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Despite Ban, Rat Poisons Still Sickening Mountain Lions

Mountain lion P41
Mountain lion P41 | Photo: National Park Service

California’s mountain lions are still being exposed to deadly second-generation rat poisons despite a 2014 ban on those rodenticides being sold over the counter. That’s the upshot of a California Department of Fish and Wildlife database of mountain lion mortalities that tallies the results of liver tissue samples from 68 dead lions delivered to CDFW between November 2015 and December 2016.

The CDFW tested liver samples from the lions for the presence of seven different anticoagulant rat poisons. Of the 68 on the list, only five showed no evidence of exposure to any of the seven rodenticides tested for.

Of the remaining 63 pumas, all but four had detectable amounts of more than one rodenticide in their livers, with 43 showing at least traces of three or more anticoagulants. One unlucky big cat, which died in March 2016 in Orange County, had significant amounts of six different rodenticides in its liver tissue.

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The database doesn’t include all mountain lions that died in the state during 2016, many of which likely either escaped discovery or were too far decomposed when found for reliable test results. Other animals were given post-mortems by other agencies such as the National Park Service and are not included in the total here.

The anticoagulant rodenticides for which CDFW tested are the second-generation anticoagulants Brodifacoum, Bromadiolone, Difethialone, Difenacoum, Chlorophacinone, and Diphacinone, and the first-generation anticoagulants Warfarin and Coumatetralyl. All these rodenticides work by inhibiting blood clotting, which leads to internal bleeding and subsequent dehydration in animals that consume the poisons. First-generation anticoagulant rodenticides  have lost favor among exterminators because rodents have developed tolerance to them. But the newer, second-generation rodenticides are not easily broken down by their victims’ livers. Instead, they can accumulate to many times a lethal dose as an animal eats several helpings of the slow-acting poisons over the course of a week or more. The rodenticides then likewise accumulate in the bodies of predators and scavengers that eat the poisoned mice, rats, or other animals.

Mountain Lion P22 with mange
Griffith Park's P22 with mange. Recapture and prompt veterinary treatment helped him recover, for now. | Photo: National Park Service

That’s a problem for mountain lions, as well as many other wild animals. Even at doses too low to kill a mountain lion directly, anticoagulant rat poisons can cause other health problems that can themselves become life-threatening. Those subsidiary ill effects include both behavioral changes — potentially causing less caution when crossing highways, for instance — or, as in the case of Griffith Park’s renowned P-22, severe mange.

The CDFW database doesn’t record causes of death for the mountain lions listed there, so it’s not certain whether the lions succumbed to poisoning or some other harm, such as roadkill or illegal hunting. But the presence of anticoagulant rodenticides in an overwhelming majority of the pumas sampled suggests that partial ban on rat poison sales enacted in 2014 by the state’s Department of Pesticide Regulation isn’t working.

In fact, numbers of wild animals found to have been exposed to rat poisons — including species other than mountain lions — have climbed rather than fallen since the ban, according to the Bay Area-based group Raptors Are The Solution (RATS). That group analyzed data provided by CDFW and determined that while 68 wild animals were found dead with detectable levels of rat poison in the year before the 2014 ban went into effect, that count rose to 114 animals in the second year of the ban. Animals counted in those figures include coyotes, foxes (including the Endangered San Joaquin kit fox), hawks, raccoons, black bears, and fishers.

The likely reason for the ban’s failure to limit wildlife poisonings is that the 2014 regulation change was limited solely to retail sales, with licensed pesticide applicators exempted. That means anyone in possession of a Qualified Applicator’s License — which can be obtained after a home study course and an exam costing $80 — can buy any of the banned pesticides from wholesalers. Exterminators, farmers, landscaping companies, and real estate managers can thus apply those rodenticides with just a moderate investment.

For that reason, groups such as RATS are exploring ways to extend the ban to qualified applicators as well as retail customers. RATS is pinning its hopes on a bill written by Assembly Member Richard Bloom and State Senator Fran Pavley that would ban professional applicators from using second-generation rodenticides. 

“A mouse doesn’t care whose rat poison it’s eating,” says Lisa Owens-Viani, Director of RATS. “And a hawk doesn’t care whose poison the mouse ate. It’s clear that this loophole has kept the ban from working. Changes in our behavior, like not allowing trash bins to overflow, can address mouse and rat problems without putting our wildlife at risk.”

For ongoing environmental coverage in March 2017 and afterward, please visit our show Earth Focus, or browse Redefine for historic material.
KCET's award-winning environment news project Redefine ran from July 2012 through February 2017.

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