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Last Updated

24 Jun 2003

Source: Newsday, June 24, 2003

When Animal Germs Infect Humans

Rising globalization, demand for exotic pets heighten the risks


"A Mysterious Disease." "Never Seen in the West." "Doctors Baffled."

A number of such headlines have appeared since West Nile virus surfaced here in the summer of 1999. Sporadic cases of bubonic plague have been reported in New York City and mad cow disease in Britain. The Asian outbreak of severe acute respiratory syndrome became public in March and, earlier this month, monkeypox announced its foray into the Western Hemisphere - specifically, the U.S. Midwest.

What these diseases have in common is transmission into the human population through contact with animals - a process termed zoonosis.

"Every so often there is a species jump, when an infection - one we've never heard of or never described in the literature - makes a leap from one animal to another," said Dr. Dan Shapiro, a specialist in infectious diseases and an associate professor at Boston University School of Medicine, who is writing a book on zoonosis. "If the second animal is human, that can be a problem."

Federal health officials took quick action to stem the spread of monkeypox, banning the sale of domestic prairie dogs as well as six types of rodents imported from Africa - animals sold in response to Americans' taste for exotic pets. Dozens of Midwesterners had fallen ill after handling pet prairie dogs apparently infected when housed near the rodents.

Zoonotic diseases are not a new phenomenon; animals have been known to transmit a long list of illnesses, including rabies, scabies, salmonella, trichinosis, botulism, malaria, measles, yellow fever, hantavirus and a number of strains of both streptococcus and influenza. Even the pandemic of Spanish influenza that killed an estimated 20 million people in 1918 is believed to have originated in swine.

"What is potentially unique about monkeypox, and what has caught people's attention, is that monkeypox has not been introduced to the Western Hemisphere before," said Dr. Robert Kim-Farley, visiting professor of epidemiology at the University of California, Los Angeles School of Public Health.

But experts say it's hard to determine if the number of such diseases crossing the species barrier to humans has been rising in recent years. What is known is that increasing urbanization worldwide, encroachment on previously uninhabited forest and desert land and a mobile human population traversing oceans at jet speed provide ample opportunities for diseases to emerge - or re-emerge, occasionally in more virulent forms - just about anywhere.

"People are increasingly encroaching on to out-of-the-way places," said Dr. Stephen Morse, director of public health preparedness at the Mailman School of Public Health of Columbia University. "Deforestation provides more contact with forest creatures. As more land is being given over to agriculture, and there's a higher density of both animals and human beings, that puts them in contact with obscure infections that were sequestered."

And the speed of global travel heightens the potential.

"An animal can, within 24 hours, go from the jungle in the Congo to someone's bedroom in the United States," Kim-Farley said. "You just never saw that before.

"If they had been shipped by sea, they would have either no longer been contagious by the time they arrived, or have died," he said.

Some epidemiologists do believe zoonotic diseases are on the rise, but they say there's no cause for alarm because scientists today are adept at tracing new infections and eager to follow the trail.

"The conditions that favor these transfers into human populations continue to increase," Morse said.

The leap between species can be made a number of ways: by consuming diseased meat, being bitten by mosquitoes or fleas, handling a pet or having contact with animal products like blood, hides, fur or wool, or dairy products, experts say.

Britons were infected with the human version of mad cow disease by eating beef containing the microscopic protein particle that causes the disease. And health officials believe food handlers in China may have become infected with the SARS virus after handling animals at a market that supplied restaurants in Guangdong.

In the United States, the growing popularity of exotic pets led to a chain of monkeypox infection that is believed to have started when the prairie dogs were housed with imported animals that carry the illness. Federal health officials said six types of rodents have been implicated in the monkeypox outbreak in humans: the giant Gambian rat, tree squirrel, rope squirrel, brush-tailed porcupine, striped mouse and dormouse. All African rodents have been banned for sale and import, and it is illegal to release them to the wild.

This is not the first time federal health officials have taken the bold action of banning pets. In 1975, federal officials banned the miniature pet turtles kids used to win at street fairs when it became known they were the source of 14 percent of all human salmonellosis cases in the country.

The same year, officials also banned imported monkeys and other nonhuman primates as pets because they carry serious diseases like tuberculosis.

The problem with zoonotic diseases is two-fold, experts say. Once an animal population harbors a virus, it is virtually impossible to eradicate the disease. That's why public health officials have urged pet owners not to let prairie dogs or rodents free, an action that could create a persistent animal reservoir of monkeypox in this country's wildlife.

The second factor is how efficiently a new disease is transmitted among humans. HIV, for example, is transmitted very efficiently through sex, and its virulence doesn't weaken as it is transmitted time after time.