The Surprising Effects of Rat Poison in Bobcats

February 3, 2022

In this blog, Small Cats Program Project Coordinator Laurel Serieys takes us through her research on the impact that rat poisons have on bobcats. Learn about how these poisons are having detrimental effects on bobcats in the Santa Monica Mountains in southern California, and what has been done to change this.

Rat poisons don’t just kill rats; they kill wildlife too. Wildlife species are exposed to anticoagulant rat poisons when the poisons are used in urban and agricultural areas to target species such as rodents. But those who consume the poisons do not die immediately of the internal bleeding they are intended to cause. It can take more than a week for a poisoned rodent to die. In the meantime, the poisoned animal may be vulnerable to predators, and if a predator such as a bobcat preys on the poisoned rodent, the bobcat becomes poisoned too. Thus, poisons enter local food webs and become especially harmful to animals at the top of the food chain. Through a process called bioaccumulation, animals at the top of the food chain absorb toxins from eating lots of different prey animals, but their organs cannot filter out the toxins, causing the poisons to accumulate in their systems. As a result, some of the most iconic species in California – bobcats, coyotes, foxes, mountain lions, owls – are the most vulnerable to this indirect poisoning.  

The potential consequences of rat poison exposure on carnivores in the Santa Monica Mountains in Los Angeles has been the foremost question of researchers working at the National Park Service and the University of California, Los Angeles (UCLA) since 2002. I joined this team of biologists in 2006 as a wildlife intern with the National Park Service in the Santa Monica Mountains. During this internship, I witnessed firsthand the ramifications poision has on wildlife. Later that year, I was given the opportunity to earn a P.h.D. investigating the effect of rat poisons on bobcats. I could see even then that biologists and conservationists alike had no grasp of just how dangerous indiscriminate use of rat poisons is for wildlife.

By the time I came on board, there were a few things we knew. For example, the effect of the anticoagulant rat poison chemicals could vary widely among different animal species that may ingest the poisons. Domestic dogs are 100 times more vulnerable to the toxic effects of these poisons than domestic cats. Would the same trend hold for wild dog species (coyotes, foxes) compared to wild cat species such as bobcats and mountain lions? Data suggested the trend was the same: anticoagulant poisoning was a leading cause of death in coyotes in the Los Angeles area, but at the time, National Park Service biologists documented only one bobcat and two mountain lion deaths directly due to poisoning.

Another important trend was emerging for bobcats. Bobcats were dying at unprecedented rates due to uncontrollable outbreaks of mange. It was beginning to look like the population might not survive. Mange is a common skin parasite that was previously rarely fatal to bobcats because their immune systems can suppress the infection. Something seemed to be preventing the bobcats’ immune systems from doing its job effectively. Interestingly, veterinary pathologists discovered that every bobcat that died of mange had high levels of anticoagulant exposure. The pathologists suggested that mange and anticoagulants may be linked…but they could not explain how rat poisons would cause a common infection such as mange to become fatal. 

As I began studying these animals for my PhD National Park Service biologists and I proposed a hypothesis. Like domestic cats, bobcats may be tolerant to the anticoagulant effects of the rat poisons, but that low-level chronic exposure to the poisons had other weakening effects in bobcats, specifically in the way their immune systems functioned. These invisible effects likely increased their susceptibility to mange. To test this hypothesis, we would need to know how many bobcats were actually being exposed to the poisons across the Santa Monica Mountains. Then, we needed to know if there were measurable health effects that explained the link between anticoagulant rodenticides and fatal mange infections.

We faced two problems. First, we needed to test the immune systems of enough bobcats to understand how their immune systems work before and after they are exposed to rat poisons. That meant spending months capturing animals all over the Santa Monica Mountains and collecting blood and other samples to test in the laboratory. The second problem was more intractable: no one had invented laboratory tests to study the immune function of bobcats. I had to develop our own methods.  

The study involved trapping more than 125 bobcats from across the Santa Monica Mountains. I worked with researchers from all over the country to devise new ways of measuring bobcat immune function. With these new methods, we generated detailed health and genetic profiles of each bobcat we captured. With help from the NPS and US Geological Survey biologists, I also collected 172 bobcats that died of mange or vehicle collision so that I could collect liver samples which are the preferred samples for anticoagulant rat poison testing (the compounds accumulate in the liver over time).  With these samples, I then assessed prevalence of and risk factors for anticoagulant exposure in bobcats.  

The findings confirmed my worst fears. We detected that 89% of bobcats were exposed to the poisons when we used the liver samples! By comparison, we detected only 34% exposure using blood samples from the live-trapped animals (we can’t take liver samples of live animals without harming them). This difference in exposure detection reflects that the poisons are in the blood of an animal for a considerably short period of time before it is filtered from the blood stream by the liver, where the compounds can then persist for more than a year! I also found that just looking at how close a bobcat lived to residential and other urban development was the single best predictor of whether that bobcat had been exposed to rat poison. This finding indicated that poison use around residential areas was the primary contributor to environmental contamination. Finally, we discovered that bobcats that were exposed to multiple types of anticoagulant rat poisons were more than 7 times more likely to die of mange than any other source of mortality. 

Next, we used the health profile data we generated from live trapped animals that were both exposed and unexposed to the poisons. We wanted to know if we could find any evidence of the poisons altering immune function such that bobcats may become more vulnerable to deadly mange infection. Unexpectedly, we found explicit evidence that the poisons suppress certain types of immune cells, while stimulating others! In other words, their immune system has to work harder but cannot work as effectively as the immune system of unexposed bobcats. More recently, UCLA colleagues used some of the blood samples I collected to probe whether this immune dysfunction was expressed on a genetic level. The work supported my initial findings; anticoagulants promote immune dysfunction in bobcats. They also discovered other compelling links; anticoagulants change the level of expression of genes associated with skin maintenance and wound healing. Overall, it is becoming increasingly clear that even when these poisons don’t kill wildlife directly, they still have consequences that may indirectly kill animals by making them vulnerable to other stressors such as disease.  

While discovering how widespread exposure was in bobcats in the Santa Monica Mountains, and the alarming apparent consequences of exposure, the work led to some positive changes. Through making the findings of our research publicly available, activist groups, including Raptors Are the Solution (RATS), were motivated to take up the cause to get these poisons banned across the state. As a result, there have been legislative changes in California regarding the consumer availability of some of the most toxic of these rat poisons. Four of seven commercially available compounds have been banned, which will work towards protecting untold mammalian and avian species. California is the first state to ban the most toxic of these compounds, and RATS is now working in other states to try to reduce rat poison use and availability on a nationwide scale.  

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