Pesticide Use and Horses

By: Lorelei Prichard DVM
About the author

Pesticide toxicity is a rare occurrence in horses – yet no event is rare to the one who experiences it. Ingestion, inhalation, or absorption of insecticides, herbicides, certain medications and chemicals can, in some instances, lead to an accidental poisoning. Poisons, by their chemical nature, may cause damage to structure or lead to disturbances of function.

Most pesticide poisonings occur as a result of inappropriate pesticide use or administration. These intoxications are usually accidental and non-life threatening and include inadvertent ingestion of dilute fertilizers, ant-killing products, small amounts of rodenticides (mouse and rat baits), and minimal exposure to molluscides (snail bait).

Exposure to intoxicants such as organophosphate or carbamate insecticides, however, can cause severe reactions and are the insecticides most commonly reported to poison centers. Examples of organophosphates include Malathion and Parathion, whereas carbamates are often known better as Sevin dust, Furadan, and Lannate.

These insecticides exert their effects upon the nervous system, and symptoms are exhibited by the musculoskeletal system, respiratory system and gastrointestinal system. Acute symptoms of toxicity are often seen within 1-12 hours of exposure. Symptoms include anxiety, sweating and generalized muscle tremors. Abdominal pain, diarrhea, irregular muscle contractions and weakness, bronchoconstriction and increased bronchial secretions may be observed as well.

Most horse fly products utilize pyrethrins as an insecticide. Pyrethrin is a compound that originates from the chrysanthemum plant, and people have used extracts from these flowers for centuries for insect control. Although effective, sunlight and air quickly degrades the potency of pyrethrins, and its usefulness lasts only around two hours. Permethrin, a synthetic form based on pyrethrins, is more stable and lasts significantly longer. For horses sensitive to pyrethrins, Dr. Mike’s Insect Repellent is a viable alternative.

What should you do if you suspect pesticide toxicity?

• Involve your veterinarian, county extension agent and the poison control center.
• Identify and log the onset and the duration of symptoms.
• Note the number of animals (and species) affected.
• Identify any toxins/insecticides applied within the past 24 hours.
• Ask your veterinarian to draw blood if organophosphate toxicity is suspected.
• If your animal has died, have your veterinarian conduct a postmortem examination as soon as possible.

The most reliable diagnostic for organophosphate toxicity is determination of acetylcholinesterase levels in your horse’s blood. Organophosphate toxicity can be treated if discovered in a timely manner, however symptomatic treatment and respiratory support will be needed as well.

Postmortem findings are generally nondiagnostic when pesticide toxins are involved, however other causes of death may be revealed during an autopsy. Analysis of brain tissue for decreased levels of acetylcholinesterase levels may also be of value if performed within a few days of death. Realize that ambient temperature will affect natural tissue destruction (postmortem autolysis) and that samples will be more diagnostic if taken immediately.

Special care should always be taken when chemical and pesticide use is employed around horses, and precautions should be taken to prevent drift or drainage that may contaminate feed and water sources. Even when appropriate chemicals are used, improper application can lead to toxic residues. When dealing with pesticides and other chemicals, always read the label carefully, apply as directed, and follow all precautions.

Copyright © 2007 Lorelei Prichard DVM