American Scientist

When the American black-footed ferret nearly went extinct in the 1970s, it did not go down alone. Largely unnoticed, the parasites that live off the ferret found themselves deprived of a key host. A breeding program rebuilt the wild population of ferrets to about 1,000, but one of its parasites—a louse—may not have survived the ferret’s near extinction. Parasites on an endangered mammal generally are considered a threat to the host’s health. However, Nyeema Harris, a postdoc at the University of California at Berkeley, says fewer parasite species may not be good in the grand scheme and could even pose a health threat.

Mammals, particularly carnivores, share parasites with people: Nearly one-third of the diseases hosted by North American carnivores are zoonotic, meaning they infect both humans and wildlife. Carnivores host more than half of the 125 emerging zoonotic diseases, according to a 2001 study led by University of Edinburgh’s Sarah Cleaveland.

Although loss of a parasite that infects humans could be beneficial, Harris knew that the loss of one disease decreases overall disease diversity, which influences likelihood of transmission to humans. A parasite may respond to a declining host in three ways, Harris notes: It might (1) go extinct with the host, (2) adapt to a completely novel host, or (3) begin feeding on a more abundant alternate host. Parasites that are highly specialized to a particular host are prone to extinction, giving an advantage to generalists that can hitch rides on various species. As a result, the loss of one host species could change parasite distributions.

Such change could present a new risk for people. “Certainly for zoonotic diseases, where both humans and wildlife species are potential hosts, the loss of wildlife hosts could be particularly problematic,” Harris says, “because the parasites exploiting animal resources may be forced to increase use of humans.” If the number of wild hosts declines, the parasites (and diseases they transmit) can end up in humans instead. For instance, research has shown that the incidence of Lyme disease is higher in places with fewer wild species that host ticks.

In a study recently published in the Proceedings of the Royal Society B with Rob Dunn of North Carolina State University, Harris mapped the distribution of 29 species of carnivores in North America, with more than 1,300 parasite species. Harris used a broad interpretation of the term parasite, including those seen with the naked eye, such as intestinal worms, and those not, such as bacteria. She excluded parasites outside the body, such as lice and ticks, because they vector but do not cause zoonotic disease. Harris wanted to know how patterns of zoonotic disease could change if some carnivores went extinct, so she simulated random species losses across the map. In their model, parasites never evolved to infect a novel host, so they remained only if they could infect another included carnivore. Harris and Dunn’s study is a step toward a broader simulation of more complex host–parasite dynamics, including parasites hosted by noncarnivorous species.

In the model, the proportion of diseases that were zoonotic increased as more carnivores were lost overall. Moreover, the local distributions of zoonotic disease, as well as the extent of human exposure, varied substantially, depending on which carnivores remained. Areas that lost wide-ranging carnivores such as red foxes and coyotes experienced a decreased diversity of zoonotic disease. Such decrease is associated with greater risk of transmission to people, because the zoonotic diseases that are left—like the parasites that survive when their preferred hosts die out—tend to be the generalists that can find another, more available host. Although red foxes and coyotes are not threatened, the findings suggest that local losses of carnivores, which commonly occur, may increase zoonotic disease risk in humans.

Although losing wide-ranging carnivores affects zoonotic parasite distribution in the model, the loss of carnivores of conservation concern—such as the gray wolf—does not. These carnivores have more limited distribution and tend to host fewer specialist parasites and parasite species overall. Because the loss of specialists increases generalist zoonotic parasites, more limited carnivores do not strongly affect zoonotic disease. No one knows whether the diversity of parasites, particularly specialists, has declined in the carnivore species that have become rarer. If it has, then those endangered creatures could affect zoonotic disease risk not shown in the model.

“If you are a parasite, or any organism consuming or relying on another organism, you want to associate with the most abundant resource in the landscape,” Harris says. Unfortunately, the mammals that are most abundant in the landscape are often livestock, pets—or humans. —Katie L. Burke