Toxic prey

Household rodent poison is working its way into the environment, killing and injuring more wildlife than previously thought, newly released research in California shows.

Recent tests on dead animals revealed traces of poison intended for mice, rats and squirrels are commonly found in predators that consume the critters.

The victims of vermin control include some of the West’s most charismatic creatures: raccoons, bobcats, coyotes, mountain lions and the endangered San Joaquin kit fox as well as hawks, eagles and owls.

In the survey, 73% of the predators examined between 1994 and 2003 tested positive for anti-coagulant, the active ingredient in many over-the-counter poisons. Anti-coagulants prevent blood clotting and cause animals to bleed to death internally.

The sample of 123 carcasses included mostly collared wild predators involved in field research and raptors felled by windmills in Altamont Pass in the Bay Area, according to the California Department of Fish and Game.

“The number of animals being exposed we’re certain is grossly underreported,” says Bob Hosea, a pesticide toxicologist with the state Office of Spill Prevention and Response.

Preliminary findings from another long-term study link bobcat mortality to poisoning. The results reveal a significant increase in the number of bobcats dying of mange over the last five years, said Seth Riley, wildlife ecologist for the National Park Service.

“We didn’t see any of this in the first six years of the study and it appears there is a strong relationship between [the bobcats] dying of mange and having anti-coagulant in their livers,” Riley said.

Researchers say the damage is not limited to California and the West. Similar findings have come from New York and the widespread use of the poisons probably points to a national problem.

Many of the victims are species most adapted to living on the fringe of human development. But their proximity to subdivisions, strip malls and highways also puts them in the path of homeowners, ranchers and farmers who rely on rodenticides to control pests.

Domestic pets die, too, but internal bleeding leaves no signs of trauma, researchers say.

As some rodents build immunities to poisons, a new generation of more potent formulations is entering the market.

Established chemicals such as warfarin required repeated doses to reach a lethal level, but a single dose of brodifacoum -- the most common and most toxic of the anti-coagulants -- is sufficient now.

However, it takes a rodent up to 10 days to die after exposure, and because the animals typically go back for multiple feedings of poisoned bait, their bodies can become highly toxic by the time predators eat them.

Brodifacoum is the active ingredient in the widely used product, d-Con. It is one of the anti-coagulants registered for use in California; others include bromadiolone, diphacinone, chlorophacinone and difethialone. About 10 million pounds of anti-coagulants are sold in California annually, according to Syngenta International, a chemical manufacturer.

“In our opinion, they [anti-coagulants] are too dangerous to be allowed for use by a non-licensed applicator. That’s really the bottom line,” says Gavin Shire of the Virginia-based American Bird Conservancy.

Dale Kaukeinen of Syngenta acknowledges harm to wildlife, but says it’s an unintended consequence of trying to protect human health.

“We fully accept the fact and believe the evidence that there is contamination [to wildlife]. But what no one wants to do is to leave the country without tools to control rodents because of the diseases they spread and the damage they cause,” Kaukeinen says.

The Environmental Protection Agency began reevaluating registration of all rodenticides about six years ago as required by law. The agency is scheduled to release a preliminary analysis for public comment this summer.

“The question arises how significant is this to the ecosystem?” says UC Davis ecologist Rex Marsh. “I don’t think the risk is great enough to consider banning a highly effective rodenticide.”