Case of the Month - April 2026

Signalment and History

This eastern gray squirrel (Sciurus carolinensis) was found dead in a yard by a home owner. The home owner had noted a fairly large number of dead squirrels over the years. At inquiry, the owner stated that they do not use rodenticides on their property, but that they suspect that some neighbors use rodenticides lavishly.

Gross Findings

The contents in a segment of the colon of the submitted eastern gray squirrel were distinctly turquoise colored.

Morphologic Diagnosis

None

Etiologic Diagnosis

Rodenticide toxicity.

Discussion

Rodenticides are commonly used by home owners and in agricultural settings to control rodent populations. In the United States, anticoagulant rodenticides (including e.g. warfarin, chlorphacionone, diphacionone, bromadiolone, difethalione, brodifacoum) and non-anticoagulant rodenticides (including e.g. bromethaline, cholecalciferol, zinc phosphide, and strychnine) are commonly used. The rodenticide market in the United States is valued at several billion dollars. While anticoagulant rodenticides target the coagulation system of vertebrates, the non-anticoagulant rodenticides target the calcium-phosphorus metabolism (with potentially severe kidney failure) in the case of cholecalciferol, the central nervous system in the cases of bromethalin and strychnine, and cellular respiration in the case of zinc phosphide. The use of rodenticides poses a deadly risk not only to the targeted rodents but also to many other non-targeted vertebrates (including pets and humans) that accidentally ingest the bait or that prey on or scavenge rodents that have ingested rodenticides and subsequently become weak or die (e.g. carnivores and birds of prey). Because of the inherent danger of rodenticides, they are usually manufactured and sold as baits in bright, “unnatural” colors such as a bright green, turquoise, and vivid blues. Unfortunately, there are no standardized color-coding rules for the active compounds in rodenticides between the various producers so that only a chemical analysis of the compound can determine which compound was used in cases of intoxication (aka when the label of the product is not available). In the present case, such a chemical analysis was not performed. The absence of hemorrhages within the squirrel suggests that the animal ingested a non-anticoagulant rodenticide. In order to avoid the risk of harming non-target wildlife, pets, and humans, alternate methods of rodent deterrents can be used. These include sound-based repellents and non-toxic, smell based botanical rodent repellents. Furthermore, entry points allowing rodents access to dwellings should be sealed and nesting materials for rodents can be minimized in dwellings. Last but not least, attracting predators, including birds of prey and snakes to the yard can be helpful in reducing rodent populations.

References

1. Mercer MA, Davis JL, Riviere JE, Baynes RE, Tell LA, Jaberi-Douraki M, Maunsell FP, and Lin Z (2022), Mechanisms of toxicity and residue considerations of rodenticide exposure in food animals. – a FARAD perspective. Journal of the American Veterinary Medical Association, 260: 514-523. DOI: 10.2460/javma.21.08.0364
2. Nakayama SMM, Morita A, Ikenaka Y, Mizukawa H, and Ishizuka, M (2019), A review: poisoning by anticoagulant rodenticides in non-target animals globally. The Journal of Veterinary Medical Science, 81: 298-313. doi: 10.1292/jvms.17-0717