ST. GEORGE — Say a rodent ingests poison on bait placed by a property owner trying to mitigate the risk of damaged crops or disease spread, but it doesn’t die right away. Instead, it continues eating the poisoned bait over time, with multiple lethal doses building up in its small body.

What happens when it’s caught and eaten by an eagle?

Rodenticides are chemicals, such as Bromadiolone tablets, used to kill rodents, but they can also negatively affect raptors, big cats and other predators, as well as pets and people. According to All About Birds, a publication by the Cornell Lab of Ornithology, poisons like arsenic or strychnine were historically used to control rodents, but they killed the animals too quickly, leaving a dead body behind, which drove others away from the bait.

Because of this, first-generation anticoagulants, like warfarin, which act like blood thinners, were developed. However, they required multiple feedings in a week to kill a pest, and some rats developed a resistance, according to All About Birds.

“The second-generation anticoagulant rodenticides solved these problems by acting like a beefed-up, re-engineered warfarin,” the organization states. “A single feeding on brodifacoum bait is often enough to kill a rat, but the dying process still takes days. During this time, the doomed, disoriented, and desperate rat makes an easy meal for a hawk.”

Second-generation anticoagulants can kill an animal after one feeding and typically remain in the animal’s tissues longer than first-generation anticoagulants. So, they pose a greater risk to nontarget species that either eat the bait or another exposed animal, according to the U.S. Environmental Protection Agency.

Because of this, second-generation anticoagulants are restricted for consumer use but are still available for commercial use. Still, All About Birds reports they can be purchased in large-chain agricultural supply stores.

Second-generation anticoagulant rodenticides are stable and have a long half-life, so they can linger in the ecosystem, moving through the environment and food web, said Dr. Barnett Rattner, a research physiologist and ecotoxicologist at the U.S. Geological Survey’s Eastern Ecological Science Center at the Patuxent Research Refuge in Maryland.

“You have to realize, the chemicals just don’t sit there — they move around because of wind, rain, animals eating them, animals excreting them,” he told St. George News.

Additionally, in some cases, rodenticides can take more than 10 days to kill an animal, said Lauren Ross, a lifesaving and care specialist at Best Friends Animal Sanctuary’s Wild Friends facility in Kanab.

“That means that the rodents might keep eating more and more of the poisoned bait until they have built up much more than the lethal dose in their bodies, which in turn affects animals that might predate on those rodents,” she said. “The fact that it does not kill them quickly means that birds of prey could be in a whole new area before they start to succumb to it, especially if it is during migration. This means it is difficult to really pin down any problem areas using a lot of poison.”

Rattner said that while many animals may be exposed to rodent poisons, research has indicated that a small percentage show severe signs of toxicity.

“If we look closely enough, you or I might even have some rodenticides in our bodies, and we haven’t eaten the bait,” he said. “So I think there might be a substantial number of animals that get exposed that don’t get exposed very much. … That doesn’t mean they’re safe, but it doesn’t mean that they’re dying from rodenticide exposure. And they could be sublethally affected in some way.”

Ross reiterated this point.

“Rodenticides often go undiagnosed in wildlife rehabilitation centers because the symptoms are not always what you would expect,” she said. “According to The Cornell Lab, a study done looking at the effects of rat poison on raptors showed that 139 out of 161 raptors that had been brought into Tufts Wildlife Clinic in Massachusetts tested positive for anticoagulant rodent poison. Out of these, only nine showed clinical symptoms.”

At Wild Friends, there have been approximately 10 potential cases of rodenticide toxicity, including red-tailed hawks, barn owls, great horned owls, Swainson’s hawks and Cooper’s hawks, Ross said. These ranged in age from nestlings to adults. Animals affected by rodent poisoning may experience a loss of appetite and resulting emaciation, depression, lethargy or a loss of coordination.

“Since the most popular rodenticide essentially acts as an anticoagulant, the animal affected will generally end up dying of internal bleeding,” she added. “Sometimes this presents itself in very obvious ways like blood coming from the nose, mouth, eyes or ears, but oftentimes (it) is internal, and you don’t know until it is too late.”

Ross said she suspects the facility cares for many more animals adversely affected by poisons than have been confirmed and that rodenticides could be a larger problem than people realize.

