Rat Poisons Putting Predators at Risk

by Laurel Klein

Anticoagulant rat poisons—the number one method of rodent control used worldwide—may take a week or more to kill rodents. During this interim, the rodents continue to move around, leaving predatory wildlife species at risk of consuming poisoned prey. Despite the widespread use of anticoagulants in commercial and residential areas, little is known about the indirect effects of sublethal, chronic exposure of wildlife to these poisons.

In Southern California, National Park Service (NPS) and US Geological Survey (USGS) biologists have found widespread exposure of bobcats, mountain lions, and coyotes to anticoagulant rodenticides. Through the testing of livers, 92 percent of bobcats (n=48), 87 percent of mountain lions (n=8), and 83 percent of coyotes (n=24)1 tested positively for anticoagulant rodenticides. Further, NPS and USGS have documented that more than 70 percent of the animals tested for anticoagulants in Los Angeles, Ventura, and Orange counties were exposed to several different compounds, suggesting multiple exposure events.

The biologists have also documented an epidemic outbreak of notoedric mange in bobcats in these three counties highly associated with anticoagulant exposure. Notoedric mange is caused by an external parasitic mite and has generally only been reported in isolated cases in wild felids. Necropsy results of bobcats who died after contracting mange revealed that mortality was not due to direct anticoagulant toxicity, but rather from the mange itself. However, mange and anticoagulant exposure were highly associated: All 39 bobcats with advanced mange tested positive for anticoagulant compounds.

Mange is typically suggestive of an immune compromised state, and so we hypothesize that sublethal, chronic anticoagulant exposure compromises bobcat immunity, increasing their susceptibility to the disease. As a UCLA PhD student and NPS collaborator, this hypothesis is central to my dissertation research. My work focuses on how urban development can increase disease susceptibility in bobcats at the urban-wildlife interface. I am particularly interested in the effects of anticoagulants. To understand the sublethal impacts of these poisons on bobcats, I have been trapping bobcats in areas in and around Los Angeles. In order to trap bobcats, we use cage traps that we check twice daily to ensure that no animals are injured during the course of the research. We collect samples at the site of capture for a variety of assays that will help us understand whether sublethal chronic exposure to anticoagulants affects the health of bobcats, potentially increasing their susceptibility to disease.

Recently, I trapped and released 20 bobcats in areas around Beverly Hills, Bel Air, and the Hollywood Hills. Of these 20 cats, five had mange and three required rehabilitation. While it is reassuring to find so many bobcats in such urban regions, these data confirm that a mange outbreak is affecting multiple bobcat populations around Los Angeles. As this research goes forward, blood samples from these animals will be tested to determine if anticoagulant exposure is also linked with these cases of mange. Documenting the prevalence of exposure to these poisons and their potential sublethal effects may help spur the implementation of rodent control measures that pose fewer risks to predator species.

This study was funded by the Christine Stevens Wildlife Awards program. To learn more about this program or to view additional studies, click here.

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