Plague once struck mortal fear in humans, but it's a thing of the past, right? Not so, according to research published in a special issue of the journal Vector-Borne and Zoonotic Diseases, which focused solely on plague.
Two of the papers in the issue were co-authored by a U.S. Geopogical Survey (USGS) researcher, and they reveal that plague has hindered recovery of the critically endangered black-footed ferret and several species of prairie dogs (including the threatened Utah prairie dog), and it's not just an episodic problem.

Plague's impact on other wildlife populations is cause for concern, Dean Biggins, USGS wildlife biologist and co-author of the papers, said in a USGS press release:

    The impacts of plague on mammal populations remain unknown for all but a few species, but the impact on those species we have studied raises alarms as well as important questions about how plague might be affecting conservation efforts in general.

According to the USGS, plague–a bacterial disease carried by fleas–arrived in North America in the late 1800s. As it spreads across an area, it devastates wildlife populations and can infect humans.

The problem with plague is it's difficult to detect unless there is a large death toll among a species, Biggins says:

    The overall difficulty of detecting plague in the absence of a large-scale die-off serves as a warning for those dedicated to wildlife conservation and human health. Hazards from plague may exist even where there have never been epidemics that caused widespread and readily detectable levels of mortality among local rodents such as prairie dogs.

As an example, the USGS cites the death of a National Park Service employee in Arizona who had contracted plague after finding a dead cougar which had been infected with plague.

Plague Persists in the Wild
The two research papers supported the idea that plague not only persists in the wild, but it is necessary to consider the impact of plague in conservation efforts.

The prairie dog research looked at how plague persists between outbreaks. Researchers theorized that plague remains and “is transmitted at low rates among highly susceptible individuals within and between their colonies,” the abstract states. The researchers tested populations after reducing flea numbers and concluded that their research suggests plague persists in prairie dog populations regardless of outbreaks.

In the ferret study, researchers provided flea control in one area as well as flea vaccination to individuals in both the flea-controlled and non-flea-controlled areas. The results revealed vaccination and flea control both increased the re-encounter rate, suggesting that plague is a factor in ferret survival rates.

While prairie dogs are a threatened species, the black-footed ferret is highly endangered, so the research into the impact of plague on the species could be vital to recovery efforts.