Rodent pests have inhabited most parts of the world since humans started generating trash. Although mice and rats are capable of living on their own in the wild, they thrive in the presence of humans, eating our leftover, discarded, improperly stored, or spilled foodstuffs.
Mice and rats have been a part of human life since the beginning, and there is no doubt that the desire to remove them from our lives also has been there since the beginning. In recent times a variety of traps and poisons have been developed to keep our homes and lives free of unwanted rodents.
This raises a concern. Might these modern rodent-control measures pose a threat to the original, and by most measures still the best, means that humans have employed to eliminate pests?
Cats of course love to hunt, and mice especially are a favored prey. A mouse that has consumed mouse bait will become weak and subsequently will be especially easy to catch. Even well-fed cats might consume parts of mice that fall prey to them. Will they, in turn, become poisoned?
Concerns of this nature are common enough to have a name: secondary rodenticide exposure. And cats aren’t the only creatures about whom concerns arise. Birds of prey, scavenging animals, and wild felids also may have a proclivity to consume poisoned mice or rats.
Here’s the good news: Although I have treated many hundreds of dogs for rodenticide ingestion, I have yet to knowingly treat a cat for of rodent bait toxicity. (Read on for an explanation of the word knowingly.) Dogs find the bait itself to be palatable and will consume blocks of mouse bait because it tastes good. Cats generally are not drawn to the bait itself. And the risk of secondary exposure, although not zero, appears to be quite low in cats.
However, cat owners should remember that baits differ. Historically, the most common mouse and rat poisons consisted of so-called anticoagulants. The latest generations of anticoagulants are exquisitely toxic. They cause uncontrollable bleeding and slow death. Affected animals become weak (and in the case of rodents, easy to catch). They might bleed internally or externally, and they may cough up blood.
Although I cannot remember ever treating a cat for blood clotting problems linked to ingestion of rodents poisoned with anticoagulants, such problems are possible.
Anticoagulant rodenticides take several days to make rodents sick. During that time, the rodent may continue to feed on the poison, leading to very high levels of the poison in the system. There also might be bait left inside the rodent’s digestive tract at the time that it is caught and consumed.
I furthermore know that the risk of secondary exposure to anticoagulant rodenticides is not merely theoretical from personal experience. Although I’ve never treated a cat or dog for suspected secondary exposure, I did once treat a red-tailed hawk that was believed to have been exposed by consuming a poisoned rodent.
Pets poisoned (directly or secondarily) with anticoagulants usually survive if they are treated in time. Blood product transfusions can stop the bleeding and reverse anemia in the short term, and vitamin K is an antidote to the toxins.
Two other types of rodent poisons historically have been in common use. One, cholecalciferol, is a form of vitamin D that causes an overdose (yes, it is possible to suffer fatal overdoses from consumption of certain vitamins). It causes mineralization of body tissues; the kidneys are the most commonly affected organs. In other words, cholecalciferol causes kidney failure. There is no antidote.
I have never knowingly treated a cat for secondary exposure to cholecalciferol, but I have treated many hundreds, if not thousands, for kidney failure. Although I can’t state with certainty that none of them was poisoned, I can say that cholecalciferol has been reported to carry a low risk of secondary exposure.
The final common form of rodenticide is bromethalin, which causes brain swelling and leads to death in rodents through neurological compromise. In pets, bromethalin may cause hind limb weakness, tremors, seizures, and death.
Bromethalin has no antidote, and its toxicity varies by species. Cats are approximately five times more sensitive to the poison than dogs. However, most cases of bromethalin toxicity occur in dogs who consume the bait directly, and I have never treated a cat for known or suspected secondary bromethalin toxicity. The risk of secondary toxicity for bromethalin furthermore is reported by reputable sources to be low.
In fact, bromethalin’s low secondary toxicity risk has been the cause of regulatory and legal action in the United States. Because it is less dangerous to pets, children, and wildlife, the EPA has mandated that rodenticide manufacturers replace anticoagulants with bromethalin in their products. At this time, the most dangerous anticoagulant rodenticides no longer are sold to the public in the U.S. (although professional exterminators may still have access to them).
Of course, older, dangerous anticoagulant rodenticides are still in homes and garages. Furthermore, all rodenticides have some potential to pose risk to pets. They also cause great suffering to the rodents that they target. I therefore recommend that people stick to the only rodent control measure that purrs, sits in your lap, and provides love and companionship.
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