Through centuries past, a variety of methods, active ingredients, formulations, and control measures have been used for rodent pest control. These efforts become more apparent as urban areas expand and cohabitation with wildlife evolves. As humans increasingly encounter wildlife and rodent pests, attempts at rodent control also carry increasing risk of nontarget-species exposure.
Nontarget species may be exposed to rodenticides by direct accidental ingestion of the rodenticide product, by ingestion of the target species and subsequent relay toxicity, or, less commonly, by baited ingestion secondary to malicious intent to kill either domestic animals or wildlife.
A variety of active ingredients and formulations of rodenticide products are available to consumers, pest control professionals, and those engaged in agricultural management. The most commonly used rodenticide products available on the market include bromethalin, cholecalciferol, anticoagulant rodenticides, phosphides, and corn cellulose–based products. Strychnine poisoning is addressed in a separate chapter. Rodenticides, their ingredients, and their formulations exist within a landscape that is actively evolving, and accurate identification of the product is critical to successful management of the poisoned patient.
It is relatively common to be presented with a patient that has ingested a rodenticide of unknown active ingredient. Baits should not be identified on color, shape, formulation, and size alone, because these are ever-changing in the landscape of pest control and may be quite similar across brands, ingredients, and formulations. Some manufacturers may color-code their products, and a known manufacturer may be a source for further information. Obtaining a detailed history from the animal owner is of utmost importance because knowing the place of purchase, the target species, the bait dimensions (if a sample of the bait is available, the bait may be weighed on a gram scale for accuracy), and the bait formulation can be helpful in narrowing the list of possible active ingredients and assessing the toxic dose for the individual patient. Decontamination and treatment recommendations are often based on a combined approach to address several or all possible active ingredients. These recommendations may include induction of emesis, administration of activated charcoal in single or multiple doses, SC or IV fluid treatment, administration of vitamin K1, monitoring of calcium and phosphorus monitoring, and supportive care based on the clinical course for the individual patient. Each case can and should be assessed individually, even in the face of an unknown active ingredient.
Historical Considerations and Regulation of Rodenticide
Pet Poison Helpline internal case data, accessed Jul 2020.
The development of chemicals for consistent use in rodent control can be traced to the early 20th century, with use of zinc phosphide as a rodenticide documented in 1911. Anticoagulant products quickly followed because the association between moldy sweet clover, its dicumarol by-product, and consequent internal bleeding and death in cattle was established between the 1920s and mid-1930s. This discovery led to the development of warfarin in the 1940s and its increasing popularity as a rodenticide product in the 1950s. Warfarin and its first-generation anticoagulant cousins remained effective for some time, although notable resistance within the rodent population was documented as early as the 1970s.
Resistance to first-generation anticoagulant rodenticides led to development and popularity of both bromethalin (a neurotoxin) and a second generation of anticoagulant rodenticides (the “superwarfarins”). These anticoagulant rodenticides had median lethal doses (LD50) that were 2.5–200 times lower than those of first-generation products. With wider use, greater potency, and a narrowed safety margin, however, came increased risk to the health of wildlife via direct ingestion of prey or carrion or relay toxicity, raising concerns for secondary human exposure and poisonings, especially in young children.
As a consequence, the US EPA drafted its “Risk Mitigation Decision for Ten Rodenticides” in 2008 and attempted to implement it by 2011. In light of some resistance on the part of manufacturers, the mitigation efforts were delayed, but they were broadly accepted by 2014. The risk mitigation efforts address the following active ingredients specifically:
First-generation anticoagulant rodenticides: warfarin, chlorphacinone, diphacinone
Second-generation anticoagulant rodenticides: brodifacoum, bromadiolone, difenacoum, difethiolone
Bromethalin
Cholecalciferol
Zinc phosphide
The following regulations were established. Exempted from these regulations are field use, tracking powders, and island conservation efforts.
Available to consumers for purchase:
Prohibited rodenticide classes: second-generation anticoagulant rodenticides
Bait size according to intended species:
Mouse: across active ingredients: 0.25–1 ounce
Rat:
First-generation anticoagulant: 4–16 ounces
Cholecalciferol: 2–8 ounces
Bromethalin: 1–6 ounces
Zinc phosphide: 0.15–0.3 ounces
Package size: Packages must be less than or equal to 1 pound of bait in total.
Ready-to-use (RTU) bait stations: All baits must be sold with at least one RTU bait station, with the intent that the bait remains in the station unless removed in small quantities by the target species.
Pet-resistant RTU bait stations: A station is labeled as "pet resistant" if it remains unbroken after 2 hours of placement near normal pet feeding stations and in normal pet environments, whether or not pets showed interest in or disturbed the station. For each product, this factor is studied across two groups of six dogs each. However, companion animals may be able to break through pet-resistant stations to access bait.
Formulations:
Allowed: blocks, pastes
Prohibited: meal, grain, pellets, liquid
Exceptions: Zinc phosphide is sold as a gray pellet or powder and without an RTU bait station, but it is intended for belowground use only.
Available for agricultural or pest control professionals only:
Allowed rodenticide classes: first-generation anticoagulant rodenticides, second-generation anticoagulant rodenticides, bromethalin, cholecalficerol, and phosphides
Bait size limitations: 0.25–16 ounces, depending on the intended target species (as above)
Package size and labeling requirements for anticoagulant rodenticides:
First generation: minimum of 4 pounds
Second generation:
Agricultural use: minimum of 8 pounds, labeled to be placed within 50 feet of agricultural buildings
Pest control professionals: minimum of 16 pounds
RTU bait stations: While not required to be sold directly with the product, RTU bait stations must be used if nontarget species, pets, and children < 6 years old can access the product.
Formulations: blocks, pellets, pastes, meal, grain, liquid
Availability: Sold only in agricultural or farm and tractor stores, or directly to pest control officers. Of note, these products remain readily available through online sources and may unintentionally be diverted directly to consumers and pet owners.
Since enactment of the mitigation efforts, suppliers were allowed to sell procured product that may not yet have adhered to these regulations. Therefore, exposure to “older” products placed in or around homes or buildings may not abide by these standards for some time, until supplies of said products are exhausted.
Companion animal exposure will more likely be to either bromethalin or cholecalciferol, and less commonly to anticoagulant or phosphide rodenticide products. Data procured through the Pet Poison Helpline since 2009 show a notable shift in the active ingredients to which companion animal patients have been exposed. Accurate product identification remains imperative in guiding appropriate efforts to decontaminate and treat the poisoned patient.
