Last year, Maria Colby noticed a worrying increase in the number of owls and hawks coming to her wildlife sanctuary in Henniker with signs of poisoning.

“You're lethargic,” Colby said. “Your red blood cell count is low.”

She knew what she was seeing. “Birds of prey eat dead things,” she said. “If there's a dead animal on the ground, they'll eat it – free food.”

That's a problem.

Poisons known as second-generation anticoagulants (SGARs) are commonly used to kill mice and rats in homes, businesses and barns. But the rodents do not die immediately and are vulnerable to being eaten by other wildlife.

Sick animals and birds can be treated with vitamin K if they get to her quickly enough, Colby said. But often they have to be euthanized. “Their organs fail,” she said.

A recent study by a researcher at Tufts University found 100% of red-tailed hawks, New Hampshire's bird of prey, to be contaminated with SGARs.

It's not just the birds.

Dr. David Needle, a senior veterinary pathologist at the University of New Hampshire Veterinary Diagnostic Laboratory, was working on a study of distemper virus in fisher martens several years ago when concerns about these newer poisons first arose.

After a study in Vermont found SGARs in fisher martens, Needle's team repeated the study here, conducting necropsies on fisher martens and captured gray and red foxes. “We also found anticoagulant rodenticides in almost all of our captured animals,” he said.

The more they searched, the more they found.

A large, multi-state study in which Needle participated tested fishermen in New Hampshire, Maine, Vermont, New York and Pennsylvania. The highest exposure rates were found in New Hampshire and Vermont.

“Mesocarnivores” are small to medium-sized mammals like foxes, raccoons and bobcats — “all the cute things that run around,” Needle said.

They all eat rodents.

Far-reaching effects

“If you use anticoagulant rodenticides in your home, the mice in your home will die, but we also know for sure that they will get into the landscape and the mesocarnivores that eat them will be impacted,” Needle said.

Biologists don't yet have a complete picture of how these poisons affect mammals, he said. “They don't die from it, but there is historical evidence that it can impair reproductive capacity and disrupt immune status in some small mammals,” he said.

If some of these predators disappear from the landscape, more rodents — and the ticks and diseases they carry — would likely find their way into our homes, Needle says.

Patrick Tate, a wildlife biologist and fur project manager at New Hampshire Fish and Game, shares the concern. “We don't know the long-term effects of these compounds on animals.and how it can influence the individual animal's ability to resist viruses,” he said.

In many cases, animals found to contain residues of these chemicals also suffered from other diseases. Tate wonders what comes first: Is an animal's death caused by a virus, or do the chemicals weaken the animals and make them susceptible to disease?

The SGAR poisons are available online and in hardware stores. Tate said he has seen buckets of them outside homes and businesses. “Overall, I think there is a tremendous lack of information and misconception among the public,” he said.

“Many people believe that rodenticides only affect the animals that eat them,” he said. “They don't realize that they affect all life forms.”

Mischievous rodents

In New Hampshire, pesticide regulation is the responsibility of the state Department of Agriculture, Markets and Food. A 2023 bill to ban SGARs except for certain public health or agricultural uses was defeated in committee.

Instead, the State Pesticide Control Agency has set up a task force to study the impact of rodenticide use on wildlife. In its report, the group recommended further research into the effects of the chemicals on birds of prey and wildlife and better documentation of cases of “secondary” poisoning.

Meanwhile, the U.S. Environmental Protection Agency is close to issuing new rules for the use of SGARs. The agency is expected to require that they be applied only by licensed pest control professionals.

That would be acceptable to Adam Carace, owner and CEO of Pest End Exterminators in Plaistow, who was a member of the task force last year.

Carce said his industry has changed a lot in the last 20 years. “Back then,” he said, “all we did was spray paint.”

The first step now is to inspect a property and seal off places where rodents can get in. “We're really trying not to use as many rodenticides as we have in the past,” Carace said.

But sometimes using SGARs is the best solution – especially with rats, which are “very, very smart,” he said. “We've seen videos of rats using their tails to trigger baited snap traps.”

David Rousseau, director of the state's Division of Pesticide Control, said rats are a growing problem in many areas of New England. He is waiting for EPA guidance on SGARs.

But he also believes that the public needs to be better informed about the potential risks to wildlife.

His agency urges homeowners to take the first step of “rodent-proofing” their homes by removing all food sources and repairing areas through which the animals can enter.

“Rodenticides are poisons,” Rousseau said. “They should be the last option considered.”

Finding a balance

The problem is complicated, said Tate of the Fisheries and Game Commission. A ban on rat poison could have negative effects on human health, as diseases could be transmitted from animals to humans.

“In my opinion, the best thing would be not to apply rodenticides outside of a building,” he said.

Colby, a wildlife rehabilitator, said she understands the use of poison to keep rodents away from hospitals and food service establishments. “But that doesn't apply to regular homeowners,” she said, noting that safer products made from corn gluten are available.

Pest control expert Carace said there are alternatives, such as “multi-traps” that can catch multiple mice until they die. But some methods are unaffordable for low-income residents, he said. “If I go into someone's house and they can't afford that option, I don't want to leave them with no other choice,” he said.

Banning SGARs would mean “losing a tool from our toolbox,” Carace said.

“Unfortunately, yes, there are deaths in what we are dealing with,” he said. “But if we didn't do that, there would be an epidemic.”

Bait traps are attractive to many homeowners because the mice usually run outside to die – out of sight, out of reach. But experts say that while kill traps are inconvenient, they are a better option for most households.

“My message is that for environmental reasons we have to accept the hard fact that we want to kill the animals, that we have to be careful to cause as little damage as possible to the environment and the ecosystem, and that we have to use killing traps instead of rodenticides,” said Tate of the Fisheries and Game Commission.

UNH veterinarian Needle uses snap traps to kill mice in his own home. “It's not fun, but … using bait stations doesn't mean you're not killing them,” he said.

His advice: Take responsibility yourself. “That’s the New Hampshire way.”

[email protected]