Plague conjures images of Gothic horror—rough wooden carts piled high with pestilent bodies—but it is more than a medieval memory. The disease, caused by the bacterium Yersinia pestis, kills several hundred people every year by attacking the lungs, lymph nodes or blood. Less obviously, plague also ravages wildlife around the world.
Introduced to the U.S. a century ago, it is creeping into the upper Midwest, wiping out prairie dogs and threatening the black-footed ferret, one of North America’s rarest species. Confined to rural regions, the disease so far is not a major threat to people—only a few Americans die from it annually. But things could change if the bacterium spreads to urban-loving rodents such as rats. Now some researchers think that another species could provide the information needed to contain plague’s spread in the U.S.: the giant gerbils of Kazakhstan.
Inhabitants of the vast steppes of Central Asia, the gerbils grow to one foot in length. They are natural hosts for Yersinia, and many researchers believe that the plague bacterium, carried by fleas hitching rides on the Mongols centuries ago, spread from these gerbils. Until World War II, plague killed scores of people every year in Kazakhstan. “Whole villages were being wiped out,” recounts Stephen Davis, an Australian researcher who recently joined Yale University’s School of Public Health.
The former Soviet Union, which controlled the region at the time, cracked down on the disease: beginning in 1949, it sent teams into the steppe to collect gerbils and fleas to determine the extent of the outbreaks. They fumigated infected burrows with insecticide, killing the fleas but sparing the gerbils. Plague fatalities among humans dropped to a few cases a year. (Antibiotics can cure infected individuals if they are treated promptly.) The control program has continued relatively unchanged to this day, although funding from the Kazakh government has tailed off in recent years.
All these data have become a rich resource, says Michael Begon, an ecologist at the University of Liverpool in England. The archive, housed by the Kazakh Scientific Center for Quarantine and Zoonotic Diseases in Almaty, came to the attention of Western researchers in 1996, when Herwig Leirs, an ecologist at the University of Antwerp in Belgium, reviewed a funding application from the center. Leirs was astonished. “We said, ‘This is potentially a gold mine for doing research,’ ” he recalls. The archive was “literally in big books, handwritten in thick ledgers,” stored in Almaty and 12 regional stations, he describes. Using a small set of plague data, Davis, Begon, Leirs and others published their initial findings in Science in 2004, namely, that when the gerbil population exceeded a certain threshold, plague outbreaks occur two years later.
Scientists are now focused on developing an early-warning system using this threshold, Begon says. This group includes Davis, who published a paper in Nature last summer that furthered the theoretical understanding of how plague spreads by employing percolation theory. In physics, it can explain, for example, how a fluid spreads through a porous medium—without enough connecting pores, the fluid remains in pockets. In epidemiology, percolation theory can model how disease spreads in situations that do not have random mixing, such as the widely spaced burrows of the gerbils.
The fixed nature of the burrows means that not every infected site may have to be fumigated. Such a targeted approach could require less funding—perfect for Central Asia and potentially useful information about the spread in other rodents, such as prairie dogs.
The U.S. has had little endemic pockets of plague since 1898, when it arrived from Asia. In the past two years, plague outbreaks have pushed into South Dakota, says Christopher Brand of the National Wildlife Health Center, who coordinated a November symposium on the issue. Having no natural defense, prairie dogs are especially vulnerable to plague; the mortality rate hovers around 90 percent. Already the disease has killed one third of the prairie dog population in South Dakota’s Conata Basin.
Conservationists are in particular worried about plague’s effect on the black-footed ferret, an endangered species that preys on prairie dogs, its primary food source. Desperate to save North America’s only native ferret, the U.S. Fish and Wildlife Service is spraying prairie dog burrows with insecticide. The service has also begun a capture-and-release vaccination program for the ferrets.
These labor-intensive tasks could cost less if the threshold model derived from the giant gerbils holds up; Kazakh scientists are now testing the concept. Davis says he is “quite encouraged” by the similarity in data between the prairie dogs and the gerbils. To further compare notes, he and other concerned researchers plan to meet in Kazakhstan this spring, as the harsh winter thaws and the gerbils emerge from their burrows.
Note: This article was originally printed with the title, "Plague in the Prairie".