MSPCA-Angell Headquarters

350 South Huntington Avenue, Boston, MA 02130
(617) 522-7400
Email Us

Angell Animal Medical Centers – Boston

350 South Huntington Avenue, Boston, MA 02130
(617) 522-7282
More Info

Angell West

293 Second Avenue, Waltham, MA 02451
(781) 902-8400
For on-site assistance (check-ins and pick-ups):
(339) 970-0790
More Info

Angell at Nashoba – Low-Cost Wellness Care

100 Littleton Road, Westford, MA 01886
(978) 577-5992
More Info

Animal Care and Adoption Centers – Boston

350 South Huntington Avenue, Boston, MA 02130
(617) 522-5055
More Info

Animal Care and Adoption Centers – Cape Cod

1577 Falmouth Road, Centerville, MA 02632
(508) 775-0940
More Info

Animal Care and Adoption Centers – Nevins Farm

400 Broadway, Methuen, MA 01844
(978) 687-7453
More Info

Donate Now


More Ways to Donate

From an online gift to a charitable gift annuity, your contribution will have a significant impact in the lives of thousands of animals.


What are rodenticides?

A rodenticide is a pesticide used to kill “pest” animals, such as mice and rats. Rodenticides are problematic not only because they cause immense suffering in target animals, but also because they can sicken and kill non-target animals, such as owls or cats who eat mice who had ingested the poison. There are several different types and classes of rodenticides, some of which are slightly less inhumane or less dangerous to non-target animals. However, all rodenticides inflict pain and suffering and none are guaranteed to spare non-target animals should they consume it.

Types of rodenticides

There are currently three classes of rodenticides: first generation anticoagulants, second generation anticoagulants (SGARs), and acute toxicants. The first two groups — first and second generation anticoagulants — are  poisons used for rodent control that work by stopping the blood clotting processes, causing lethal hemorrhage. Animals bleed internally and suffer severe pain for several days before dying. While it’s unclear exactly how long it may take for an animal to die, as it is dependent on the amount consumed, some studies conclude 1-3 days for rats, others 4-8 days, and still others have found that it can take up to 11 days for mice to die. Animals typically remain conscious until close to the time of death.

Anticoagulants are also the most dangerous to non-target animals (i.e., pets and non-target wildlife). First generation anticoagulant rodenticides last for approximately 7 days in an animal’s system, but SGARs are more potent and generally last for 4 weeks, making them more dangerous to other animals who ingest poisoned prey. Predators tend to prey on poisoned dying animals due to their inability to move quickly. Animals who eat these poisoned animals (or the poison itself) may die a similarly painful death, or they may survive, but suffer hemorrhaging and detriments to their ability to reproduce, thus having broader population impacts.

The third category, acute toxicants, are considered slightly less cruel and less risky to non-target animals, but they are still dangerous and inflict suffering.

One type of acute toxicant — alpha-chloralose — is relatively easy to treat in non-target animals if discovered quickly, and appears to cause relatively less suffering in rodents. Also, animals who ingest sublethal doses rapidly recover to normal functioning. However, mice can build up a tolerance to alpha-chloralose and therefore carry a large quantity in their bodies that could severely impact non-target animals who eat the mice. And non-target animals must receive treatment quickly if they are to survive.

Another commonly used acute toxicant, zinc phosphate, usually kills the animal who ingests it within a few hours, but it causes severe pain. There is also no antidote to zinc phosphate, making it deadly to non-target animals. A third type, Calciferol, though classified as an acute toxicant, typically takes several days to kill the animal, causing pain and illness during that time. Its effects can be reversed, but with difficulty.

Another type, strychnine, also causes death. In early 2022, two dogs tragically died after ingesting what their veterinarian suspects to be this type of rat poison.

Is rat poison really that dangerous to other wildlife and to pets?

Yes. It usually takes days for a rodent to die from rat poison, during which time non-target species can ingest one or more contaminated animals. Non-target animals can also consume the poison directly.

The impact of rat poison on wildlife has been documented across Massachusetts. A 2011 study, for example, showed that a stunning 86% of 161 birds of prey had some form of second generation anticoagulants in their liver tissue. An additional study, published in 2017, found rates of 96%. Dr. Murray, researcher at the Tufts Wildlife Clinic at Cummings Veterinary Medical Center at Tufts University, notes that these birds are representative of the state, and that “the food chain is extensively contaminated.” And while the rodenticide may not always kill these non-target animals, it can impact their ability to reproduce and sicken them. Animal control officers and veterinarians in the Commonwealth have seen, for example, birds of prey fall out of trees from muscle weakness and choke on their own blood from internal bleeding as a result of having ingested rodenticide.

