Rodenticides: Background & Hazards
Rats and mice have been living with humans for centuries, and humans have been working for just as long to keep them at bay. Rodents such as gophers, moles, voles, and squirrels are also viewed as pests in some settings.
Poisons have been used as rodent-control measures for many years. Before the 1940s, rodenticides contained heavy metals such as arsenic and thallium or poisons such as strychnine and red squill. Most of these chemicals are no longer used as rodenticdes with the exception of strychnine, which is currently registered for use only below-ground as a bait application to control pocket gophers. For more details, see US EPA’s RED Facts on Strychnine.
In the 1940s and 1950s, the first kind of anticoagulant rodenticides known as first-generation rodenticdes were developed. This class of rodenticides work by disrupting the normal blood clotting or coagulation process so that dosed individuals or animals suffer from uncontrolled bleeding or hemorrhaging. Anticoagulant rodenticides that are in wide use today are members of a more potent class of anticoagulants known as second-generation rodenticides. The following section provides a more detailed comparison between first and second generation rodenticides.
Other rodenticides are non-anticoagulants and work in different ways. Non-anticoagulant rodenticides in use include bromethalin, cholecalciferol, and zinc phosphide.
Using rodenticides for control is frequently the cause of unintended consequences, from the stench of rodents decomposing in the walls to poisonings of non-target organisms. Primary poisoning can result from wild birds, pets, or even children eating baited rodenticides, while secondary poisoning of birds and mammals (including dogs and cats) can accompany the ingestion of poisoned rodents.
First-generation anticoagulant rodenticides listed in the table below require rodents to consume the bait for several consecutive feedings for delivery of a lethal dose. There are three US EPA-registered first-generation rodenticides, including warfarin (also used as an anti-clotting drug for coronary artery disease), chlorophacinone, and diphacinone. The first-generation compounds are excreted fairly rapidly by mammals, usually within a week. However, the use of any rodenticides pose a poisoning risk to children, pets and wildlife.
|First-Generation Rodenticides||Types||Acute Oral Toxicity||Primary Poisoning Risk||Secondary Poisoning Risk|
|Chlorophacinone||Anticoagulant, multiple dose treatment||High||Low (birds and mammals)||Low (birds), High (mammals)|
|Diphacinone||Anticoagulant, multiple dose treatment||High||Low (birds and mammals)||Moderate (birds), High (mammals)|
|Warfarin||Anticoagulant, multiple dose treatment||Moderate to high||Low (birds), Moderate (mammals). Highly toxic to cats.||Moderate (birds and mammals)|
The second-generation anticoagulant rodenticides (SGARs) are substantially more potent than the first-generation compounds, and a lethal dose can be ingested in a single feeding. Included in this class of rodenticides are the compounds difenacoum, brodifacoum, bromadiolone and difethialone. SGARs are not excreted easily, persisting in bodily organs such as the liver. Some SGARs are especially problematic to birds and mammals. For instance, brodifacoum and difethialone pose the greatest potential risks to avian predators and scavengers that feed on target or nontarget animals poisoned with bait.
EPA studies indicate that the first-generation anticoagulants are less hazardous than the more highly toxic and persistent second-generation anticoagulants. SGARs compounds described in the table below are much more likely to poison predatory wildlife that eat live or dead poisoned prey and have a higher risk of severe poisoning for children, pets, and other non-target wildlife.
|Second-Generation Rodenticides||Types||Acute Oral Toxicity||Primary Poisoning Risk||Secondary Poisoning Risk|
|Brodifacoum||Anticoagulant, single dose treatment||High||High (birds and mammals)||High (birds and mammals)|
|Bromadiolone||Anticoagulant, single dose treatment||High||Moderate (birds), High (mammals)||Moderate (birds and mammals)|
|Difethialone||Anticoagulant, single dose treatment||High||High (birds), Moderate (mammals)||High (birds), Moderate (mammals)|
|Difenacoum||Anticoagulant, single dose treatment||High||Moderate (birds), High (mammals)||Moderate (birds), Data gap (mammals)|
Non-anticoagulant rodenticides, including bromethalin, cholecalciferol, and zinc phosphide, are US EPA-registered and frequently used in controlling pest rodent populations. The potency of these rodenticides is highly variable, with rodent mortalities typically occurring on the order of several hours to days following ingestion of a lethal dose. These rodenticides belong to three different chemical classes that differ from one another as well as the anticoagulants in their mode of action, or anatomical change leading to rodent death.
As a neurotoxicant, bromethalin poisons the central nervous system and ultimately causes respiratory distress following ingestion of a single dose. Cholecalciferol, the biologically active form of vitamin D, is completely nontoxic in small amounts, but massive single doses or prolonged low level exposure can prove toxic. Ingestion of excessive amounts of cholecalciferol induces hypercalcemia (elevated blood calcium levels), which ultimately results in heart problems and kidney failure. Zinc phosphide, which functions to liberate toxic phosphine gas upon interaction with stomach acid, is highly toxic and can lead to rodent mortality within hours of ingestion. Unfortunately, zinc phosphide can also poison birds, such as hawks and owls, following ingestion of treated baits.
|Non-Anticoagulant Rodenticides||Type||Acute Oral Toxicity||Primary Poisoning Potential||Secondary Poisoning Potential|
|Bromethalin||Non-anticoagulant, single dose treatment||High||Low (birds and mammals)||Low (birds and mammals)|
|Cholecalciferol||Non-anticoagulant, multiple or single dose treatment||High||Low to moderate (birds and mammals)||Low (birds and mammals)|
|Zinc Phosphide||Non-anticoagulant, single dose treatment||High||High (birds and mammals)||Low (birds and mammals)|
For more information on different types of rodenticides, see the National Pesticide Information Center’s (NPIC) fact sheet.
The first line of defense against rodents should be exclusion and trapping. These methods do not pose a poisoning risk to children, pets and wildlife. If you plan to use rodenticides, be sure to follow all label directions.
Remember, removing rodents with traps or poisons will not keep rodents out of your property in the future. To permanently keep rats and mice out of your home or business, you will need to prevent access by sealing all possible entry points. It is equally as important to eliminate rodent attractions such as food and water by keeping food in tightly sealed glass or plastic containers and repairing leaky pipes.
- Erickson, W. and D. Urban. 2004. Potential Risks of Nine Rodenticides to Birds and Nontarget Mammals: a Comparative Approach, US EPA
- Fishel FM. 2012. Pesticide Toxicity Profile: Coumarin and Indandione Rodenticides . University of Florida IFAS Extension.
- Fishel FM. 2012. Pesticide Toxicity Profile: Miscellaneous Rodenticides. University of Florida IFAS Extension.
- Hadler MR, Buckle, AP. 1992.Forty-five years of anticoagulant rodenticides—Past, present and Future Trends. Proceedings of the Fifteenth Vertebrate Pest Conference 1992. Paper 36.
- Thacker, JL, Willhite L. 2009. Long- Acting Anticoagulant Rodenticides. Iowa Statewide Poison Control Center.
- Watt BE, Proudfoot AT, Bradberry SM, Vale JA. 2005. Anticoagulant rodenticides. Toxicology Review 24(4): 259–269.
- US EPA. 2007. Pesticide Fact Sheet: Difenacoum. US Environmental Protection Agency.