Poisons Used to Kill Rodents Have Safer Alternatives

Clinical assistant professor Maureen Murray of the Tufts Cummings School of Veterinary Medicine in central Massachusetts was doing a good job of keeping her emotions under wraps as she clicked through photos of her recent necropsies. But I was watching her eyes as well as her computer screen, and they revealed anguish. Like her colleagues here and at similar clinics around the country, Murray is a wildlife advocate as well as a scientist.

Each image was, in her word and my perception, “sadder” than the last. There was the great horned owl with a hematoma running the length of its left wing; the red-tailed hawk’s body cavity glistening with unclotted blood; sundry raptors with pools of blood under dissected skin; the redtail with a hematoma that had ballooned its left eye to 10 times normal size; and, “saddest of all,” the redtail with an egg. The well-developed blood vessels in her oviducts had ruptured, and she had slowly bled to death from the inside.

All these birds were victims of “second-generation anticoagulant rodenticides” used by exterminators, farmers, and homeowners. They’re found in such brand names as d-Con, Hot Shot, Generation, Talon, and Havoc, and they sell briskly because of our consuming hatred of rats and mice. The most pestiferous species are alien to the New World and therefore displace native wildlife; they contaminate our food and spread disease. We also hate them for their beady eyes, their naked tails, and their vile depictions in literature, from Aesop to E.B. White. So the general attitude among the public is “if a little poison’s good, a lot’s better.” But even a little second-generation rodenticide kills nontarget wildlife.

Both first- and second-generation rodenticides prevent blood from clotting by inhibiting vitamin K, though the second-generation products build to higher concentrations in rodents and are therefore more lethal to anything that eats them. The second generation was developed by Imperial Chemical Industries of London at the request of the World Health Organization, because rats appeared to be developing tolerance to warfarin, a first-generation rodenticide.

U.S. Fish and Wildlife Service contaminants specialist Michael Fry makes this point about the widespread use of second-generation rodenticides by people oblivious to the dangers: “One good reason for using first-generation poisons is that if you do have a problem, like developing tolerance, you want a backup. If you go in with your strongest thing first, there’s no backup.”

For a rodent to get a lethal dose from a first-generation rodenticide it has to eat it more than once, but that’s not a problem. Leave first-generation baits out for a week and they’re just as efficient as the second generation. What makes second-generation rodenticides so non-selective is that they kill slowly, so rodents keep eating them long after they’ve ingested a lethal dose. By the time they expire, or are about to, they contain many times the lethal dose and are therefore deadly to predators, scavengers, and pets.

Because they are weapons of mass destruction, second-generation rodenticides are the preferred tool wildlife managers use to restore native ecosystems to rat-infested islands. But the EPA has declared them too dangerous for public use and ordered them off the general market. They’re still widely available, however, because stores have huge stocks and because a recent court decision has allowed three of the largest manufacturers to defy the order.

Many of Murray’s patients don’t have enough red-blood cells to deliver oxygen to their tissues, so they are logy. Their heads droop, the linings of their mouths are pale; some bleed from their eyes, nose, lungs, or other organs. In 2011 she found rodenticides in 86 percent of the raptor livers she examined, and all but one contained brodifacoum, especially deadly to birds. She rehabilitates some patients by injecting them with vitamin K, but the birds still retain rodenticides and are likely to accumulate more after they are released.

There’s no safe place or safe delivery system for second-generation rodenticides. After a rodent partakes, it stumbles around for three to four days, displaying itself as an especially tempting meal not just for raptors but for mammalian predators, including red foxes, gray foxes, endangered San Joaquin kit foxes, swift foxes, coyotes, wolves, raccoons, black bears, skunks, badgers, mountain lions, bobcats, fishers, dogs, and house cats—all of which suffer lethal and sublethal secondary poisoning from eating rodents. Deer, nontarget rodents, waterfowl, waterbirds, shorebirds, songbirds, and children suffer lethal and sublethal poisoning from eating bait directly.

A four-year survey (1999 to 2003) by the Environmental Protection Agency found that at least 25,549 children under age six ingested enough rodenticide to suffer poisoning symptoms. Currently about 15,000 calls per year come in to the Centers for Disease Control from parents whose children have eaten rodenticides. Even if you place bait where children can’t get it, rodents are apt to distribute it around your house and property.

In California, the only state other than New York that has looked carefully, rodenticides showed up in 79 percent of fishers (one fisher even transferred poisons to her kit via her milk), 78 percent of mountain lions, 84 percent of San Joaquin kit foxes, and, in San Diego County, 92 percent of raptors.