Organophosphate Toxicity in Sheep: Tremors, Weakness, and Collapse

Organophosphate toxicity is poisoning caused by a class of insecticides that overstimulate the nervous system. These chemicals block acetylcholinesterase, an enzyme that normally helps nerves reset after sending signals. When that enzyme is inhibited, nerves keep firing. In sheep, that can lead to drooling, diarrhea, tremors, weakness, breathing problems, collapse, and sometimes death.

Sheep may be exposed during parasite control, premise spraying, or accidental contact with concentrated products. Risk rises when products are mixed incorrectly, used too often, applied to stressed or very young animals, or combined with other cholinesterase-inhibiting chemicals. Some sheep are exposed through contaminated feed, water, or equipment rather than direct treatment.

This is a true emergency because the lungs and breathing muscles can be affected early. Some sheep show obvious muscarinic signs like salivation and diarrhea first, while others may present with tremors, weakness, or sudden recumbency. Your vet will decide how aggressive treatment needs to be based on the exposure history, exam findings, and how quickly signs are progressing.

Most cases happen after exposure to organophosphate insecticides used for external parasite control or premise treatment. Examples in veterinary and agricultural settings include products containing compounds such as coumaphos, diazinon, dichlorvos, chlorpyrifos, malathion, trichlorfon, and related chemicals. Sheep can be poisoned by dips, sprays, dusts, premise applications, or accidental access to concentrates.

Common causes include mixing errors, using a product at the wrong dilution, repeating treatment too soon, applying more than one cholinesterase-inhibiting product, or treating animals that are already stressed, dehydrated, or ill. Lambs and debilitated sheep may be less able to tolerate exposure. Contaminated feed, water, or equipment can also be involved.

Not every sheep on the property will look equally affected. Dose, route of exposure, fleece contamination, weather, and time since exposure all matter. Your vet may also consider carbamate insecticide poisoning because it can cause a very similar cholinergic syndrome.

Diagnosis usually starts with the story and the physical exam. Your vet will ask about recent dips, sprays, pour-ons, premise insecticides, feed changes, and any chance of accidental access to concentrates. The pattern of signs matters too. Salivation, diarrhea, pinpoint pupils, tremors, weakness, and respiratory distress after pesticide exposure are strongly suspicious.

Your vet may recommend blood testing for cholinesterase activity, which can support the diagnosis when levels are decreased. In some cases, samples from blood, rumen contents, feed, or the suspected product may be submitted for toxicology. Response to treatment can also help support the diagnosis, especially when atropine and decontamination improve signs.

Because several emergencies can look similar, your vet may also work through other possibilities such as lead poisoning, salt toxicity, polioencephalomalacia, hypomagnesemia, plant toxicosis, or carbamate exposure. If the sheep dies, necropsy and laboratory testing may help confirm the cause and protect the rest of the flock.

Prevention starts with careful pesticide handling. Use only products labeled for sheep, follow the label exactly, and measure concentrates accurately. Do not guess at dilution rates or combine products unless your vet specifically advises it. Store concentrates away from feed, water, and children, and keep mixing equipment clearly labeled.

Before treating a flock, check whether any sheep are very young, thin, dehydrated, sick, or stressed from transport, weather, or heavy parasite burden. Those animals may need a different plan. Avoid repeat treatments sooner than directed, and do not use multiple cholinesterase-inhibiting products close together without veterinary guidance.

If a treatment error happens, remove animals from the source right away and call your vet promptly. Save the product container, lot number, and mixing details. Good records matter. Write down what was used, how much was mixed, when it was applied, and which animals were exposed. That helps your vet protect both the affected sheep and the rest of the flock.