By Dr. Sonnya Crawford
If you have had a mouse or rat problem, you’ve probably picked up poison at the local hardware store to fix the issue. Unfortunately, rodenticides are killing far more than mice and rats.
The biggest-selling rodenticides, like D-con, are classified as second-generation anticoagulant rodenticides. These chemicals work their way up the food chain, meaning the animals that eat rats and mice end up dying from secondary poisoning. Hawks, owls and cats are the most vulnerable because they feed primarily on rodents.
Because of the potential of resistance to the first-generation rodenticides, companies began manufacturing the stronger second-generation products. Both types work by inhibiting Vitamin K, which prevents blood from clotting. First-generation products require multiple doses to build up high enough levels to cause death. The second-generation products only require one meal to be toxic.
Still, with either generation, the mouse or rat doesn’t die for several days. It can continue to feed on the poison for days and build up super-toxic concentrations of the second-generation anticoagulant. By the time they die, they contain many times the lethal dose and are therefore deadly to scavengers and predators, including pets.
There is no safe place or safe delivery system for second-generation rodenticides. After a rodent ingests the poison, it stumbles around for several days, displaying itself as an especially tempting meal. The predator then suffers lethal or sub-lethal secondary poisoning. Studies have shown that wildlife suffering from mange, internal parasitism or general ill-thrift commonly have sub-lethal doses of anticoagulant poisoning. Secondary wildlife poisonings have been documented in over 25 wildlife species.
California is the only state other than New York that has looked carefully at poisonings of non-target wildlife by second-generation anticoagulants. California found that rodenticides showed up in 79 percent of fishers, 78 percent of mountain lions, 84 percent of San Joaquin kit foxes, and 92 percent of raptors. There is also some concern — and compelling evidence — that these second-generation products are concentrating in the soils and potentially being ingested by worms, which are food for songbirds, voles and other animals.
The Environmental Protection Agency has declared second-generation rodenticides 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.
Prevention is the best form of rodent control, but if you are having rodent problems, the old-fashioned snap traps are still the most humane method. Glue traps are not recommended because they cause undue suffering to the rodent and are a potential danger to reptiles, birds and children.
A newer device, the electronic trap, is battery-powered, delivering an electric shock that kills rodents quickly.
Sonnya Crawford, DVM, is a veterinarian at Grays Harbor Veterinary Services in Montesano. Her pets include two cats, numerous parrots, a giant bunny and saltwater fish. Her special interests are in avian medicine, veterinary dermatology and dentistry. Reach her at drsonnya@gmail.com.
