Case of the Month - May 2026

Signalment and History

Two, approximately 4 week-old great horned owl (Bubo virginianus) nestlings were admitted to The Raptor Center of the University of Minnesota after being found outside a nest site. 

Owlet #1: At presentation, one animal was bright, alert to responsive and feisty when handled, and was able to stand. The owlet had a normal respiratory pattern. Physical examination revealed a small to moderate amount of blood within the oral cavity and subjectively pale mucous membranes, raising concern for a coagulopathy. Radiographs identified radio-dense material consistent with bone within the caudal gastrointestinal tract, without evidence of musculoskeletal abnormalities. The animal was treated with subcutaneous lactated Ringer’s solution, meloxicam (0.9 mg/kg), and vitamin K (2.5 mg/kg). Despite supportive care, the clinical condition deteriorated. By the following morning, the animal had developed dull mentation, sternal recumbency, and increased respiratory effort despite supplemental oxygen. Due to progressive clinical decline, the animal was euthanized. 

Owlet #2: The owl was bright, alert to responsive and feisty when handled. It was in a sternal posture. Physical examination revealed moderate to severe hemorrhage within the oral cavity, blood within the nares, and subjectively pale mucous membranes. Radiographs identified radiodense material consistent with bone within the caudal gastrointestinal tract, without evidence of musculoskeletal abnormalities. The owl was anesthetized with isoflurane and intubated (ET tube size 2.5 mm) for supportive care. During induction, open-mouth breathing with increased respiratory rate and effort was noted; the patient stabilized under anesthesia but experienced a transient episode of hypoxia during recovery associated with suspected airway obstruction of the endotracheal tube by fluid. Treatment included subcutaneous lactated Ringer’s solution, meloxicam (1 mg/kg), and vitamin K (2.5 mg/kg). Despite supportive care, the owl died overnight.

Gross Findings

Owlet #1: The animal was in a good nutritional state based on the small to moderate amount of subcutaneous and internal fat (BW:792 grams). There were a generalized pallor of mucosa membranes, skeletal muscle and internal organs (liver, spleen, and heart). A large blood coagulum covered the right liver lobe.

Owlet #2: The animal was in a good nutritional state based on the small to moderate amount of subcutaneous and internal fat (BW: 1009 grams). The subcutis and fascia over the pectoral muscles were expanded by acute edema and hemorrhage. The left lung lobe was diffusely dark-red (hemorrhage). The liver was beige discolored and the heart was pale.

Figure 1: Owlet #1 (left) and owlet #2 (right) are depicted. Please note the pallor of the musculature of owlet #1 and the acute hemorrhage over large portions of the pectoral muscles of owlet #2. 

Toxicology

Diphacinone was detected in the liver of owlet #1 (performed at Michigan Veterinary Diagnostic Laboratory, Michigan State University). Brodifacoum, bromadiolone, chlorphacinone, dicoumarol, difenacoum, difethalione, and warfarin were not detected in the liver of owlet #1. The liver of owlet #2 was not examined. 

Morphologic Diagnosis

Owlet #1: - Anemia, marked.

• hemocoelom, marked, acute

Owlet #2: - Anemia, marked.

• Skin/muscle, subcutaneous hemorrhage, focally extensive, acute
• Lungs (left lobe), hemorrhage, diffuse, acute

Etiologic Diagnosis

Diphacinone intoxication.

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. While anticoagulant rodenticides target the coagulation system of vertebrates, 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 birds and mammals (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. free-ranging carnivores and birds of prey). An abundance of research shows that birds of prey are particularly commonly exposed to rodenticides. Thirty one of 36 great horned owls (86%), 7 of 16 red-tailed hawks (43%), and 5 of 11 Cooper’s hawks (45%) examined at the Minnesota Veterinary Diagnostic Laboratory between 2001 and 2005 had an analytically detectable concentration of anticoagulant rodenticides (exclusively brodifacoum in owls and mainly brodifacoum and/or rarely bromadiolone) in the liver regardless of evidence of hemorrhage or the cause of death (Dr. Arno Wuenschmann, personal communication). In the present cases, both nestlings were likely fed a rodent by the parents that had died or was predisposed to predation due to ingestion of diphacinone.

Because of the inherent danger of rodenticides to non-target species 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. Elliott JE, Rattner BA, Shore RF, and van den Brink NW (2016), Paying the pipers: mintigating the impact of anticoagulant rodenticides on predators and scavengers. BioScience 66: 401-407. doi:10.1093/biosci/biw028

2. Gomez EA, Hindmarch S, and Smith JA (2022), Conservation letter: raptors and anticoagulant rodenticides. Journal of Raptor Research, 56: 147-153. 

3. 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