Closantel: veterinary dose and monograph

At a glance

Drug class

Antiparasitic

Also known as

Virclos, Flukiver, Closantil

Species with doses

Cattle, Goats and Sheep

General dose range

5–10 mg/kg PO once — check the species tables for the indication

Routes

PO

Last reviewed

1 October 2026

Indications

• Adult & late-immature liver fluke (Fasciola hepatica) in cattle, sheep, goats
• Haemonchus contortus (incl. some BZ-resistant strains)
• Bunostomum, Oesophagostomum, Chabertia (limited)
• Oestrus ovis (sheep nasal bot)

Closantel dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Cattle

• Adult liver fluke: Label-sourced — closantel has no dosing monograph in Papich 5e, Plumb’s or Merck; follow the product label. Merck notes closantel binds strongly to plasma protein (long persistence, activity against late-immature flukes) with slaughter withdrawals of 21–77 days and no use in lactating animals whose milk is for human consumption.

Goats

• Liver fluke / Haemonchus: Label-sourced — not dosed in Papich 5e, Plumb’s or Merck; follow the product label. Overdose causes blindness — dose accurately by weight.

Sheep

• Adult / late-immature liver fluke: Label-sourced — not dosed in Papich 5e, Plumb’s or Merck; follow the product label. Merck: slaughter withdrawals for closantel range 21–77 days; not for ewes producing milk for human consumption. Toxic (blindness) in overdose — weigh accurately.
• Haemonchus contortus: Label-sourced — not dosed in Papich 5e, Plumb’s or Merck. Merck notes closantel’s prolonged plasma persistence gives extended protection against Haemonchus. Follow the product label dose and withdrawal (21–77 days for closantel products).

Buffalo

No buffalo-specific doses are on file, so buffalo are dosed as cattle.

• Adult liver fluke: Label-sourced — closantel has no dosing monograph in Papich 5e, Plumb’s or Merck; follow the product label. Merck notes closantel binds strongly to plasma protein (long persistence, activity against late-immature flukes) with slaughter withdrawals of 21–77 days and no use in lactating animals whose milk is for human consumption.

Contraindications

NOT for lactating dairy animals producing milk for human consumption (long milk withdrawal). Hypersensitivity. Severe ocular toxicity if overdosed (transient blindness). Avoid in heavily pregnant ewes.

Species-specific warnings

Cattle: 10 mg/kg PO single for adult fluke. NOT for lactating dairy. 28 d slaughter withdrawal.

Sheep: 7.5–10 mg/kg PO single. 42 d slaughter withdrawal. Excellent fluke + Haemonchus activity.

Goats: 7.5–10 mg/kg PO. Same withdrawal as sheep.

Horses: NOT used in horses — no established safety / efficacy data.

Dogs: NEVER use in dogs — toxicity reports.

Cats: NEVER use in cats.

Adverse effects

• Transient blindness at overdose (>20 mg/kg)
• Mild GI upset
• Photosensitivity (rare)

Drug interactions

• Other flukicides (triclabendazole, albendazole): complementary spectrum — sometimes sequential use
• Albumin-displacement drugs (sulphonamides, NSAIDs): may transiently increase free closantel — caution in sick animals
• Other salicylanilides (rafoxanide, oxyclozanide): same mechanism — do NOT combine
• Bromsalan flukicides (dibromsalan): combination has caused fatalities in cattle — AVOID

Pregnancy and lactation

Generally safe in pregnant sheep, goats, cattle at therapeutic doses. Crosses placenta in low amounts. Long withdrawal periods (cattle 28 d slaughter, sheep 42 d) reflect tissue residue persistence rather than acute toxicity.

Monitoring

• Exact body weight before dosing — narrow safety margin demands precision
• Withdrawal periods strictly: cattle 28 d slaughter; sheep 42 d slaughter; NOT for lactating dairy
• Faecal egg count reduction test at 14 days post-treatment — confirm efficacy and resistance
• Visual examination — discontinue at first sign of vision change
• Liver enzymes on chronic / repeated dosing

Toxicity and overdose

NARROW THERAPEUTIC MARGIN — doses >15 mg/kg cause BLINDNESS (optic nerve atrophy / retinal damage) in cattle and sheep. Permanent. Idiosyncratic blindness has been reported at therapeutic doses (uncommon). Hepatotoxicity at very high doses. NEVER in cats or dogs (toxicity reports). NOT for lactating dairy cattle — long tissue residue contaminates milk.

Management: No specific antidote. Discontinue. Supportive care: IV fluids, hepatic protection (SAMe, silymarin). Blindness: usually permanent once established — no treatment reverses optic nerve atrophy. Hemodialysis NOT effective (highly protein-bound).

Clinical pearls

• Sheep liver fluke (acute / chronic): 10 mg/kg PO single — kills adult + late-immature (>6 weeks) flukes; immature flukes <6 weeks not killed
• Cattle adult liver fluke: 10 mg/kg PO single — not effective against immature stages
• Haemonchus contortus in sheep / goats: 5–7.5 mg/kg PO single
• NEVER exceed 15 mg/kg — risk of BLINDNESS from optic nerve damage
• Long protein-binding gives prolonged efficacy — useful in seasonal fluke control
• Triclabendazole is preferred for immature flukes (<6 weeks); closantel for adults
• NEVER use in lactating dairy cattle — long milk residue contamination
• NEVER in cats or dogs

Mechanism of action

Salicylanilide flukicide / nematocide. Uncouples mitochondrial oxidative phosphorylation in adult and late-immature Fasciola hepatica and Haemonchus contortus — disrupts ATP synthesis, leading to parasite death. Highly protein-bound (~99% to plasma albumin) → very long half-life (~14 days in sheep) → effective against immature flukes for up to 4 weeks post-treatment. Also has anti-ectoparasitic activity against blood-feeding nasal bot fly (Oestrus ovis) larvae.

Formulations and products

• Virclos oral solution 5 % w/v (Virbac)
• Flukiver injection 5 % (50 mg/mL)
• Virclos (Virbac) — 5 % w/v (50 mg/mL), Oral solution
• Santaworm (Mankind Veticura) — 15 % w/v (150 mg/mL), Oral solution
• Clositel (Indian Immunologicals (IIL), Hyderabad) — 15 % w/v (150 mg/mL), Oral solution
• Endorid (Micro Animal Health Care (Micro Labs group), Bengaluru) — 150 mg/mL, Oral solution

Storage

Oral solution: 15–25 °C, shake well before use, protected from light. Multidose bottles: discard 28 days after broaching. Do NOT freeze.

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

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Frequently asked questions