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.
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."
Learning From Experience
Anthrax attacks and SARS outbreaks have made us warier if not wiser, says George A. Pankey, MD, director of infectious diseases research at Ochsner Clinic Foundation, New Orleans.
"I think the surveillance is better now as a result of the bioterror threat," Pankey tells WebMD. "Local labs and a lot of infectious disease doctors are attuned to unusual things happening. And I think public health services around country are very aware.... In general we are a lot better off then we were 20 years ago."
That's true -- but more must be done, says Lawrence T. Glickman, VMD, DrPH, professor of veterinary epidemiology and environmental health at the Purdue University School of Veterinary Medicine.
"Our ability to diagnose viruses is better now," Glickman notes. "But I think we need better reporting systems for both people and animals. I think both sides are working on that. Unfortunately we don't have a CDC for animals, so a lot of what happens in pets -- especially exotic pets -- is unknown."
Watching the Animals
If new diseases come from animals, it's a good idea to keep an eye on them. That's exactly what Glickman is doing. In collaboration with the CDC and the nation's largest chain of pet hospitals, he and his colleagues are amassing a huge database on cat and dog health.
It's called the VMD-SOS: Veterinary Medical Data-Surveillance of Syndromes. Data comes from the 60,000 dogs and cats seen weekly at Banfield Pet Hospitals' 300 veterinary facilities in 43 states.
"Every night that information is processed, and with the right programming we could be alerted to a disease outbreak in cats or dogs," Glickman says. "Human health surveillance systems are more regional and less standardized. Our system may let us give public health officials their first warning of an outbreak."
Other systems already are in place. The USDA is keeping track of West Nile virus in birds. The CDC is upgrading its national laboratory network. And the World Health Organization -- taking stock of what it learned from the SARS outbreak -- is taking a much more active role in responding to disease outbreaks.
Back to From Animals to Humans: Tracing the Path of Infectious Disease.
Published July 8, 2003.
