazy?' and didn't touch it. He got sick and she didn't." The man developed typhoidal tularemia.

Another case involved an epidemic in war-torn Kosovo; patients presented with fever, pharyngitis, cervical adenitis and suppurating nodes. "This is as an example of how the pathogen was probably on the wild-animal level," he explained, "and rodent-contaminated food and water allowed it to enter the domestic plane; then human disease followed." Once rodents were controlled, the epidemic disappeared.

Each case resonated with his theme: "Conservation medicine is an idea whose time has come, and none too soon," he said. "Earth is the only planet — that we know of — that supports life. We should do everything in our power to preserve it. We want to keep people healthy, and also protect the diversity of all life."

back to top of page