Anthelmintics as Hidden Toxins in Small Ruminants and Cattle: A Systematic Review
Background: Gastrointestinal nematode infections remain a major constraint on productivity and welfare in sheep, goats, and cattle. Routine anthelmintic administration is central to parasite control; however, reports of intoxication associated with overdose and misuse have raised safety concerns. Aims: This study aimed to synthesize published evidence on documented anthelmintic intoxication in domestic ruminants. Methods: A systematic literature search was conducted in Scopus, PubMed/MEDLINE, and Web of Science following PRISMA guidelines. Studies reporting confirmed or strongly suspected intoxication in sheep, goats, or cattle were included. Fifteen studies met the eligibility criteria and were analyzed qualitatively. The search strategy used combinations of keywords related to anthelmintics, toxicity or intoxication, and domestic ruminant species. Titles and abstracts were screened for relevance, followed by full-text evaluation of potentially eligible articles. Data extracted from the included studies covered animal species, type of anthelmintic involved, exposure circumstances, clinical manifestations, and reported pathological findings to enable a descriptive synthesis of intoxication patterns. Results: Most intoxication events involved overdose or inaccurate weight-based dosing. Macrocyclic lactones and levamisole were primarily linked to neurological dysfunction, including ataxia and tremors. Closantel was consistently associated with optic nerve degeneration and blindness, whereas nitroxinil exposure resulted in acute hepatic and renal injury with high mortality in outbreak settings. High extra-label benzimidazole dosing was reported to cause intestinal crypt necrosis and bone marrow suppression. Conclusion: The findings indicate that clinically significant toxicity arises mainly under misuse conditions rather than routine labeled administration, emphasizing accurate dosing and veterinary oversight to improve drug safety in ruminant production systems.