Secondary Poisoning Concerns With Rodent Baits

November 1, 1998

If my cats eat mice that have died from your rodent baits, can they be secondarily poisoned?” This is one of the most frequently asked questions by our residential clients. But similar questions are also of interest when we conduct rodent control around livestock farms, zoos, exotic animal farms and various animal-rearing facilities.

What is secondary poisoning? How does it differ from primary poisoning, or from secondary ingestion? By understanding the basic terms and concepts associated with the hazards and risks of using rodenticides, PCOs can remain informed and decrease the occurrences of harming non-target animals during rodent control programs.

PRIMARY VS. SECONDARY POISONING. Primary poisoning refers to an animal directly consuming a rodenticide bait. This can occur when a bait is installed unprotected (i.e. in accessible areas, or not within a tamper-resistant bait station) into an area where non-target animals such as dogs or cats may encounter and feed upon the bait. Or, when a homeowner buys mouse bait in the supermarket and tosses the bait into a garage where his dog finds and eats the bait. Primary poisoning is the principle means by which companion animals and other non-targets are hurt by rodenticides.

Secondary poisoning refers to one animal being poisoned after consuming the flesh of another animal which has digested the poison. But are the anticoagulant rodenticides such as bromadialone, brodifacoum, difethialone and diphacinone toxic enough to cause secondary poisoning? According to experts on this subject, residues of digested anticoagulants can be found in the livers of poisoned rodents. Thus, animals consuming the entire carcasses of poisoned animals can ingest the anticoagulant compounds when they consume the livers.

But whether or not actual secondary poisoning will occur (i.e. an animal becoming ill or dying) depends on various factors such as the inherent toxicity of the rodenticide, the sensitivity of the animal ingesting the poisoned carcass, and of course, the amount of the toxicant and the time sequence of the bait being ingested. Unlike primary poisoning, secondary poisoning typically involves repeated feedings on poisoned animal carcasses over several days or longer.

Secondary ingestion is when one animal (e.g. a dog) consumes a rodent which still has undigested bait in its gut or mouth. Technically, the dog is directly consuming the bait along with the carcass, the same as it would if it were to eat the bait directly. Secondary ingestion is much more of a concern with rat carcasses than with mice. An adult rat can consume up to 30 grams of bait per day, whereas a mouse typically ingests up to only 4 grams of bait. In real-world situations, whether or not a non-target animal is poisoned by true secondary poisoning or via secondary ingestion (or both) is a moot point because for all practical purposes, it is indeterminable. For the purposes of this discussion, we will consider secondary ingestion a component of secondary poisoning.

SECONDARY POISONING IS UNLIKELY. Back to the original question posed by the homeowner about the cat. Is secondary poisoning possible either via ingestion of the livers of dead rodents, or via secondary ingestion of rodent baits? Well, theoretically it is possible, but realistically, it is highly unlikely. Let’s examine why.

First, most of the anticoagulant baits used for rodent control are formulated with low dosages of active ingredients ranging from 25 to 50 parts per million. Even with primary poisoning or secondary ingestion of bait, a 20-pound dog, for example, would need to consume anywhere from a minimum of 1.6 to 96 ounces of our two most popular bait actives (brodifacoum and bromadiolone) to obtain the value needed for a single-dose poisoning. The range depends on the particular active ingredient, the dog species and several other factors.

Multiple feedings of these baits over a prolonged period would require significantly less dosages. Still, consider the chances of the average client’s cat, dog, exotic animal, etc., encountering and entirely consuming enough rats on a periodic basis to accumulate enough poison to cause true secondary poisoning — not to mention enough rats dying above ground in areas accessible to a foraging non-target animal. Moreover, I personally cannot imagine any companion animal with this type of appetite being taken care of as a “beloved pet” around a typical dwelling.

All this is not to say, however, that secondary poisoning is not possible. The most likely scenario conducive to secondary poisoning would be in those cases of severe or chronic rodent infestations where many rodents (particularly rats) would be poisoned over the course of days or weeks. This would need to be coupled with hungry dogs, cats, or some other free-ranging animal exhibiting a daily opportunistic foraging strategy.

TAKE SPECIAL PRECAUTIONS. My own experience has been one of keeping an eye on farm situations where the farmer allows “farm cats and dogs” to occupy the farm premises, but feeds them only intermittently or not at all. Certainly, these dogs and cats might take full advantage of feeding on any dead rodents which happen to die in accessible areas.

In zoos, exotic animal farms, and some confined livestock operations, it might also be easy to envision scenarios conducive to secondary poisoning situations. Thus, for these situations, special precautions must always be considered. Too, some captive mammals and carnivorous birds may have special sensitivities to anticoagulant baits — even in small amounts. Working with the zoo vets and rodenticide technical specialists is always the wisest approach to avoid secondary rodenticide hazards.

The bottom line is first to prevent primary poisoning from occurring by not allowing your baits to be accessible to any companion or non-target animals. To prevent secondary poisonings, ensure that poisoned rodents during rodent cleanouts are picked up daily and either buried or incinerated. For sensitive environments, attempt to have any free-roaming non-target animals (cats, dogs, livestock, exotic animals, etc.) confined or moved during the peak time when poisoned rodents might be available. In cases of accidental ingestion of bait, or even when a homeowner observes a pet eating a dead rodent , there are likely to be questions and concerns. And perhaps a trip to the vet just to be sure. So it pays to always play it safe.

Dr. Robert Corrigan is president of RMC Pest Management Consulting.