Top: Courtesy of Thomas Strömberg; Bottom: Courtesy of David J. Jefferies http://www.ee.surrey.ac.uk/Personal/D.Jefferies/bird/

Any time animals are brought together in unnatural densities, it raises the potential for disease disaster. Bullfrogs, mass farmed in South America, are shipped to the United States without disease inspection. Their discarded skins might spread amphibian fungal plagues. Outbreaks of House Finch conjunctivitis and salmonellosis in song birds have spread in the US over the past 15 years helped along by the popularity of bird feeders.

A dozen bullfrogs slump together in a dark green mass at the bottom of a plastic bucket, looking more dead than alive. Some have blotchy skin and none has strength left to move. They have traveled thousands of miles to await their fate in back of a New York China-town market. A species native to North America, these frogs are raised by the tens of thousands on South American...


The domestication of wildlife species such as frogs, fish, and deer has realized explosive growth during the past 30 years. This so-called alternative agriculture along with the burgeoning exotic pet trade has outpaced the monitoring capabilities of animal and public health systems.

Even healthy exotics can become part of a pathogen reservoir matrix. In the United States, feral cats play an essential role as the reproductive host that maintains Toxoplasma gondii in the wild. Toxoplasmosis, which can cause pregnant women to miscarry, has also been linked recently to Pacific sea otter deaths. Toxo oocysts excreted in cat feces, can survive the marine environment, where they are likely eaten by mollusks, an otter staple.

The feral pig population in the United States is estimated at 2 million. A meal of a feral pig with pseudorabies can be fatal to a Florida panther. With less than 100 panthers left in the wild, even a single death affects species survival; the panthers are now vaccinated.

Generally, only wildlife diseases that threaten humans grab the headlines. Last summer, monkey pox virus was in the US limelight after giant Gambian rats crossed paths with American prairie dogs at a pet shop. The virus jumped from rats to prairie dogs to people, ultimately spreading across six states. The CDC and the Food and Drug Administration (FDA) quickly banned the import of African rats and put a hold on the prairie dog trade, but it is unknown how many rats or prairie dogs are still in circulation or have been dumped back in the wild.

Only a year before, prairie dogs were at the center of another health scare involving tularemia. Although the particular bacterial type, type B, is not considered life-threatening to humans, more than 100 wildlife species are susceptible. The animals had been trapped in the wild and sent from a distributor of exotics in Texas to pet stores as far away as the Czech Republic.2

"The trouble is, the US attitude has been that unless there's a known problem, we'll let [the animal] in," says veterinary epidemiologist Victoria Bridges at USDA's Center for Emerging Issues in Fort Collins, Colo. "Oftentimes there [are] not a lot of data. How do you analyze the key gaps?"


These pet-driven problems are stunning for their sheer number and bizarre details. Yet, short-sighted if often well-meaning wildlife management practices have triggered some of the most serious disease outbreaks.

A century ago conservationists focused on rebuilding decimated herds of game species, explains Steve Torbit, senior wildlife biologist with the National Wildlife Federation. They built fish hatcheries to restock streams, managed hunting seasons to build up herds, and translocated animals to start herds elsewhere. Exotic game birds such as Chinese pheasants and Hungarian chuckers were imported for sport hunting. If there was a disease concern, the prevailing thought was to deal with it later, says Torbit. "And, of course, now it is later."

Bison brought in to restock Yellowstone have picked up brucellosis, a cattle disease that causes miscarriages. Elk have also contracted it, and fostered its spread by crowding onto winter feed grounds designed to keep them out of ranchers' hay.

Meanwhile, millions of game birds raised and released for hunting mix with wild flocks. "The average mortality for North American waterfowl before 1960 was about 55,000 birds annually," says Friend. "Now we've got single events that take a million birds or more." Likewise, raising elk and deer for meat and so-called canned hunts may have helped move the prion malady called chronic wasting disease across the country; modern cervids often migrate by truck.


The latest wildlife plague is more pernicious than prions. According to several small studies, antibiotic-resistant bacteria have been found in everything from sparrows to sharks, providing a ubiquitous natural reservoir for dangerous antimicrobial traits.

At the wildlife clinic of Tufts University School of Veterinary Medicine, associate professor Mark Pokras' students tested animals on admission and release. "Whether they were rabbits or red-tailed hawks or porcupines, they would have some bacteria that showed resistance," reports Pokras. And the levels went up during their stay even if they hadn't been treated with drugs.

Further testing on dozens of wild-trapped sparrows at three locations – the Tufts clinic, a Boston hospital, and a wilderness area – revealed another pattern. Bacteria in birds near the clinic showed more resistance to animal drugs, whereas testing near the hospital revealed resistance to human drugs.

Veterinarian Mark Mitchell's team at Louisiana State University's Wildlife Hospital near Baton Rouge found that bacteria in top-of-the-food-chain raptors were resistant to drugs ranging from doxycycline and tetracyclines to fluoroquinolones. The group also studied wild sharks, taking samples off the coasts of Louisiana, Florida, Martha's Vineyard, and Belize. "We found resistance in all of them. And finding it in small samples sizes, about a dozen sharks at three locations," says Mitchell.

The germs are not just cycling between wildlife, livestock, and humans, they are becoming more dangerous with each pass. For example, Salmonella enterica variant Typhimurium DT104, a fairly common serotype first seen about 15 years ago, is now resistant to eight drugs. Although the number of food-borne salmonella outbreaks in the United States has decreased, the percentage of outbreaks involving multidrug-resistant bacteria has risen, notes Linda Tollefson, deputy director of the FDA's Center for Veterinary Medicine.

For most pathogens, there simply are no boundaries between animals and people. Almost every human disease that has emerged in the past 30 years has originated from wildlife, and now human diseases and those of their companion and food animals now threaten wildlife. The globe-trotting frogs in Chinatown are literally drops in the bucket when it comes to the modern spread of disease. Everything, pathogens included, is on the move.

Janet Ginsburg taotm@earthlink.net is a freelance writer in Chicago, Ill.

Interested in reading more?

Magaizne Cover

Become a Member of

Receive full access to digital editions of The Scientist, as well as TS Digest, feature stories, more than 35 years of archives, and much more!
Already a member?