Rodenticides are pesticides used to control rodent populations. Rodenticides fall into two general categories, anticoagulants and non-anticoagulants. Anticoagulants stop normal blood clotting and cause the animal to slowly bleed to death. Common anticoagulant rodenticides include warfarin, diphacinone, chlorphacinone, brodifacoum, difenacoum, and bromadiolone. There are several different types of non-anticoagulants, and they all vary in their mechanism of action and include bromethalin, strychnine, cholecalciferol, and zinc phosphide.
Rodenticides are toxic to many species of birds and mammals including pets, farm animals, and wildlife species. Birds and other wildlife are at risk of secondary poisoning caused by eating poisoned prey. Even pets and children are at risk of consuming rodenticides used in the household.
Anticoagulants work by thinning the blood and cause the rodent to slowly bleed to death. However, it takes a few days for the rodent to die, and in the meantime, they may continue to feed on the poison. During that time they become slow and sick, making easy targets for predators, such as hawks and owls. Even after the rodent dies, its carcass contains rodenticide residues that can be lethal for scavengers.
Newer or “second generation” anticoagulant rodenticides are the most harmful because a lethal dose of can be ingested in a single feeding. If the rodent continues to feed on the single-dose anticoagulant after it eats a toxic dose during the first day, it may build up more than a lethal dose in its body before the clotting factors run out and the animal dies. Residues of single-dose anticoagulants may remain in liver tissue for many weeks, so a predator that eats many poisoned rodents may build up a toxic dose over time.
Ingestion of anticoagulant rodenticides results in interference with blood coagulation and spontaneous bleeding. Clinical signs can include widespread bruising, bleeding into body cavities, and blood in the urine or feces; if the bleeding is sudden and significant, then cardiovascular shock and death can result. Bleeding can occur internally or externally and can affect any part of the body. Vitamin K is used to treat anticoagulant rodenticide intoxication and help restore normal coagulation. Because the second generation anticoagulant rodenticides can be very long acting, treatment may be required for weeks to months.
Diagnostic tests are expensive and not always accessible for all rehabilitators who care for raptors. Blood tests are not a viable option to screen for second generation anticoagulant exposure in red-tailed hawks, one of the most common species to experience secondary poisoning, because blood is not sensitive enough to detect the compounds found in the poison—except in birds that ingested a lethal amount.
The clinical signs of non-anticoagulant poisoning include rapid onset of seizures, muscle tremors, limb weakness, ataxia, neurologic signs, respiratory paralysis, anorexia, nausea, vomiting, diarrhea, and lethargy. Diagnosis of non-anticoagulant rodenticide toxicity is based on detection of the chemical in the digestive system or tissues of the animal. The treatment for non-anticoagulant rodenticide poisoning is typically only supportive care.
Those birds admitted into rehabilitation are often too far gone to be helped. This is a sad reality that wildlife rehabilitators see all the time. Flaco, the famous Eurasian Eagle-Owl that lived in Central Park after escaping his enclosure in the Central Park Zoo, was found upon his death to have been exposed to four different anticoagulants that are commonly used for rat control.
In 2022, the EPA proposed new mitigation measures for rodenticides. These measures are intended to reduce exposure to non-target organisms such as mammals and birds that may inadvertently consume rodenticides through their prey, or animals that may consume the rodenticide directly. Proposed measures include requiring bait to be placed in tamper-resistant bait boxes to ensure it is contained and requiring users to collect carcasses of rodents that may have consumed rodenticides to prevent further exposures to non-target organisms that could consume the carcasses.
Other preventative measures can be taken to get rid of rodents without using rodenticides. They include:
- Keep all garbage and food in tightly sealed containers.
- Do not leave food for pets or other animals outside.
- Remove standing water—fix leaky pipes and holes where water pools.
- Check crawlspaces and vent covers for openings.
- Clean barbecues following use.
No one wants to live with rodents or have them destroy property. Choosing alternatives to rodenticide poisons will help protect birds and wildlife, as well as our pets.
Written by Judy Cadmus
For more information:
Cornell University College of Veterinary Medicine
https://rethinkpriorities.org/publications/paths-to-reducing-rodenticide-use-in-the-us
https://now.tufts.edu/2020/09/16/understanding-risks-rodent-poisons-birds-prey
Rodenticide alternatives:
https://www.yournec.org/you-dont-need-to-use-poison-non-toxic-alternatives-to-rodenticide/
https://santabarbaraaudubon.org/natural-alternatives-to-rodenticides/
