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Leap From Animals to Humans

Pets from the wild can pose health risks.

June 15, 2003|Wendy Orent | Wendy Orent writes frequently about infectious disease and is coauthor of the forthcoming book "Biowarrior: Inside the Soviet/Russian Biological War Machine."

ATLANTA — It appeared at first we'd dodged the bullet. Now it's not so clear. Monkeypox, a close cousin to the smallpox virus, unexpectedly appeared this month in the Midwest, far from its natural home in the rain forests of Congo, Liberia and Sierra Leone. For the last couple of decades, monkeypox in Africa has been an elusive threat. It has erupted in Congolese villages after someone has become infected through killing, skinning and eating a rat, squirrel or monkey. Sometimes the disease has been passed on to the victim's family or fellow villagers and sometimes it hasn't. Experts from the Centers for Disease Control and Prevention and other health officials have been telling us that monkeypox isn't a very infectious disease. The truth, however, is a bit more complicated.

To understand why, we have to look at the biology of the disease. Unlike smallpox, monkeypox isn't a human specialist. It's an animal disease, and no one is sure what range of animals it infects. It kills monkeys, hence the name. Someone once found a dead squirrel on a Congo highway with suspicious-looking lesions on its skin. It turned out to have died of monkeypox, so some scientists decided that the disease is more a rodent disease than a primate disease. Apparently the disease infects giant Gambian rats, and now we know that prairie dogs, frequent victims of plague and tularemia, also can die from African monkeypox.

What has emerged from the remarkably rapid and effective investigation into the Midwest monkeypox outbreak is that African rodents, unsurprisingly, are the source of the disease. It appears that one giant Gambian rat, infected with monkeypox and housed with prairie dogs in a pet store, transmitted the disease to the prairie dogs, which were sold and swapped across several states.

Up to 68 people as of Friday had been infected. In the great bulk of those cases, the affected people appeared to have had direct contact with a sick pet. What doctors are more concerned about is person-to-person transmission, which indicates a more virulent strain, one better adapted to humans. At least three possible cases of monkeypox have now been found in people not exposed to animals. In Congo, where the death rate is up to 10%, monkeypox often spreads from person to person -- in part, public health officials say, because of primitive conditions.

But there's more to it than that. In the Congo province of Kasai-Oriental, where monkeypox occasionally breaks out, people inhabit fairly large, airy huts within a family compound, which is usually some distance from other compounds. These are not such terrible conditions for keeping the disease in check. We can't automatically assume, as some health experts contend, that it is better sanitation and living conditions in America that are keeping monkeypox from being so deadly here. Several other factors, hidden and not so hidden, are also at work.

As Peter Jahrling, the army's chief virologist, explains it, monkeypox, like other diseases, is not a single strain; there are lots of variants. Like all forms of life composed of DNA or RNA, the monkeypox virus is subject to evolution, mutation, the force of natural selection. Whatever afflicted that Gambian rat, it probably wasn't a single pure strain but a swarm of variants. And some of those strains possessed the ability to transmit from animals to people. We don't know yet whether some of the strains also were capable of spreading from human to human, but that would not be surprising given what we know about the disease in Africa. It is the inevitable logic of natural selection at work: Those variants that are best at transmitting to people will be the ones that make the jump from animal to human. Then, among those strains, those even better at transmitting might jump again from human to human. This is a sort of winnowing-out process, where the best jumpers win.

In one well-documented outbreak in Congo, the chain of person-to-person monkeypox transmission was seven links long. If it hadn't stopped moving, whether through medical intervention or because it ran out of hosts in the village, the disease would have become better and better at spreading among people. It would have become a more "humanized" disease.

It would have looked more and more like smallpox.

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