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The origin in the species
MED prof studies transmission of disease from animals to humans
By Brian Fitzgerald
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Daniel Shapiro |
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When a disease makes a “species jump” from animal to human, it also makes headlines, especially when it kills people. Avian influenza, for example, claimed another victim in Vietnam on February 1, bringing the total number of deaths in the country to seven.
Known as bird flu, the virus has appeared in 10 nations, including China. It joins Ebola virus, West Nile virus, SARS, mad cow disease, and monkeypox as recently emerging zoonoses — diseases that are transmitted to humans through contact with animals. Daniel Shapiro, a MED associate professor, is writing a book on the topic of zoonoses, which he says have afflicted humans for millennia.
Animals can transmit many illnesses to humans, and it can happen in a variety of ways. “These include rabies, via bites,” says Shapiro, “salmonella, through the fecal-oral route; plague, by the bite of an infected rodent or by contact with infected animals; anthrax, from inhalation, contact, or ingestion of contaminated animals or animal products, such as wool or animal hides; and hantaviral infection by inhalation of rodent excreta.”
In addition, “AIDS is caused by HIV-1, which originated in chimpanzees,” says Shapiro. “HIV-2, which is less common, originated in a different nonhuman primate, the sooty mangabey.”
The outbreak of influenza that killed an estimated 20 million people worldwide in 1918 and 1919 — the worst infectious pandemic in history — is believed to have originated in swine. And let's not forget that bubonic plague, which killed 25 million Europeans between 1347 and 1352 — one-third of the population — was transmitted from rodents to fleas to humans.
However, it seems that there has been a spate of recent diseases that have jumped the species barrier to infect people. Last June, dozens of people in the United States contracted monkeypox after handling pet prairie dogs. “The prairie dogs had apparently become infected when housed with rodents — fellow exotic pets — imported from Africa,” says Shapiro. “What was unique about monkeypox was that for the first time it had spread to the Western hemisphere.”
Experts say it's hard to determine if zoonoses have been on the rise in recent years. But they point to several factors that contribute to the spread of zoonotic agents, including increasing trade between nations, “as in the case of cattle crossing borders,” says Shapiro — borders that once kept diseases more isolated. Also, as more humans encroach on traditional animal habitats, they are catching animal-borne diseases. “In the northeastern United States, for instance, people are moving onto previously uninhabited land,” he says. “If a new subdivision of homes is built on a wooded area, there is the potential for people to contract Lyme disease after being bitten by infected ticks.” He also points to international travel as a vehicle for spreading disease: “In today's world, a person can go from one side of the planet to the other in a matter of hours.”
Shapiro says that more than half of newly emerging diseases striking humans have made the leap from animal to man. “Very few diseases are human-specific,” he says. The jump between species can be made in several ways: by consuming diseased meat, by being bitten by fleas or mosquitoes — as in the case of the West Nile virus — or simply by handling an animal. In China, officials have seized and killed thousands of civets after they contributed to the reemergence of SARS. The small game mammal is sold as food in Asian markets and restaurants.
Factory farming is also to blame, according to Shapiro. “When animals are overcrowded, they are more likely to get sick, just as disease spreads quickly among humans in a refugee camp,” he says. “And a bacterium such as E.coli can contaminate hundreds of thousands of pounds of ground beef at one company. The beef is then distributed to wholesalers in several states, as opposed to, say, a century ago, when a family farm fed a small number of people, and an outbreak was limited to a local, rural population.”
Shapiro, the director of the clinical microbiology and molecular diagnostic laboratories at Boston Medical Center, has long been interested in zoonoses, and cowrote a research paper on a 72-year-old Boston woman who caught a Cryptococcus neoformans infection from a pet cockatoo in 1998.
Zoonoses will be with us “as long as there is human-animal contact,” says Shapiro. And as humans continue to change the environment, the migratory patterns of animals will also change, bringing more zoonotic diseases. “Global warming raises temperatures and the location of water,” he says, “which will help spread mosquito-borne diseases.”
In the meantime, in all countries “there needs to be a superb job of monitoring diseases that are known to be transmittable to humans,” Shapiro says. Increasing diagnostic capabilities enable scientists to track the course of a disease quickly. But, he points out, “veterinary public health today is woefully underfunded, and that has to be taken more seriously. The weak link in all this may be in those countries that can't afford even adequate human public health. In terms of the future of zoonotic infections, that is the major concern.”
6
February 2004
Boston University
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