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Last Updated

18 Jun 2003

Source: New York Times, June 17, 2003

Beyond Cute: Exotic Pets Come Bearing Exotic Germs


Epidemiologists can be such killjoys. Consider, for instance, Dr. Michael T. Osterholm, who has been publicly denouncing prairie dogs since 1997. A prairie dog in a burrow is one thing, but a prairie dog in the house makes Dr. Osterholm a bit edgy.

The fact that the United States has exported thousands of prairie dogs to Japan, where they are not found in nature and where people find them adorable, gives Dr. Osterholm a full-blown case of the willies. Japan banned prairie dog imports in March, and the European Union halted them yesterday, but researchers still worry about what havoc may be wrought by the animals that have already been shipped overseas.

Where some people see a cute and cuddly ball of fur, scientists like Dr. Osterholm see a vector: a ball of disease-causing viruses, bacteria, parasites and who knows what other germs. Dr. Osterholm, who is director of the Center for Infectious Disease Research and Policy and a professor of public health at the University of Minnesota, said that until recently, his main objection to prairie dogs was that they and their fleas sometimes carried bubonic plague. He had not even thought about monkeypox, the disease brought to the Americas for the first time last month, presumably by a three-pound African rat, which infected its fellow inmates in a pet shop, prairie dogs, which may then have spread the disease to as many as 82 people in five states.

Though Dr. Osterholm had not predicted monkeypox, its arrival did not entirely surprise him. The worldwide trade in so-called exotic pets has done two things that are practically a recipe for spreading exotic diseases. First, the trade has transported animals like giant Gambian rats across oceans and brought them together with species that they would never encounter naturally, like prairie dogs. Not much is known about what microbes those animals might spread to each other, or what the microbes might do inside a new host. Second, the trade has brought people close to animals and to diseases they had little or no contact with before.

"It clearly stacks the deck in favor of infectious agents," Dr. Osterholm said, and he rattled off a list of agents that have animal origins and can cause severe illness in people: H.I.V., Ebola virus, a highly virulent form of the bacterium E. coli, the Nipah virus that spread from bats to pigs to people in Malaysia in 1998, and the current epidemic of the respiratory disease SARS.

Like SARS, which has been traced to a previously unknown coronavirus carried by palm civets and badgers in the jam-packed live-animal markets of southern China, the outbreak of monkeypox in the United States is a reminder of how little is known about infectious diseases in wild animals and the threat they may pose to humans.

Dr. Frank Fenner, an expert on pox viruses and other viruses at Australian National University in Canberra, said, "Quite a lot of new viruses have been turning up, all coming out of animal hosts."

He added: "I think we know so little about the viruses of wild animals."

Dr. Fenner said scientists were familiar with hundreds of viruses carried by people and domestic animals, but had much less information about the many viruses that are probably carried by wild animals.

"With all the animals in the wild," he said, "we really know so little about what virus diseases they have unless they get into livestock or humans."

Dr. Fenner suggested that every species of wild animal probably carried its own distinct viruses, many more than are known. Most do not infect people, but the ones that do can lead to nasty surprises.

Monkeypox is not new. It was first identified in monkeys in 1959, but its ability to infect people was not recognized until 1970. The disease is usually milder than smallpox in humans, causing a death rate up to 10 percent in Africa, compared to 30 percent for smallpox. Although the disease was named for monkeys (because it was first found in them), scientists later came to realize that its real host is a rodent. Dr. Fenner said three or four species of African squirrels were thought to be the main hosts, and infections in monkeys and people were considered accidental. Squirrels are commonly eaten in some parts of Africa, and people are probably infected from handling sick animals.

Monkeypox outbreaks in people in Congo were detected in the 1990's, and a 1999 report by the World Health Organization suggested that the disease might have found a foothold when health experts said people no longer needed smallpox vaccinations, which can prevent monkeypox because the vaccine and monkeypox viruses are closely related.

Despite that theory, the health organization did not recommend that smallpox vaccination be resumed in Africa, because H.I.V. rates are high there, and the smallpox vaccine can be quite dangerous for people with H.I.V. or AIDS.

The viruses that cause smallpox, monkeypox and cowpox, because they infect people, are among the best-known members of the pox virus family. But the family has several dozen other members that infect a broad range of animals, causing diseases not found in people, like camelpox, skunkpox, raccoonpox, rabbitpox, mousepox and bird poxes specific to canaries or juncos. Suipoxvirus infects pigs, taterapox infects naked-soled African gerbils, and still another pox virus, thought to have an unknown main host, causes a sickness called Uasin Gishu disease in horses in Africa. Chickenpox, despite its name, is not caused by a pox virus; the microbe that causes it belongs to the herpes family.

Some pox viruses, as far as researchers can tell, infect only a single host. Camelpox, for instance, has been found only in camels. And yet of all the pox viruses, it is the one most closely related to smallpox. Smallpox also has only one natural host, people, which explains why it could be eradicated: since there was no animal host in the wild, once the virus was stamped out in people, it had nowhere else to go.

Cowpox, on the other hand, might be called promiscuous: it infects not only cows, but also people, rodents, cats, elephants, rhinoceroses and okapis. In people, it generally causes a very mild disease, and secretions from people and animals with cowpox were among the earliest substances used to vaccinate people against smallpox.

Scientists do not know why some pox viruses are limited to a single species while others infect a multitude, but Dr. Fenner said the ones with more than one host were likely to be the most enduring.

"If a virus affects only one rare species of animal and that animal becomes extinct, the virus becomes extinct with it," he said. "But if it infects several species of animal it may survive, as cowpox does. We know it occurs in gerbils in Russia and in field mice and voles in England as a natural infection in the wild. And there may be other rodents as well, so it's very unlikely to be wiped out except by a new ice age or something like that."

The most familiar member of the pox virus family is in some ways the most mysterious. Many people assume that vaccinia, the virus used to make smallpox vaccine, is the same virus that causes cowpox and that was first used by Dr. Edward Jenner in 1796 to vaccinate people against smallpox. In fact, vaccinia is not the cowpox virus. It is a distinct species, and scientists do not know where it came from. But in the early days of vaccination, there was no way to store a vaccine, so people were usually vaccinated with secretions taken from other people or animals. Scientists have speculated that such arm-to-arm passage may have created a hybrid of smallpox and cowpox, or perhaps even brought in a type of horsepox that no longer exists in nature.

Researchers say it should be no surprise that a virus capable of infecting multiple rodents in Africa could find a ready host in a rodent here.

And yet even those who ardently oppose exotic pets in principle may succumb to their furry charms. Dr. Osterholm admitted that he gave in to his son's insistence on having an African dwarf hedgehog.

"Everyone had them," Dr. Osterholm said.

But for a public health expert, letting this animal into the house was as bad as smoking. The moment it relieved itself, Dr. Osterholm collected the droppings and whisked them off to his lab to analyze. He found that the hedgehog was carrying three strains of salmonella bacteria. He let his son keep the pet, but imposed extensive hand-washing requirements any time a family member touched it.

"It was no fun at all," Dr. Osterholm said.