Rat Poison May Be Increasing Deadly Disease in California Bobcats

Bobcats in Santa Clara County | Photo: Marcel Holyoak/Flickr/Creative Commons License

Bobcats are dying off in open spaces on the San Francisco Peninsula, and wildlife experts are asking for the help of park volunteers to find out what's to blame -- though they have their suspicions.

Two years ago at Rancho San Antonio County Park near Cupertino, staff from the Midpeninsula Regional Open Space District (MROSD) observed a number of dead or seriously ill bobcats, and sightings of the predator starting to drop at about that time. The park, in the foothills of the rugged Santa Cruz Mountains, has long been prime habitat for the cats, as evidenced by a major geographic feature there having been named "Wildcat Canyon."

Park staff and wildlife fans are understandably concerned about the drop in bobcat numbers, and MROSD has asked its core team of volunteers to help solve the mystery behind the deaths. But there's a prime suspect, which has been implicated in similar deaths elsewhere in California: a terminal parasitic infection spurred by exposure to household rat poison.

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In an email sent out October 23 to the MROSD's Trail Volunteer mailing list, the District's volunteer coordinator Paul McKowan appealed for help collating visitor reports of sick or dead bobcats.

"We are monitoring scat using GPS so that individual bobcats can be more closely observed," McKowan wrote. "A study in the Santa Monica Mountains last year found that the bobcats were dying of Notoedric mange, coupled with chronic exposure to anti-coagulant rat poison. We believe the same combination may be at work here, but we need data and evidence... We are asking for your help in reporting all sightings of bobcats in and near Rancho San Antonio Preserve to staff[.]"

Notoedric mange is a chronic disease caused by the parasitic mite Notoedres cati, with symptoms similar to the sarcoptic mange suffered by dogs -- skin lesions, scabbing, hair loss, and itching. Generally assumed to be introduced into wild cat populations by domestic and feral housecats, notoedric mange was once thought to pose only a minor threat to wild cats.

But as the MROSD email mentions, a serious outbreak of notoedric mange in the Santa Monica Mountains starting in 2002 resulted in a startling number of mortalities, with death rates possibly reaching 50 percent of the local population. Investigations led by biologist Laurel Klein Serieys found that Santa Monica Mountains bobcats who had died of mange all held trace amounts of anticoagulant rat poison in their livers, sometimes at levels far too low to cause outward symptoms of poisoning.

Rat poisons, especially those containing the active ingredient brodifacoum, have increasingly been implicated in the poisonings and deaths of an increasing number of California wildlife species. Used to kill rodents in settings from homes and restaurants to illegal cannabis farms, the poisons persist in the bodies of their victims. When a predatory animal eats a poisoned rodent, it gets the full dose of that rat poison -- and most predators won't just eat one poisoned rat or mouse.

Rat poison affects predators ranging from feral cats to fishers, birds of prey, and coyotes. Bobcats are theoretically as susceptible to bioaccumulative rat poison as any other carnivore, though it's been noted the cats seem to be able to withstand higher doses of the poison than other predators.

But the link between low levels of rat poison and terminal mange is troubling, especially since cats not exposed to the poison seem not to be dying of mange. As Laurel Klein Serieys says on her website Urban Carnivores,

The bottom line is that using our data we have collected since 2002, bobcats are 7 times likely to die of notoedric mange if they are exposed to greater than 0.05 ppm of anticoagulant rat poisons. Further, we have never found a bobcat that died of mange that was not exposed to anticoagulant rat poisons. We therefore hypothesize that chronic exposure to the anticoagulant poisons decreases the immune competence of the bobcats, making them more susceptible to severe cases of mange.

Serieys points out that in addition to the Santa Monica Mountains and Bay Area outbreaks, bobcats with serious cases of mange have been reported from San Diego, Santa Barbara, and Orange counties.

Less-advanced cases of mange in bobcats can be treated with commonly available veterinary anti-parasite drugs, and Serieys' team has taken to giving prophylactic doses to bobcats they capture even if they show no signs of infection. But she points out that without removing rat poisons from the environment, treating mange is a band-aid solution. "One bobcat that had mange that we successfully rehabilitated... died later from mange and was exposed to anticoagulant rat poisons," she writes. "Thus, the treatment of every animal with mange is not going to solve the greater problem of mange in bobcats if it is related to anticoagulant rat poisons."

Groups like the Bay Area-based Raptors Are The Solution are working to press the EPA to ban or reformulate the deadliest of the rat poisons on the market, and to educate the public on the danger rat poisons pose to wildlife, pets, and children. Such campaigns seem to be gaining traction. In the meantime, Bay Area hikers who spot sick bobcats in the MROSD's parks and reserves are encouraged to report the information to the nearest ranger.

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About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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