Banishing Plague from the Prairie

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Vaccinating prairie dogs may be the key to saving rare black-footed ferrets

By Sharon Oosthoek

© Wendy Shattil and Bob Rozinski

© Wendy Shattil and Bob Rozinski
Behind the brick walls of the National Wildlife Health Center, past security doors leading to an isolation room, black-tailed prairie dogs dine on peanut-butter-flavored pellets. These tan-colored rodents with black-tipped tails were captured near Wall, South Dakota, and now live in burrows of stainless steel boxes connected by plastic pipes. Normally, they would be eating alfalfa pellets, carrots and broccoli. But on this summer day in Madison, Wisconsin, the only thing on the menu is peanut butter snacks, served up by the center’s scientists.

The prairie dogs—13 in all—gobble up the new offering and that’s good news because the pellets contain a vaccine against plague. Three weeks from now they will be exposed to Yersinia pestis, the bacterium that causes the deadly disease. Remarkably, nine of them will live.

The vaccine-laden pellets are the handiwork of U.S. Geological Survey (USGS) epizootiologist Tonie Rocke, who believes they hold the key to saving an animal once thought extinct.

The imperiled animal is not a prairie dog, but a black-footed ferret—a sleek, cream-colored creature about the size of a housecat, with black feet and tail tip. It is a member of the weasel family and the only ferret native to North America. This endangered rodent eats prairie dogs almost exclusively and uses their burrows for shelter. Without prairie dogs, black-footed ferrets haven’t a hope.

But both critters are extremely susceptible to plague—the same plague that killed millions of people across Europe in the 1300s. It invades the lymph nodes, blood or lungs, killing within days. Called sylvatic plague when found in wildlife, it has cut a swath through black-footed ferret and prairie dog colonies across North America.

“It’s always surprising to people to find out we have plague in this country,” says Rocke. In fact, Yersinia pestis has been killing people and animals since the turn of the last century, after it hitchhiked here on flea-infested rats aboard ships in San Francisco’s harbor. But these days, the bacterium is mostly a problem for wildlife.

Humans no longer have much contact with infected fleas. According to the Centers for Disease Control and Prevention, there is an average of 13 cases of human plague every year in the United States. Caught in time, antibiotics will cure it.

But among prairie dogs and black-footed ferrets, which have far more contact with fleas and no access to antibiotics, outbreaks spread with lethal speed, says Randy Matchett, a U.S. Fish and Wildlife Service biologist at the Charles M. Russell National Wildlife Refuge in Montana.

“Back in 1992, I watched a prairie dog colony on our national wildlife refuge go from 1,300 acres to nine acres in three weeks,” he says. “And in clinical trials of exposing ferrets to plague, they are pretty much dead within three days.”

That’s a big deal, given that out of tens of thousands of black-footed ferrets that once lived across North America’ prairies, there are only about 800 left. Every single black-footed ferret alive today is a descendant of a small population discovered near Meeteetse, Wyoming, in 1981, after a rancher’s dog deposited a dead ferret on his owner’s door step. When disease struck this last population of about 130 ferrets in 1985, USGS biologists, correctly believing they would never find others, gathered up the 18 remaining creatures for captive breeding. Just seven of those were able to reproduce.

For at least 20 years before that, the animals had teetered on the edge of extinction. By the 1960s, only a small colony in South Dakota remained, and after it died out in the mid 1970s, the species was thought extinct.

The ferrets are victims of drastic declines in their prairie dog prey. A healthy ferret population needs roughly 10,000 acres of prairie dog colonies. But entire prairie dog colonies were wiped out in the early part of the last century with the widespread conversion of prairie to farmland.

Considered a farm pest because they eat grass that could otherwise feed livestock, prairie dogs were also poisoned or shot. Many still are. (For a hopeful exception to that trend, see the sidebar.) Disease, such as plague, wiped out many of the remaining colonies and today, prairie dogs occupy less than 2 percent of their original habitat.

Since plague is the most immediate threat to their survival—and by extension to the survival of black-footed ferrets—it is essential that prairie dogs be vaccinated, says Rocke. Black-footed ferrets raised at the six captive-breeding facilities across North America are vaccinated before being released into the wild. But it does them no good if their prey dies.

That much was made clear when ferrets at Russell wildlife refuge survived the 1992 plague outbreak. “We protected the ferrets from plague because they were vaccinated so that they could starve to death when plague killed their prey base,” says Matchett, sarcastically.

That’s where Rocke and her peanut butter pellets come in. She has spent the past 10 years perfecting an oral vaccine for prairie dogs—the first version used mashed sweet potato molded into Jell-O-like cubes. Once Rocke proved the vaccine worked, she then set her sights on creating bait that wouldn’t dissolve in the rain and could easily be distributed in the wild. “It’s pretty hard to drop Jell-O from a plane,” jokes Rocke.

The newest version uses a hard peanut-butter-flavored pellet, about the size of a marble. Rocke is now applying for government permission to launch field tests of the pellets, which she hopes to begin in about two years.

“A lot of people think, ‘Big deal, prairie dogs are a pest.’ But they are very important to the ecosystem. They’re eaten by almost everything,” she says. “It isn’t just mammalian predators that need prairie dogs—it’s even birds like the mountain plover that totally depend on prairie dogs for their burrows. They tend to go away if prairie dogs go away.”

