Fenbendazole
Fenbendazole is a highly effective, broad-spectrum **benzimidazole anthelmintic** widely used in veterinary medicine across multiple species. **Key Clinical Features:** * **Nematode Efficacy:** Highly effective against gastrointestinal roundworms, hookworms, and whipworms. * **Cestode Efficacy:** Effective against *Taenia* species tapeworms, but **not** effective against the common flea tapeworm (*Dipylidium caninum*). * **Protozoal Efficacy:** Considered a first-line treatment for **Giardia** infections in dogs and cats. * **Respiratory Parasites:** Frequently utilized for various lungworm infections (e.g., *Aelurostrongylus*, *Capillaria*, *Paragonimus*). **Clinical Pearl:** Fenbendazole has an exceptionally wide margin of safety and is considered the drug of choice for treating parasitic infections, including Giardia, in pregnant animals.
Mechanism: Fenbendazole is a **time-dependent** antiparasitic agent, which is why multi-day dosing is typically required in small animals. * **Primary Mechanism:** It selectively binds to parasite **ฮฒ-tubulin** โ prevents tubulin polymerization โ inhibits **microtubule formation**. This disrupts essential intracellular transport mechanisms within the parasite. * **Secondary Mechanism:** At higher concentrations, it disrupts metabolic pathways by inhibiting key enzymes such as **fumarate reductase** and **malate dehydrogenase** โ depletes ATP/energy reserves โ parasite death.
Dosing by species
- For susceptible parasites ยท 5 mg/kg in feed for 3 days ยท PO ยท q24h ยท 3 days
- CAMELIDS: For susceptible parasites in new world camelids ยท 10-20 mg/kg PO for 3-5 days ยท PO ยท q24h ยท 3-5 days
- CAMELIDS: For adjunctive treatment of meningeal worm infestation (Parelaphostrongylus tenius) ยท Fenbendazole 50 mg/kg PO for 5 days with ivermectin 0.3 mg/kg SC for 5 days. ยท PO ยท q24h ยท 5 days ยท Also use prophylactic treatment for stress ulcers, anti-inflammatories, fluids, nutritional support, etc.
- For susceptible parasites ยท 5 mg/kg in feed for 3 days ยท PO ยท q24h ยท 3 days
- For susceptible ascarids, hookworms, strongyloides, and tapeworms (Taenia spp. only) ยท 50 mg/kg, PO for 5 days ยท PO ยท q24h ยท 5 days
- For Giardia ยท 50 mg/kg PO once daily for 3 consecutive days; if this does not clear the infection, a longer course of therapy (5-7 days) is used ยท PO ยท q24h ยท 3-7 days
- For Giardia ยท 50 mg/kg PO q24h for 3-5 days ยท PO ยท q24h ยท 3-5 days
- For lungworms (Aelurostrongylus abstrusus) ยท 25-50 mg/kg q12h for 10-14 days ยท PO ยท q12h ยท 10-14 days
- For lungworms (Aelurostrongylus abstrusus) ยท 50 mg/kg, PO for 10 days ยท PO ยท q24h ยท 10 days
Routes of administration
Contraindications
- Not FDA-approved for use in horses intended for food purposes
Adverse effects
- Hypersensitivity reactions (secondary to antigen release by dying parasites)
- Salivation (infrequent)
- Vomiting (infrequent)
- Diarrhea (infrequent)
- Pancytopenia (rarely reported in dogs)
- Bone marrow hypoplasia (rarely reported in dogs)
Drug interactions
- Bromsalan flukicides (dibromsalan, tribromsalan) ยท Abortions in cattle and death in sheep have been reported after concurrent use
Monitoring
- Fecal examinations to confirm parasite clearance
- Monitor for hypersensitivity reactions if treating heavy parasite burdens
- Fecal examination for parasite clearance
- Complete blood count (CBC) if bone marrow hypoplasia is suspected
Overdose
Fenbendazole is exceptionally well tolerated, even at doses up to **100X the recommended dose**. * The LD50 in laboratory animals exceeds 10 grams/kg when administered orally. * It is highly unlikely that an acute overdosage would lead to any significant clinical signs.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.