Plague-riddled prairie dogs a model for infectious disease spread
Every now and then, colonies of prairie dogs are wiped out by plague, an infectious disease most often associated with the Black Death of the 14th century.
Plague doesn't usually kill people these days, but it's alive and well among the millions of ground-dwelling rodents of Colorado and other western states, notably the black-tailed prairie dog. They're resilient critters, though: following wholesale destruction of colonies, they seem to repopulate with a vengeance.
Colorado State University biologists say this sporadic ebb and flow of prairie dog plague is an ideal model for the study of rare infectious zoonotic disease – disease that can jump from wildlife to humans – like MERS (Middle East Respiratory Syndrome) and Ebola.
Plague, in all its terrible forms, is caused by the Yersinia Pestis bacterium, usually spread through flea bites. Last year in Colorado, there were a handful of human cases, including at least two deaths.
A multi-year, CSU-led study that involved trapping and testing thousands of prairie dogs across the Pawnee National Grassland, and tens of thousands of their plague-carrying fleas, was conducted by CSU biologists Daniel Salkeld and Michael Antolin, and is published Jan. 13 in BioScience.
Research scientist Salkeld and Antolin, professor and chair of biology in the College of Natural Sciences, assert that the swirl of ecological factors driving plague outbreaks in prairie dogs can lend key insights into the study of zoonotic diseases. Such diseases, among them Ebola, which swept through west Africa in 2014, are notoriously hard to study. Their outbursts are sporadic at best, making their spread trajectories elusive.
"Plague is deadly – it's not like the common cold. It kills its host," Salkeld said. "It affects different hosts, including rats, prairie dogs and grasshopper mice. It is reasonably rare to watch an outbreak, and doing so can give us insight into other rare diseases like Ebola."
Throughout their analysis of plague in prairie dogs, they concluded that such diseases may "smolder" unnoticed in a population for years, rather than jump from species to species immediately before an outbreak. They also found that investigations occurring after outbreaks can be too shallow or yield false information about which host was chiefly responsible.
In other words, there's no simple transmission model of diseases like Ebola and plague. Throughout their study, Salkeld and Antolin found that grasshopper mice and coyotes that scavenge plague-killed prairie dogs can speed transmission of the disease by spreading the disease-carrying fleas.
They also found that an outbreak in a prairie dog colony might go unnoticed for years, because the animals are dying underground. Furthermore, the mechanism that drives the spread of plague during the the smoldering, unnoticed period might be different than during a full-blown epidemic.
Ecological conditions come in bursts, too – for plague, a cool, wet season is best for the pathogen to proliferate. Prairie dogs are well adapted to surviving drought. "Prairie dogs spread like crazy during drought in Colorado," Antolin said. "Without plague, they would probably spread faster during wet periods."
The parallels with Ebola are without question, Antolin said. Ebola became a pandemic due to a combination of factors, including exposure of the virus in densely populated urban centers with little access to health care and sanitation. Similarly, the Black Death in medieval Europe spread due to things like concentrations of people living with animals; the pathogen was given a pathway to persist.
In the case of Ebola, they argue that the sampling of fruit bats after human outbreaks may have biased subsequent investigations toward bat-Ebola virus ecology, and other, possible host species may have been overlooked.
The researchers hope their study leads to better measures for modeling and predicting infectious disease transmission, but there are still open questions about the human-wildlife interface of disease. In future studies, Salkeld will continue to investigate this question with other human-wildlife diseases, including Lyme disease and Colorado tick fever.
The multi-year effort, supported by the National Science Foundation, was joined by other CSU researchers including evolutionary ecologist Colleen Webb, and mammologist Paul Stapp, a CSU Ph.D. now at Cal State Fullerton. Other partners were the Centers for Disease Control, and the Ranger District for the Pawnee National Grasslands, which provided long-term prairie dog town size and location data.