Now some experts suspect there may be even more at stake. USGS wildlife biologist Dean Biggins says plague may be to blame for declines of other species of concern.

The problem, says Biggins, is that plague only makes it onto the radar during large-scale epidemics. But a recent study he conducted with Matchett shows plague can “simmer along” between outbreaks, spreading slowly but fatally.

“Pygmy rabbits and pikas and marmots—some of these animals we haven’t thought much about (in terms of plague),” says Biggins. “But they’ve declined and it’s starting to make me wonder if plague isn’t responsible but it’s slipped by us.”

While Rocke understands plague’s potential implications for other wildlife, for now she is focused on the black-footed ferrets living at 19 reintroduction sites in the United States and Canada. The animals are spread across public, private and tribal lands in their former habitat throughout the Great Plains and the intermountain West. Defenders of Wildlife is heavily involved at many of these sites, offering expertise and providing funding.

Defenders is especially focused on South Dakota’s Conata Basin site, which was—until recently—home to the world’s most successful black-footed ferret reintroduction program. The basin is a 73,000-acre area within the Buffalo Gap National Grassland and in early 2008, about 30,000 acres of it were occupied by prairie dogs. But plague broke out in May of that year, killing prairie dogs in about half the area.

It also killed a third of the basin’s black-footed ferrets. At last count—the fall of 2009—there were just 200 ferrets left.

Shortly after plague was discovered in the basin, the U.S. Forest Service, the U.S. Fish and Wildlife Service and the National Park Service began dusting prairie dog burrows with pesticide powder to kill plague-infested fleas. It is currently the only way to control the spread of plague. Defenders of Wildlife pitched in, buying powder and dusters and donating them to the $300,000-annual Conata Basin ferret program.

In 2008 and 2009, ferret biologist Travis Livieri was part of the team that dusted a half-million prairie dog burrows, a hugely labor-intensive undertaking. “Picture getting on a four-wheeler and driving up to every single prairie dog burrow, sticking a wand down the side of it, spreading five grams of dust in each burrow and doing that half a million times,” says Livieri, executive director of Colorado-based Prairie Wildlife Research, a nonprofit group he founded in 2001 to conserve wildlife on the prairie.

It’s little wonder that Livieri would like to see Rocke’s oral vaccine for prairie dogs approved sooner rather than later. But plague control doesn’t stop there. It must be paired with an injectible vaccine for black-footed ferrets, also designed by Rocke.

The vaccine is based on one developed by the U.S. Army for soldiers deployed in regions where plague is a problem. Ferrets need the injectable vaccine because bait-based vaccines aren’t as effective for them, just as injectable vaccines don’t work well in prairie dogs.

However, making sure ferrets get their needle is a lot of work. While in captivity, it is easy enough to trap and inoculate them. But those born to parents reintroduced into the wild are more difficult to get at.

That means on any given spring night, Livieri can be found bouncing around the basin in his truck, shining a roof-mounted spotlight across the dark prairie. The light reveals the nocturnal ferrets’ green eye shine, and hence their location.

Once Livieri has them in his sights, he hurries to the burrow and places a cage over the entrance. When he returns an hour later, he has his ferret. Livieri then settles down in the beam of his truck’s headlights with the cage on his lap and a syringe in his hand.

“Then I gently pull up a ball of skin through the cage in my left hand, and with the syringe in my right hand, I pop it under the skin and give them half a milliliter of plague vaccine,” says Livieri, who has vaccinated more of these creatures than anyone else.

Unlike the oral vaccine, the injectible version requires a booster a month later. So Livieri keeps track of which ferrets need a first vaccine and which require a booster with the help of hair dye: Clairol Nice and Easy natural black #122. After the ferret’s first needle, Livieri marks the animal between the ear and shoulder with a Q-tip dipped in dye. When he returns to the area a month later, the black stripe will tell him the ferret needs a booster.

Livieri is willing to go through all of this because he believes it will work. “I have a lot of hope we are going to recover the black-footed ferret,” he says. “We have tools available to us now and tools on the horizon that are going to give us an even greater chance to save this species.”

And Rocke’s vaccine will be essential to that effort, says Jonathan Proctor, Defenders of Wildlife’s Rocky Mountain region representative.

“Defenders of Wildlife and many others have spent a lot of time and money over the last 30 years to save the black-footed ferret from extinction, but without a successful plague vaccine for both prairie dogs and ferrets it may all be for naught,” says Proctor. “This work is absolutely critical to save our nation’s investment in black-footed ferret recovery.”

Back in Madison, Rocke is now perfecting her oral vaccine. She is testing the pellets’ safety profile on non-target animals, in case they like peanut butter as much as prairie dogs do. Rocke also has to work out the pellets’ ideal size and how many of them need to be spread across a given area to be effective.

So far, the vaccination program has cost hundreds of thousands of dollars, says Rocke, and she estimates by the time it wraps up it will have cost close to $1 million.

That’s a lot of money, admits Livieri. “I’m not going to trivialize it,” he says. “I think most of America would be willing to support the project if they knew what was going on. This is an amazing story. People tend to be captured by this animal once they learn more about it. It’s a charismatic little creature.”

Sharon Oosthoek is a freelance journalist who writes about the environment from her home in Toronto.

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