Feature Archive

Diseases from Animals -- Why Now?

Emerging disease detectives have a global job to do.

By Daniel DeNoon
WebMD Feature

Reviewed by Charlotte Grayson

They call them emerging diseases. Ebola in Africa. Monkeypox in the American Midwest. Dengue hemorrhagic fever in the Americas. SARS in China and Canada. West Nile virus throughout the U.S. AIDS all over the world.

And this is just the tip of the iceberg. A quick check of the web site of the International Society for Infectious Diseases shows that humans aren't the only species plagued by emerging diseases.

Why so many? Why now? It's not a sign of the apocalypse, says T.G. Ksiazek, DVM, PhD, chief of the special pathogens branch of the U.S. Centers for Disease Control and Prevention.

"There's lots of emerging diseases out there," Ksiazek tells WebMD. "But if you add up all the numbers of actual human cases in the last year or so, I doubt whether there has been a significant increase in either cases or deaths. But some of these newer diseases are more spectacular and got a lot of media attention."

Our Changing World

The world may not be ending, but it certainly is changing. Many of these changes force humans and animals into ever-closer contact.

Some changes are local. Your new suburban community, for example, may build a new subdivision in that woodland down the road. Suddenly, it seems, there are raccoons in the back yard and opossums sniffing the trash bin.

Now look at a slightly larger scale. Ranchers and hunters kill off coyotes and wolves, which usually kill off sickly deer. Instead, deer populations increase -- and deer diseases increase even faster, says George A. Pankey, MD, director of infectious disease research at Ochsner Clinic Foundation, New Orleans.

"We got rid of the predators, so now we have more deer," Pankey tells WebMD. "They bring deer ticks, and the ticks bite people. So you see more Lyme disease. It is the consequence of things being thrown out of balance."

Pull back to a global scale and the same thing is happening. The nonprofit Rainforest Alliance says the Amazonian rainforest is vanishing at a rate of 33 million acres a year. That as much real estate as in all of Delaware, Massachusetts, New Hampshire, New Jersey, Rhode Island, and Vermont combined.

"Anything that increases contact between people and other species, or with environments in which other species are active increases, the potential for something happening," Ksiazek says.

On a Wing and a Prayer

Environmental change is alarming, but it's not new. Something else is: air travel.

"The one factor that probably does have some degree of rapid change is that we live in a collapsing world," Ksiazek says. "How long it takes to travel from one continent to another has drastically changed during my lifetime."

In the case of SARS, business travelers carried the disease from one continent to another. But sometimes it's animals themselves that do the traveling. In the case of the recent U.S. monkeypox outbreak, it was exotic pets that made the trip.

On April 9, 2003, a shipment of 800 small mammals arrived in Texas from the West African nation of Ghana. They included nine different species, including rope squirrels, tree squirrels, Gambian giant rats, brushtail porcupines, dormice, and striped mice. A CDC investigation found that monkeypox had already spread widely through these animals.

Will monkeypox end up running wild in the U.S.? It's too soon to tell. Pet prairie dogs housed near Gambian giant rats got infected and spread the disease to humans. Not all of the animals in the April shipment are accounted for. As of June 11, 2003, the CDC and FDA have banned import of African rodents and trade, transport, or release of prairie dogs.

It's a scary episode -- and more are likely, says animal disease expert Lawrence T. Glickman, VMD, DrPH, professor of veterinary epidemiology and environmental health at Purdue University School of Veterinary Medicine, West Lafayette, Ind.

"International trade in these exotic animals has increased," Glickman tells WebMD. "If people in the U.S. think Gambian giant rats are good pets, there's going to be trade. The people catching them are making a fortune if they get 50 cents apiece. And then they sell here for three or four hundred dollars. Think of the profit. And recently we had a problem with people capturing raccoons and transporting them across state lines. It led to a big rabies epidemic."

The big problem with wild-caught or exotic pets: Wild, exotic bugs.

"If monkeypox were more highly transmissible from person to person, that outbreak would have been a disaster," Glickman says. "And there are a lot of things lurking in Africa that are a hell of a lot more dangerous. Ebola is one of them. There are still enough animals over there that someone would consider a pet that could have virtually anything. That is a major concern."

What If?

When new diseases do emerge, time is of the essence. It's important to find out who is infected, what causes the infection, and where the infection came from.

Infectious disease specialists such as Pankey are on the front line. Pankey stresses the importance of old-fashioned medicine.

"You have to have a basic knowledge of what might something might be, and try to take a good history from the patient," Pankey says. "So if you find out a kid has run over a rabbit with lawnmower, you worry about [a disease called] tularemia. It is a matter of knowledge."

Ksiazek, chief of the CDC's special pathogens branch, is in charge of tracking down emerging disease outbreaks. How does it work?

"It's complicated and depends on the circumstances," Ksiazek tells WebMD. "The first order of business is to sort out what it is. If you can find out that it's something someone has seen before, you may know what to expect. If it seems strange and you can't describe any known cause for the disease you're seeing, the situation is more complex.

Until the recent outbreak, monkeypox had never been seen in the U.S. But when people started breaking out with pox-like blisters, it didn't take long to figure out what disease they had. On the other hand, SARS didn't look exactly like anything doctors had ever seen before.

"With SARS, we had no ideas of, 'Well, it's probably this or this or this' to help us narrow our focus," Ksiazek says. "That's why we took a broad approach. It turned out to be something entirely new and opened up a lot of questions. We have an entirely new agent without a lot of previous experience of how it is going to act. You have to feel your way through it."

The first step is for teams of medical doctors to get a complete medical description of the new illness. Is it a disease of the lung or the liver? Do patients' lab tests have any consistent abnormalities? What is the pattern of transmission?

"Those are the kind of things that help in determining what initially should be done so preventive measures can be taken," Ksiazek says. "If it's a bacterial infection, we can look at antibiotics. If it's viral, we look at whether there are effective antiviral [drugs] or vaccines. Those are the immediate questions."

Another big issue is where in the world the patients are. This affects how quickly blood and other clinical materials (blood, bodily fluids, and other human tissue) -- possibly carrying dangerous germs -- can be taken to sophisticated laboratories. International guidelines restrict the movement of these materials.

Once the samples are in the lab, a different team takes over. Their job: to isolate the germ that causes the disease and to see whether it's like any known agent.

Outside the lab, epidemiologists try to separate fact from a burgeoning body of rumor. Ruling out these red herrings is as important as finding real clues. Eventually, data from each of the teams gets put together into a working theory -- and a plan of action.

"Sure, there are a lot of things out there that are scary, but even things like Ebola hemorrhagic fever we know how to contain," Ksiazek says. "We have gained experience in past outbreaks and know how to contain them.

Back to From Animals to Humans: Tracing the Path of Infectious Disease.

Published July 8, 2003.


SOURCES: Emerging Infectious Diseases, April 2003, March 2003, and December 2002. CDC Morbidity and Mortality Weekly Report, July 4, 2003. Rainforest Alliance web site. Lawrence T. Glickman, VMD, DrPH, professor of veterinary epidemiology and environmental health, Purdue University School of Veterinary Medicine, West Lafayette, Ind.. George A. Pankey, MD, director, infectious disease research, Ochsner Clinic Foundation, New Orleans. T.G. Ksiazek, PhD, DVM, chief, special pathogens branch, CDC, Atlanta.

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