“Because a lot of animals are not found after succumbing to the poison,” she said. “Wild animals are fearful of people. When they are sick, they are going to try to get as far from us as possible, meaning that they most likely will not be encountered and brought into care.”

It can be difficult to tell when an animal has ingested a rodenticide, Ross said. For instance, while a necropsy might show internal hemorrhaging, it could have resulted from various potential causes, and testing for the poison can be expensive.

Martin Tyner, the CEO and co-founder of the Enoch Wildlife Rescue, said animals showing signs of poisoning could have contracted a disease, such as West Nile virus, Hantavirus or rabbit hemorrhagic disease, as the symptoms could manifest similarly.

Additionally, Rattner said that animals could be affected by many other chemicals, including metals, pesticides or industrial chemicals.

“You might detect rodenticide in the animal, but maybe it didn’t die from a toxic chemical, or maybe it died, and there was some other chemical it was exposed to, like lead, that could have been responsible for the death of the animal,” he said. “And sometimes, we talk about the ultimate cause of death and the proximate cause of death.”

For instance, if a researcher locates an animal that was hit by a car, they could conclude its proximate cause of death was a traumatic injury, Rattner said. However, if it had a high amount of rodenticide in its system, the poison could have impeded its blood’s ability to clot, causing it to bleed to death — its ultimate cause of death.

“Had it not had rodenticide in its body, maybe it wouldn’t have died,” he added.

Rodenticides and birds of prey

In a 2023 study by the Raptor Research Foundation, researchers, including Rattner, investigated anticoagulant rodenticides as a possible cause of declining American kestrels (small birds of prey) across North America.

While the cause “remains a mystery,” six out of eight liver samples and seven of 12 blood samples collected from adult kestrels contained evidence of rodenticide exposure, with 59% of juvenile nest samples also testing positive, the study reports. Samples were collected from wild kestrels found in Utah, with liver samples taken from deceased animals found opportunistically and blood obtained from live birds.

“Based on these findings, we recommend that (these rodenticides) be further investigated as a potential cause of kestrel declines,” the study states.

In another study published in 2022, researchers found low concentrations of second-generation anticoagulant rodenticide residues in 42% of California condors and 93% of turkey vultures. Approximately 16% of free-flying condors tested experienced prolonged blood clotting.

“Exposure to (anticoagulant rodenticides) may complicate recovery efforts of condor populations within their current range and in the soon to be established northern California experimental population,” the study states.

How can Southern Utahns mitigate their impact?

Rattner said it’s important to learn and use best management practices when dealing with a pest problem. Rather than using rodenticides immediately, he suggested changing the environment, such as cleaning up waste or spilled feed.

“There can be places in the foundation of our home that a rodent can get in and that’s causing a problem in the house,” he added. “And rather than immediately just trying to get rid of the rodents with the rodenticide, you should maybe be trying to look and see where the animals getting in your house and seal it up in some way.”

Other steps can be taken to reduce a pest problem without using chemicals. These include removing potential rodent nesting sites like leaf piles, removing outdoor bird feeders and containing garbage in lidded containers, according to the EPA.

Rattner said rodenticides are sometimes a “necessary evil.”

“In parts of the world, rodents cause a lot of damage to homes, to infrastructure, they can carry disease that can affect people and other animals,” Rattner said. “And in some instances, you really have to do things that protect the people, but there could be some collateral damage or problems, and that’s sort of a balancing act.”

Additionally, Rattner said that those choosing to use rodenticides should follow the instructions on the label and dispose of any resulting carcasses.

Others, like Ross and Tyner, advise against any rodenticide use.

“With as many threats as there are to wild populations, we should be doing everything in our power to try to protect wildlife,” Ross said. “Not using poison and encouraging others to not use poison is a simple and incredibly important step that we can take that would have a huge impact on our native wildlife.”

Tyner said that individuals should also avoid using other chemicals.

“Rodenticide is not any worse than any other number of indiscriminate poisons that are used,” he said. “We say, ‘Rodenticide — don’t do this.’ And then people say, ‘Well, I won’t do rodenticide, but we’ve got this arsenic product over here, we’ll use that instead.’ And they just trade one poison for another. Just don’t poison.”

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