Closantel for Cow: Uses, Dosing & Side Effects

This information is for educational purposes only. Never give your pet any medication without your veterinarian's guidance. Dosing, frequency, and safety depend on your pet's specific health profile.

Closantel is a prescription antiparasitic medication used in cattle to treat certain internal parasites, especially liver flukes and some blood-feeding worms. It belongs to the salicylanilide drug class. This group works by disrupting parasite energy metabolism, which is why closantel is mainly useful against flukes and a narrower set of worms rather than every common cattle parasite.

In practical herd medicine, closantel is usually considered a targeted dewormer, not a one-size-fits-all product. Your vet may choose it when fecal testing, regional parasite patterns, grazing history, or slaughter timing suggest flukes are a likely problem. In some countries, closantel is sold alone, while in others it is available in combination products such as closantel plus ivermectin.

For U.S. producers, product availability can be more limited than in some other countries, so your vet may discuss whether an approved product is available for your class of cattle or whether another flukicide is a better fit. Because cattle are food animals, meat and milk withdrawal times matter, and those decisions should always be made with your vet.

Closantel is used most often for liver fluke infections, especially Fasciola hepatica, in cattle. Liver flukes can reduce weight gain, feed efficiency, and overall productivity. In some products and settings, closantel is also used against late immature and adult flukes, which can make it helpful when timing treatment around seasonal fluke exposure.

It may also help control certain blood-feeding gastrointestinal parasites, especially Haemonchus species. These worms can cause anemia, weakness, poor thrift, and bottle jaw in affected cattle. Because closantel has a narrower spectrum than many broad dewormers, it is usually chosen when those specific parasites are the concern rather than for routine blanket deworming.

Your vet may recommend closantel when cattle have a history of grazing wet, snail-friendly pasture, when liver condemnation has been a problem, or when testing suggests a fluke burden. It is not the right medication for every parasite problem, and resistance patterns, stage of parasite development, and the animal's production class all influence whether it makes sense.

Closantel dosing in cattle depends on the exact product, concentration, route, and parasite being targeted. Always follow your vet's instructions and the product label for your region. A commonly referenced cattle dose for closantel-only pour-on products is 20 mg/kg topically, which corresponds to 1 mL per 10 kg body weight for a 200 mg/mL formulation. Other formulations, including oral drenches or combination products, use different concentrations and directions.

Because underdosing can reduce effectiveness and encourage parasite resistance, cattle should be weighed as accurately as possible rather than guessed by eye. Group dosing based on an average weight can leave heavier animals undertreated. Your vet may also time treatment around pasture exposure, fecal results, or local fluke season so the medication is used when it is most likely to help.

For food animals, dosing decisions also include withdrawal periods for meat and sometimes milk. Withdrawal times vary by product and country. Some closantel products are not labeled for animals producing milk for human consumption or have restrictions for dairy cattle and replacement heifers. If your cow is lactating, pregnant, or close to slaughter, tell your vet before treatment.

If a dose is missed or spit out after drenching, do not redose on your own. Call your vet and share the product name, concentration, route, estimated body weight, and how much medication you believe the animal actually received.

At labeled doses, many cattle tolerate closantel well, but side effects can still happen. Mild problems may include temporary irritation at the application or injection site, reduced appetite, or brief digestive upset depending on the formulation used. Combination products can also carry side effects related to the second active ingredient.

The most serious concern with closantel is toxicity from overdose or dosing errors. Reported signs can include depression, weakness, incoordination, recumbency, dilated pupils, vision problems, and blindness. Vision loss can be permanent. This is one reason accurate weight-based dosing is so important.

Call your vet promptly if a treated cow seems weak, stops eating, acts neurologic, bumps into objects, or develops worsening swelling after treatment. See your vet immediately if there are signs of blindness, severe ataxia, collapse, or multiple animals showing illness after the same batch or dosing event.

There is no home antidote for closantel toxicity. Care is typically supportive and may include stopping further exposure, examining the eyes and nervous system, and reviewing the exact product and dose used.

Published interaction data for closantel in cattle are limited, but that does not mean interactions are impossible. The biggest real-world risk is often stacking parasite products without a clear plan. Using multiple dewormers, pour-ons, or combination products too close together can increase the chance of dosing mistakes, residue concerns, or unnecessary treatment.

Closantel is also highly relevant to food-safety planning. If your vet uses a medication in an extra-label way in food-producing animals, federal rules require a valid veterinary-client-patient relationship, careful records, and a scientifically supported withdrawal period. That is especially important if other medications are being used at the same time.

Tell your vet about all recent treatments, including ivermectin products, pour-ons, injectables, oral drenches, mineral supplements, and any medicated feed. Also mention whether the cow is producing milk for human consumption, is pregnant, or is scheduled for slaughter soon. Those details can change which option is safest and most practical.