Oxibendazole
Oxibendazole is a **benzimidazole anthelmintic** primarily utilized in equine medicine for the eradication of various internal nematodes. * **Key Targets**: Large roundworms (*Parascaris equorum*), large strongyles (*Strongylus edentatus, S. equinus, S. vulgaris*), small strongyles, threadworms, and pinworms (*Oxyuris equi*). * **Clinical Pearl**: Benzimidazole resistance is a widespread and growing concern in equine parasitology, particularly among small strongyles (cyathostomins) and *Parascaris equorum*. Routine **Fecal Egg Count Reduction Testing (FECRT)** is highly recommended to monitor farm-specific efficacy. A reduction value of less than 90% in 5-10 horses suggests resistance on a given farm. * While primarily labeled for horses, it has historical and off-label applications in cattle, sheep, and swine.
Mechanism: Benzimidazoles exert their anthelmintic effect by selectively binding to nematode **β-tubulin**. * **Primary Mechanism**: Binds to tubulin → prevents tubulin polymerization → inhibits **microtubule formation**. This disrupts essential intracellular transport systems within the parasite. * **Secondary Mechanism**: At higher concentrations, it disrupts metabolic pathways by inhibiting key enzymes such as **fumarate reductase** and **malate dehydrogenase**, ultimately starving the parasite of energy (ATP depletion). * *Pharmacological Note*: The selective toxicity arises because benzimidazoles have a much higher affinity for nematode tubulin compared to mammalian tubulin.
Dosing by species
- Susceptible parasites · 10-20 mg/kg PO · PO · Once
- Susceptible parasites · 10-20 mg/kg PO · PO · Once
- Susceptible parasites · 10 mg/kg PO; 15 mg/kg PO for strongyloides · PO · Once · Retreat in 6-8 weeks if reinfection is likely · Horses maintained on premises where reinfection is likely to occur should be retreated in 6-8 weeks.
- Susceptible parasites · 15 mg/kg, PO · PO · Once
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Severely debilitated horses
- Horses suffering from colic
- Toxemia
- Infectious disease
Adverse effects
- Hypersensitivity reactions (secondary to antigen release by dying parasites)
- Periportal hepatitis (historically noted in dogs when combined with diethylcarbamazine)
Monitoring
- Efficacy (via Fecal Egg Count Reduction Test - FECRT)
Overdose
Doses of 60 times those recommended elicited no adverse reactions in horses tested. It is unlikely that this compound would cause serious toxicity when given alone to horses.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.