Species extinctions may be due to disease, not man

Published March 1, 2001

“Biologists usually blame pollution and predation for species extinction, but the real culprits may be humans spreading disease, according to the New Scientist.

In “Sick to Death,” published in the August 5, 2000 issue, Deborah MacKenzie quotes Andy Dobson of Princeton University, “Forgetting disease was a big omission in our thinking about wild populations.” In many instances, that omission has been due, at least in part, to the ready availability of an apparently more sinister culprit to shoulder any blame: chemical pollution.

Peter Daszak, a wildlife disease specialist at the University of Georgia at Athens, helped identify the fungal plague that is killing off the world’s amphibians. “When there is a sudden die-off of wild animals, people usually suspect pollution,” Daszak noted. While an enormous amount of time has been spent pursuing environmental explanations for amphibian deaths, Daszak said diseases are the primary cause of mass mortality.

In India, scientists have been searching for signs of chemical poisoning in dead vultures . . . but until last year no one froze a carcass for pathological examination. Andrew Cunningham of the Zoological Society of London told MacKenzie it’s about time emerging diseases in wildlife got the same respect they do in humans.

Even ordinary pathogens can be dangerous

It is widely believed that infectious diseases are merely part of the natural order, and that damage is rarely permanent when wild populations are invaded by a new infection. A growing body of research, however, suggests that even ordinary pathogens can dramatically alter wildlife populations.

In 1998, Dobson helped to demonstrate that fact with British grouse. Like many temperate species, grouse populations experience boom and bust cycles. Dobson showed that in grouse, the cycle was caused by cyclic variations in gut parasites. Dobson’s work has been pointed to as a possible turning point in thinking about disease.

Although some ecologists and wildlife biologists are beginning to accept the notion that pathogens can cause extinctions, few were prepared for some of the many apparently innocuous forms–such as livestock populations and even pet animals–a pathogen reservoir can take.

In an article in the April 19, 1997 issue of New Scientist, S. Pain reported that domestic ducks harbor duck plague, a herpes virus that causes massive die-offs in wild ducks; African village dogs spread distemper and rabies to lions and wild canids. In 1994, 1,000 lions–one-third the population in Tanzania’s Serengeti National Park–died from canine distemper . . . a dog disease they weren’t supposed to get.

Even feeding birds in your garden can contribute to the problem, since the bird table provides a mixing bowl for droppings and food. Birds congregate at feeders in numbers and a mix of species that would never occur in the wild. According to MacKenzie, the growing popularity of bird-feeding may explain the worsening epidemics of Salmonella and E. coli that have killed thousands of songbirds, especially finches, in Europe and North America.

Exposure to new diseases

One of the reasons wild animals are so much at risk from the diseases of domestic animals is simply unfamiliarity. Humans react to new diseases the same way. History teaches us, for example, that Native American populations were decimated by their new exposure to common European diseases, such as measles, flu, and smallpox, in the early years following the Spanish conquest. Some degree of immunity evolves over long association with a virus or other agent of disease. A population that has never encountered the disease before may have no defense against it.

Animals, like humans, are exposed to more new diseases today than ever before. For one reason, animals and humans alike are much more readily transported across borders these days. Daszak warns that our techniques for providing this transport must be improved. Animals that are reintroduced into the wild should be checked for infection and any deaths investigated thoroughly.

It may even be the case that people who work with animals may be responsible for their exposure to new diseases. Noting that a certain frog-killing fungus had turned up at about the same time in Panama, Australia, and the United States, I. Anderson suggested in the June 27, 1998 issue of New Scientist that the fungus may have been spread by the very herpetologists who spend their lives studying endangered amphibians.

More research is needed

Far fewer studies have been undertaken on how pathogens affect wildlife than have been done with humans and pathogens. Research into wildlife pathogens is usually conducted only when something has produced obvious population losses. In the January 21, 2000 issue of Science, Daszak and colleagues sum it up concisely: “There are few regulations concerning exotic disease threats to wild animals, and a few systems for surveillance are in place. Current measures for the detection and control of human and livestock emerging infectious diseases are inadequate for the identification of similar threats to wildlife.”

In the furor over volatile organic compounds, pesticides, and other chemicals, we tend to forget that infections diseases are in fact the leading killer of human beings worldwide. Dr. Jane Orient notes in the latest edition of McGraw-Hill’s Standard Handbook of Environmental Science, Health, and Technology that more than 740 million infectious disease events occur each year, resulting in 200,000 deaths and more than $17 billion in direct care.

It shouldn’t be surprising that wild animals may have a similar problem. Perhaps if more time and resources were studying infectious diseases in animals, rather than blaming pollution, some relief might be found.

Jack Dini is a scientist and environmental writer in Livermore, California. His email address is [email protected].