Coumaphos for Cow: Uses, Lice Control & 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.

Coumaphos is an organophosphate external parasite control product used in cattle. It is not an antibiotic or dewormer. Instead, it is used on the hair coat or in application systems such as labeled sprays, dusts, backrubbers, or ear tags, depending on the product your vet recommends.

In cattle medicine, coumaphos has been used for lice, horn flies, face flies, ticks, and some other ectoparasites. Merck notes that coumaphos is active against a range of external parasites in large animals, but use has become more limited over time because organophosphates carry important animal, human, and environmental safety concerns.

This matters because the product label and formulation matter a lot. “Coumaphos” is the active ingredient, but the exact directions, age restrictions, retreatment interval, and safety precautions depend on whether your vet is using a spray, dust, backrubber product, or another labeled form. For food animals, your vet also has to consider milk and slaughter instructions on the specific label.

In cows, coumaphos is mainly used for external parasite control, especially lice and flies. Current US labels for Co-Ral products include control of horn flies and lice on beef and dairy cattle, reduction of face flies, and with spray formulations, control of ticks as well.

Lice control is often the reason pet parents and producers ask about coumaphos in winter or during periods of crowding, stress, or poor coat condition. Merck describes lice infestations in cattle as causing itching, rubbing, hair loss, rough hair coat, skin irritation, and reduced thrift, and severe blood-sucking louse infestations can contribute to anemia.

Your vet may choose coumaphos when the parasite type, herd setup, handling facilities, and residue considerations make it a reasonable option. It is not the only option. Depending on the situation, your vet may instead recommend another topical product, a pour-on, an injectable parasite plan, or management changes such as treating the whole group, improving nutrition, and reducing reinfestation pressure.

Coumaphos dosing in cattle is product-specific and label-specific, so your vet should direct exactly how it is used. For example, the 2025 EPA-stamped label for Co-Ral Fly & Tick Spray directs cattle spray mixing at 1 gallon of product per 100 gallons of water, with no more than 6 applications per year and at least 10 days between treatments. The same label gives a backrubber mixture of 9.75 ounces (95 mL) per 1 gallon of approved backrubber oil for horn flies and face flies.

For Co-Ral 1% Livestock Dust, the label allows direct application to beef and dairy cattle at no more than 2 ounces per animal per treatment, with no more than 12 treatments per year and at least 10 days between applications. Dust bags may also be used according to label directions. These are not interchangeable directions, which is why your vet and the exact product label matter.

There are also important restrictions. The spray label states do not use on animals less than 3 months old, and do not apply to sick, convalescent, or stressed livestock. Dust labels also warn against use in sick, stressed, or convalescent animals. Even when a label notes no milk or slaughter interval for a specific use, your vet still needs to confirm that the exact product, class of animal, and use pattern fit your herd.

See your vet immediately if a cow shows signs of a reaction after coumaphos exposure. Because coumaphos is an organophosphate cholinesterase inhibitor, overdose or sensitivity can cause toxic effects involving the nervous system, gut, lungs, and muscles.

Mild to moderate problems may include excess salivation, tearing, diarrhea, abdominal discomfort, muscle tremors, weakness, and unusual restlessness. Merck notes that organophosphate toxicosis can also cause miosis, frequent urination, respiratory distress, and muscle fasciculations. In more serious cases, cattle can develop ataxia, recumbency, paralysis, seizures, or death.

The Co-Ral spray label also tells veterinarians to watch for bloat, excessive salivation, and posterior paralysis after treatment. Some reactions may reflect a host-parasite reaction rather than straightforward overdose, which is one more reason not to treat without veterinary guidance. Young calves are more sensitive than adult cattle, and Merck warns that concentrations that adult cattle may tolerate can be dangerous or even lethal in younger animals.

The biggest interaction concern with coumaphos is other cholinesterase-inhibiting drugs or pesticides. The current Co-Ral Fly & Tick Spray label specifically says not to use this product simultaneously or within a few days before or after exposure to cholinesterase-inhibiting drugs or pesticides. Combining products in this class can raise the risk of toxicity.

That means your vet should know about all parasite products, premise sprays, ear tags, feed-through fly control products, and any recent pesticide exposure before coumaphos is used. Even if two products are both labeled for cattle, they may not be safe to layer together.

If a cow has liver disease, severe illness, dehydration, or recent stress such as shipping or weaning, your vet may be more cautious because those factors can complicate tolerance and recovery if a reaction occurs. Do not mix coumaphos with other on-animal insecticides unless your vet confirms the combination is appropriate and label-compliant.