Why is rat poison showing up in so many fisher cats?

Picture a fisher cat hunting mice under the hemlocks of southern Vermont, miles from the nearest barn. It has never set foot in a city. Odds are it still carries rat poison in its liver, and probably more than one kind.

That is the picture from a survey of 597 fishers taken by licensed trappers across the northeastern United States. Nearly four in five, 78.6%, tested positive for at least one rat poison, and most carried two to six different ones. Every fisher sampled in Vermont was exposed. The fisher cat rat poison problem didn’t track farms or cities. It tracked something much quieter: houses tucked into the woods.

A census taken one liver at a time. The work was led by Georgianna Silveira of the State University of New York College of Environmental Science and Forestry in Syracuse, with wildlife agencies in Pennsylvania, New York, Vermont, New Hampshire and Maine and the Cornell Wildlife Health Lab. It was published Feb. 8, 2024, in the journal Frontiers in Ecology and Evolution. Most of the livers came from the regulated fall fur harvest between 2018 and 2022, and a few dozen came from carcasses sent to wildlife health labs.

What a fisher cat is, and why it eats so many rodents

Not a cat, and it doesn’t fish. The fisher (Pekania pennanti) is a forest member of the weasel family, about the size of a house cat, with a long body and a bushy tail. New Englanders call it a fisher cat, though it is no more a cat than a ferret is. For a real small wild cat, think of something like the clouded tiger cat that camera traps unmasked. Fishers climb well, roam big home ranges and live mostly on rodents and other small prey. That diet is exactly what puts them in the path of rat poison.

The comeback came with a catch. Over many decades, much of the Northeast has grown back into forest. Fishers have followed the trees and spread into woods that now hold scattered homes, cabins and outbuildings. The region is an odd mix. Forest covers from 58.6% of Pennsylvania to 89.5% of Maine, yet the Northeast also has the highest rural population density in the country. Lots of trees, lots of people living among them. The authors wrote that the fisher’s expansion into these more human places “apparently comes at a cost.”

How rat poison moves from mice to predators

The poison works by stopping blood from clotting. Most rat poisons sold in the U.S. are anticoagulants. They block the vitamin K cycle the liver uses to build clotting factors, so a poisoned animal can bleed to death from a small injury or even hard exertion. Older “first-generation” compounds such as diphacinone need several doses and are sold off the shelf to homeowners. “Second-generation” compounds such as brodifacoum and bromadiolone were developed in the 1970s, after rats grew resistant, and a single feeding can kill. In the U.S. they are meant for licensed pest control and farm use.

A poisoned mouse is easy prey. The poison doesn’t care whether it was eaten from a bait box or inside a mouse. Lab rats dosed with these drugs shift their daily rhythms and wander away from cover, which likely makes them easier to catch. A predator that eats several of them builds up the poison. The second-generation kinds can linger in the liver for hundreds of days. Short of killing, exposure has been tied in other studies to more disease and parasites, smaller litters and more deaths among newborns.

Each compound is a separate receipt. Let me explain why the count matters. Commercial products contain just one active compound. So a fisher whose liver holds three different poisons ate, at minimum, three separate poisoned meals from three different sources. In this sense the liver works like a drawer of receipts. In the survey, 55.3% of fishers had two to six compounds, 23.3% had one, and only 21.4% had none. The lab screened for 11 compounds and found eight.

Where fisher cats in New England pick up rat poison

Three poisons did most of the damage. Diphacinone, the over-the-counter kind, turned up in 64.3% of fishers. Brodifacoum was next at 53.8%, then bromadiolone at 28.0%. That diphacinone result stands out. A California survey found it in only 14% of fishers there. Restricted or not, the authors noted, second-generation poisons can still be bought at hardware stores and online, since federal rules govern where they are sold and used but not who buys them.

The hot spot ignored state lines. Maine had the lowest rate, with 52.8% of fishers exposed. Vermont hit 100% and New Hampshire 93.3%. The team expected exposure to climb steadily from Maine toward more crowded Pennsylvania. It didn’t. Mapping the samples instead revealed a hot zone across southern New Hampshire, southern Vermont and southeastern New York, the belt of rural homes that spreads outward from big metro areas.

Houses in the woods were the strongest signal. To find the cause, the team compared each sample with the land around it. The best predictor was what planners call wildland-urban intermix, low-density homes scattered through land that is mostly forest. For every 10% more intermix around a fisher, its odds of exposure rose about 1.4 to 1.9 times for most poisons, and the number of compounds in its liver rose by about 15%. Farmland showed no clear link. The authors said their samples probably missed the most farmed parts of New York, where fishers are too scarce to trap.

Protected forests offered no refuge. The Northeast holds nearly 15 million acres (6 million hectares) of protected land, and fishers living around it were no less exposed. A fisher’s home range is larger than many of those protected patches, so the animals likely forage across their edges into yards and fields. Buildings inside the forest, such as logging camps, maple sugaring operations and maintenance sheds, could also be sources.

Why backyard rat poison is the main suspect

The pattern fits do-it-yourself pest control. Here’s the thing. Earlier surveys found that up to 80% of households in California and farmers in Ireland handled rodents themselves, without a contractor, and that rules on outdoor use are widely ignored. Another California study found that people in lower-density neighborhoods had more rodent trouble and used more of these products. Put a bait box by every cabin, garage and woodshed in a forested county, and you get the map the fishers drew. The authors concluded that off-label, non-professional use of rat poison likely drives much of the exposure in the Northeast.

The livers can’t say exactly where the poison was eaten. The study shows a strong association, not proof of cause. Trappers reported where each fisher was caught only down to the township, and townships range from under 1 square mile to more than 480 square miles (from about 2 to 1,255 square kilometers). The team ran its maps ten times with randomized locations to account for that, but it still can’t tell where a given animal ate its poisoned meal. The concentrations measured don’t reveal the original dose or any health harm, because they fade with time and vary from animal to animal. And the team had no data on illegal marijuana farms in northeastern forests, which researchers have blamed for poisoning fishers in California.

The harvest makes a cheap early warning system. Because trappers bring in hundreds of fishers every fall, states could repeat this survey and see whether new rules or public campaigns actually cut exposure. The authors want to test other carnivores next, such as bobcats and red foxes, which use more open country, and to learn more about how homeowners and professionals in the region actually use these products.

Source: Silveira, G., Frair, J. L., Murphy, L., Ellis, J. C., Needle, D., Cunningham, S. A., Watson, A., Facka, A., Tate, P., Webb, S., Royar, K., Bernier, C., Keller, T., et al., & Schuler, K. (2024). “Drivers of anticoagulant rodenticide exposure in fishers (Pekania pennanti) across the northeastern United States.” Frontiers in Ecology and Evolution, 12, 1304659. https://doi.org/10.3389/fevo.2024.1304659

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