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Diseases From Animals -- What's Next?

As experts are learning, the list of infectious diseases borne from animals is by no means complete.
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We've watched West Nile virus fly from coast to coast. We've seen SARS spread from Asia to North America. We've had several close calls with killer bird flu viruses. And don't forget the still-unsolved anthrax bioterror attacks. All in less than a decade.

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We can't help waiting for the other shoe to drop. In this we're like infectious disease experts. They don't wonder whether another disease will emerge. They only wonder when.

Fortunately, they are watching more carefully than ever before. And there's a newfound sense of respect for nature, says T.G. Ksiazek, DVM, PhD, chief of the CDC's special pathogens branch.

"Earlier in my career, there was a general attitude we'd conquered infectious diseases -- but that is certainly not the case," Ksiazek tells WebMD. "Diseases from animals and mosquito- and tick-borne diseases have been out there for a long time. 'Emerging disease' is a buzzword now, but there really are things out there with the potential for introduction on the scale of West Nile virus or for being the next pandemic flu strain."

Future Influenza

What's next? Nobody knows. But there are several excellent candidates. At the top of everyone's list is influenza. Not the run-of-the-mill influenza that already kills 36,000 Americans every year. What worries experts is that human influenza will mix with what's called "highly pathogenic" bird influenza. It would be new to humans, so existing immunity wouldn't help. And it could retain the factor that makes it fatal to nearly 100% of chickens.

"Flu is not considered to be transmitted to humans from animals -- but the mixing bowl for new human flu is animals," Ksiazek says. "That is an example of what is out there."

Experts are so tuned to flu that when the CDC heard the first reports of the disease that turned out to be SARS, flu experts were rushed to the scene.

"Flu certainly is on our radar screen," Ksiazek says. "CDC is making a large effort to plan for the next pandemic."

What Else?

"It's hard to put your finger on any given thing that is most dangerous," says Ksiazek. "Some cultures have a pretty universal diet -- all kinds of animals get exposed to each other, and to humans, in many of the world's markets. It's not the source of the next great plague of mankind, but we have to keep an eye on what's happening there."

One old disease is emerging as a new threat: Dengue virus. Dengue, spread by mosquitoes, has been around for a long time. But now several types of dengue virus circulate in the same tropical regions. That's a problem. A person who's had one kind of dengue can get a much more serious disease -- dengue hemorrhagic fever -- if infected with a second kind of dengue. "Dengue is always a concern," Ksiazek admits. "We have gone from a situation where in the late 50s, the mosquito that carries dengue was under control. That ended in the late 60s or early 70s. Now dengue hemorrhagic fever has appeared. Initially it was in Southeast Asia, now it has jumped into the Americas and into other parts of Asia. It is something that CDC ... [is] watching closely."

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