Benzimidazole‐associated pancytopenia in a dog with presumed Heterobilharzia americana infection

Adverse drug events are classified into type A (dose‐dependent and common) or type B (idiosyncratic and uncommon) reactions. Benzimidazole toxicosis in veterinary medicine typically induces nonspecific signs such as gastrointestinal distress and infrequent myelosuppression. Pancytopenia from benzimidazoles is exceptionally rare, with only one reported case. We present a 12‐year‐old, female, spayed, mixed‐breed dog with acute lethargy, anorexia, pyrexia and severe leukopenia after high‐dose benzimidazole therapy for suspected liver fluke infection. Despite hospitalisation, the dog remained unresponsive to antimicrobial therapies, developed neutropenic sepsis and was euthanized. Bone marrow analysis indicated marked haematopoietic hypoplasia suggestive of a type A adverse drug event secondary to an incidental, high‐dose fenbendazole administration.