The risk is so great to both wildlife and household pets that wildlife organizations have urged homeowners to only use poison as a last resort and, if hiring a pest control company, ask them to use non-SGAR materials. Some of the many tragic examples of wildlife dying from rat poison — either from eating it directly or eating a prey animal that had ingested poison — include MA’s first confirmed eagle death caused by rodenticides, another bald eagle only 5 months later, Ruby the Red-Tailed Hawk of Fresh Pond, a family dog, and a pregnant raccoon and her babies.

In addition to non-target wildlife, household pets are at risk of ingesting these toxins. The MSPCA’s Angell Animal Medical Centers alone see dozens of cases of companion animals poisonings each year. The cases peak during spring and fall, when landlords and homeowners are most affected by the presence of mice or rats. The cost of veterinary care to treat an animal who has ingested rat poison can exceed $2,000 and take weeks of supportive care and medication, putting a significant strain on families financially and emotionally.

The MSPCA is currently conducting a survey of licensed Massachusetts wildlife rehabilitators, and the preliminary results are striking. In 2019 and 2020, roughly a third of respondents treated animals that had been poisoned by rodenticide, and roughly an additional third treated animals that they suspected had been poisoned by rodenticide. Rehabilitators treated hawks, owls, chipmunks, skunks, and more; half of these animals had been poisoned by consuming a poisoned prey animal, while 33% ate the poison directly (and it was unclear in the remaining 17% of cases). Many of these animals needed to be humanely euthanized. Much work remains to be done in educating the public, as rehabilitators report that they are seeing an increase in rodenticide poisoning cases.

To address this serious issue, legislation has been filed at the Massachusetts State House. H. 3991: An Act relative to pesticides was filed by State Representative Jim Hawkins in early 2021. While personal use of SGARs is already banned in Massachusetts, licensed pest companies can still use it when hired to deal with rodent problems. This bill requires digitization of pesticide use forms for better monitoring, requires pesticide companies to provide customers with written information on Integrated Pest Management (IPM) strategies and the effects of SGARs on wildlife and the environment, and requires the increased use of IPM strategies in Massachusetts. We hope educating home and business owners on the dangers of rodenticide will reduce their use in our communities.

What is the alternative?

As with virtually all wildlife conflicts, prevention is the best solution. While rodenticides may kill rodents, they do not solve rodent problems, as they merely create vacant habitat for new animals to fill. Trapping mice and rats is an endless cycle if the habitat is not altered, and so it is critical to address the source of the problem via prevention. Rather than turning to rodenticides, leaders, business owners, homeowners, etc. should adopt IPM, an approach used to solve issues with “pests” while minimizing risks to people, wildlife, and the environment. IPM involves establishing site-specific goals, consensus building, and human behavior change. Using an adaptive management approach, project results are evaluated and revisited if goals are not met.

More specifically, Massachusetts entities should do the following: remove all nesting materials from the problem area, such as old clothing, books, boxes, and papers; remove all food debris and store food in secure containers; and moisten rags with peppermint oil and place them under sinks, cabinets, and in cracks. After all the rats and/or mice have left, seal all holes as small as a dime with quick drying cement, foam insulation, or wire mesh, and attach rubber or metal runners at the bottom of all doors. If traps must be used, choose the Havahart live trap. Use the MSPCA’s Intruder Excluder for more solutions and consult with one of these humane exclusion businesses.

How can I help?

As an individual, there are many things you can do. Choose prevention instead of rat poison in and around your home. If you live in a multi-unit complex, encourage the building manager to do the same. You can also educate others in your community. And keep an eye out for bills like the one mentioned above, H. 3991, for which you can lobby on the state level.

In the news:

Related Topics

An Act relative to pesticides

An Act relative to pesticides

Help us regulate rodenticide use in MA!

Learn More

About Mice and Rats

About Mice and Rats

While often thought of as “pests,” mice and rats, like all animals, have rich social and emotional lives.

Learn More

Intruder Excluder

Intruder Excluder

Intruder Excluder can offer you advice and solutions that will help you solve your conflicts with wildlife in a humane, long-term, and cost-effective manner.

Learn More

Rodenticides and Pets

Rodenticides and Pets

The MSPCA's Angell Animal Medical Centers see dozens of cases of companion animals poisoned by rodenticides each year.

Learn the Dangers

Join the Animal Action Team to stay up to date on animal issues across the Commonwealth.

Advocacy Puppy