I just spent the past three days attending a conference on global carnivore conservation, and one of the talks that truly piqued my interest centered on a case study of the effects of plague on black-footed ferret restoration. Travis Livieri of Prairie Wildlife Research presented detailed evidence of the first incursion of plague into a colony of endangered black-footed ferrets (Mustela nigripes) and its effects.
Black-footed ferrets are slinky, nocturnal mustelids with an elongated torso and short legs. They have a creamy, dust-colored coat that turns to a smokey gray down their back, and jet black on their legs. Black streaks their face like a robbers mask. They faced extinction about three decades ago, but a successful captive-breeding program produced enough individuals that the Fish and Wildlife Service began reintroducing them to grassland sites in the Western U.S. in 1991. Today, there are 16 reintroduction sites in eight western states, plus one site in Grasslands National Park in Saskatchewan, Canada and one in northern Mexico. (Check out this website for some neat black-footed ferret research videos.)
Certain background levels of plague (Yersinia pestis) are known to exist in the western U.S., but the first instance of it in South Dakota was recorded in 2005, and three years later the gram-negative bacteria made its way into a colony of prairie dogs living in Conata Basin which is also one of the largest reintroduction sites. The Basin is a mixed-grass prairie system that supports about 300 of the slinky ferrets in a self-sustaining population that produces about 75 liters of kits per year. The ferrets feed on the extensive prairie dog colonies that undergird the grasslands.
When the plague hit, it entered the Basin in a southern portion of the park and over a matter of months it killed off the majority of prairie dogs in a 15,000 acre swath. It appeared in unpredictable and random places, boiling up in islands well beyond the line of its initial advance. Livieri estimated that 10 prairie dogs per acre were lost, or about 150,000 animals. While his team found some evidence of natural survivors among the infected portions of the prairie dog towns, they found no black-footed ferret survivors. Once infected, the ferrets succumbed within 48 hours. In a matter of months, about 33 percent of the black-footed ferret population in the park was wiped out from plague.
His team pulled together a crafty, expensive and invasive two-tiered management plan. The first step involved “dusting” prairie dog burrows with a poisonous powder to kill the fleas that were acting as a vector for the plague. They spent $313,950, to dust prairie dogs holes over almost a half-million acres within the Conata Basin. If that doesn’t sound extreme, then consider their next step: inoculating black-footed ferrets against the plague using a vaccine devised by the U.S. military for bioterrorism attacks. After performing some testing on captive-bred black-footed ferrets in lab trials, they began trapping and vaccinating the wild animals.
Unfortunately, the vaccine required two shots delivered about one month apart for the best efficacy. They attempted to re-capture vaccinated individuals, but only did so in about a third of the cases. They vaccinated 216 M. nigripes individuals. They found that about 76 percent of the animals that received one shot only showed the appropriate levels of “protective titer” in their blood – meaning, that only one vaccine dose still conferred protection in about three-quarters of the cases. And the animals that received two vaccines doses showed 100 percent protection.
Livieri’s work has important implications for the management of black-footed ferrets. First, future reintroduction sites need to be monitored for the presence of plague, and dusting of prairie dog burrows may need to be done to minimize the flea vector. Second, vaccination of captive-bred individuals needs to be considered before they are set free in the wild, and vaccination of wild-born kits should be considered. Vaccinating imperiled animals is not new. Mexican gray wolf and red wolf pups are routinely monitored and trapped to administer vaccines against rabies and parvovirus. Such intensive and invasive management interventions are unfortunate but necessary steps with extremely small populations of imperiled species. But if plague vaccines become a permanent management protocol of the ferret’s reintroduction, then in the future it will also become increasingly important for wildlife handlers to sanitize their trapping and handling equipment so that they do not inadvertently spread the plague bacterium — or other emergent pathogens — to naiveprairie dog or black-footed ferret colonies.
Based on a talk by Travis Livieri “Mitigating the effects of plague on black-footed ferrets in Conata Basin, South Dakota” on Nov. 18 in Denver, Colorado.