Plague Vaccine for Prairie Dogs Could Save Endangered Ferret

• By Hadley Leggett

Wild prairie dogs may soon get a dose of something extra in their daily diet: an oral vaccine against the plague.

The same “Black Death” that devastated Europe during the Middle Ages is still alive and well in wild rodents across the western United States. Although only a few Americans get plague each year, small outbreaks like the one reported Tuesday in Ziketan, China are not uncommon. The disease also regularly wipes out whole colonies of black-tailed prairie dogs and kills one of the most endangered mammals in North America, the black-footed ferret.

Now researchers have developed a vaccine cocktail that can be mixed with food and given to wild prairie dogs. In lab testing, the oral vaccine protected prairie dogs against plague better than a vaccine given by shot. And it was certainly a lot easier to administer.

“An oral vaccine will allow us to deliver it much more efficiently to larger number of animals,” said wildlife biologist Tonie Rocke of the US Geological Survey, who will present the work Wednesday at the Wildlife Disease Association Annual Conference. “We couldn’t capture enough prairie dogs to vaccinate them individually, but because we are putting it in bait, we can broadcast it widely from vehicles or perhaps even planes.”

Scientists want to vaccinate prairie dogs not just to protect the cute, furry rodents, but also to save their endangered predator, the black-footed ferret. Once abundant across the western United States, wild ferrets were thought to be extinct until 1981, when a small population was discovered in Wyoming. A captive breeding program rescued the ferret from the verge of extinction, but scientists estimate only about 1,000 black-footed ferrets are now living in the wild. The animals are highly susceptible to plague and depend on prairie dog colonies for food and shelter: When plague destroys prairie dogs, it also kills endangered ferrets.

Last year, an outbreak of plague decimated more than 9,000 acres of prairie dog habitat in southwestern South Dakota, which was also home to around 300 black-footed ferrets. Researchers tried to protect the ferrets by capturing them and giving them a shot of plague vaccine, but Rocke says individual vaccination isn’t a good long-term strategy.

“Black-footed ferrets only eat prairie dogs,” she said. “So if the prairie dogs die from plague, even if the ferrets are vaccinated, they are left without their food supply.”

Wildlife biologists hope mass immunization of prairie dogs with an oral vaccine will be more effective than trying to capture and give each ferret a shot. Field trials are still needed to test the oral vaccine in large populations of prairie dogs, but lab experiments showed promising results: After getting the oral vaccine, 95 percent of prairie dogs survived infection with Yersinia pestis, the bacteria that causes plague. That’s an impressive success, given that the disease usually has a greater than 90 percent mortality rate.

For the lab experiment, the researchers mixed the vaccine with mashed sweet potato, and the prairie dogs gobbled it up. “But that’s not the kind of bait that we’ll use in the field,” Rocke said. “In the field we’ll need a bait that’s not like Jell-O, that’s a little more resistant to the elements. That’s what we’re working on right now.”

The researchers are also testing the vaccine in other wild animals, including pregnant ones, to make sure that the vaccine won’t have any harmful effects if accidentally ingested by a hungry passer-by.

Although the main goal is to protect ferrets, the plague vaccine might have side benefits for people, Rocke said. Humans usually catch plague from fleas living on infected wild animals, so if fewer animals get the plague, Rocke said, fewer people will be exposed to it. Mass vaccination may also cut down on pesticide use on public lands, because park rangers won’t have to dust prairie dog colonies with flea killer.

Only about a dozen people catch plague in the United States each year, mostly because we have minimal contact with rodents and their fleas. But worldwide, several thousand people contract the disease every year.

“There are parts of the world where it causes disease,” Rocke said. “The plague is still out there.”

CORRECTION: The original article incorrectly stated that the black-footed ferret was once the most abundant mammal in North America. In fact, the black-tailed prairie dog was once the most abundant mammal, but has now declined to 2 percent of its former population.