Germs and Sickness in a Shrinking World
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A. Bird-watchers aren't the only ones keeping close tabs on feathered migrants this spring. Public-health officials, fearing a resurgence of the imported virus that gave both birds and people West Nile Fever last summer, are scanning the skies for signs of avian ill health. But biologists warn that sickly birds, beyond being harbingers of human disease, warrant concern in their own right. Last year’s West Nile outbreak, which killed seven elderly people, also felled 5,000 to 10,000 native birds — mostly crows but 19 species in all, including robins, blue jays, herons, hawks, and kingfishers.
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B. That’s not surprising. Just as measles and smallpox carried to the New World by Spanish conquistadors took a devastating toll on Native Americans who had never been exposed to the germs, transporting domestic and wild animals from one place to another introduces native creatures to new, and often deadly, infectious diseases. As globalization shuffles more people, animals, and pathogen-contaminated products around the world, biologists say such infections are increasing. According to Peter Daszek of the University of Georgia’s Institute of Ecology in Athens, “There is probably no place on Earth that is free from pathogen pollution.”
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C. In a report published last winter in Science, Daszek documents dozens of “emerging” wildlife diseases — from elk stricken with bovine tuberculosis to Antarctic penguins who show signs of exposure to a chicken virus. He believes such diseases pose a significant, yet largely unrecognized, threat to global biodiversity. And the spread of some wildlife infections also endangers human health. In last week’s Science, researchers warn that seals just discovered to be infected with influenza B could spawn a new flu epidemic among humans.
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D. Like predators, pathogens are natural components of ecosystems that help regulate wildlife populations. But humans cook up host-pathogen combinations that could never occur in nature. One early example was the introduction of cattle to Africa in the late 1800s, which sparked an epidemic of a virus called rinderpest among native buffalo, wildebeest, and other grazers. So many of these animals died that large swaths of natural savanna turned into scrubby forest.
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E. Indeed, spillover of pathogens from domestic livestock and pets may be the leading cause of emerging wildlife diseases, especially as humans encroach increasingly on wild-animal habitat. In the western United States, the black-footed ferret was nearly wiped out by distemper, a deadly viral disease of domestic dogs. African wild dogs in the Serengeti did die out after exposure to canine distemper. Today Africa’s endangered mountain gorillas are threatened by several diseases, from measles to the common cold, that they catch directly from human tourists.
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F. Sometimes, this process reverses itself in what Daszek calls “spill back”. A disease known as brucellosis, for example, was probably introduced to North America along with cattle. But infected bison roaming Yellowstone National Park are now considered a threat to livestock, and are often shot by ranchers when they wander outside park boundaries.
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G. Destroying habitat is another way humans spawn disease outbreaks. The loss of US wetlands has crowded ducks, geese, and other waterfowl into smaller bodies of water, sparking epidemics of cholera and botulism that kill tens of thousands of birds at a time. According to Robert McLean, director of the US Geological Survey National Wildlife Health Center, “Shrinking habitat is also deteriorating, which promotes growth of bacteria and other pathogens.”
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H. Occasionally, the culprit behind an emerging wildlife illness is a mystery. In 1998, biologists identified a new fungal disease that was killing huge numbers of frogs in remote upland rain forests of Australia and Central America. Since then, the same fungus has been fingered in die-offs of boreal toads in the Colorado Rockies. But even in these cases, Daszek suspects people are ultimately to blame. One possibility is that global warming, which has increased the number of dry days in tropical cloud forests, concentrated water-loving frogs in too small a habitat, sparking disease outbreaks similar to those rampant among waterfowl.