Equis ISSN 2398-2977

Toxicity: pesticides

Contributor(s): Steven Gallego, Birgit Puschner, Nicola Bates


  • Cause: pesticide exposure usually due to inappropriate use, handling or storage; exposure may be dermal, inhalation or ingestion.
  • Signs: dependent on agent, dose, individual, duration, route.
  • Diagnosis: can be difficult due to vague signs and rapid progression of poisoning.
  • Treatment: usually supportive; occasionally specific.
  • Prognosis: case by case dependent.



  • Pesticide-dependent including, but not limited to:
    • Organophosphates found in insecticides.
    • Carbamates found in insecticides and molluscicides.
    • Anticoagulants found in rodenticides Toxicity: anticoagulant rodenticide.
    • Metaldehyde found in snail and slug baits.
    • Pyrethrins/pyrethroids found in insecticides for animal and property use.
    • Strychnine found in rodenticides.
    • Phosphides found in vertebrate control products
  • Pesticide mixtures often incorporate feedstuffs (baits, such as grain) found edible by horse and pest alike.
  • Pesticide misuse (excess application, ignoring manufacturer’s withdrawal periods) with grass or grains intended for equids.

Predisposing factors


  • Wind.
  • Inappropriate use.
  • Human error, iatrogenic, malicious Toxicity: malicious intoxication.
  • Inappropriate storage.
  • Inappropriate disposal of obsolete pesticides or used pesticide containers.


  • Pesticide wind drift from neighboring properties.
  • Unintentional placement of insect or rodent baits accessible to horses.
  • Miscalculated anthelmintic dose administration.
  • Contraindicated pesticide use in foals.
  • Storage of pesticides in the same area as feeds.
  • Failure to follow manufacturer’s instructions for correct usage.
  • Illiterate caretakers unable to read pesticide manufacturer instructions.
  • Malicious intent by others Toxicity: malicious intoxication.
  • Illegal use of restricted or banned pesticides.
  • Storage activation: certain pesticides (such as malathion, diazinon) become more toxic with time and heat as toxic degradation products accumulate.
  • Use of old pesticide containers for feed or water.
  • Inadequate clean-up of pesticide spillage.
  • Contamination of feed or water source.


  • OPs, carbamates: acetylcholinesterase inhibitors:
    • Muscarinic parasympathetic stimulation.
    • Nicotinic neuromuscular junction block.
    • Central nervous system stimulation and eventual depression.
  • Anticoagulants: vitamin K1 depletion, inhibiting hepatic prothrombin production, resulting in coagulopathy.
  • Metaldehyde: uncertain pathophysiology, but excitatory neurotransmitters may be increased, or inhibitory neurotransmitters decreased.
  • Avermectins: bind to neuromuscular glutamate-gated chloride channels.
  • Pyrethroid/pyrethrins: holds nerve cell sodium channels open allowing sodium influx leading to cell excitation.
  • Strychnine: loss of spinal motor neuron inhibition resulting in skeletal muscle stiffness and rigidity.
  • Phosphides: uncertain pathophysiology, but multiple mechanisms result in oxidative injury.


  • OPs, carbamates, metaldehyde, avermectins, pyrethroids, strychnine, phosphides:
    • Onset can be measured in minutes to hours depending on animal age, route of exposure, dosage, concentration, product and particular agent.
    • Duration can be hours or several days again depending on route of exposure, dosage, concentration, product animal age and the particular agent.
  • Anticoagulants:
    • Signs do not manifest for up to 7 days following exposure to anticoagulants but could be longer if dose is low and there is repeated ingestion, as the half-life of these compounds is long, and they accumulate.


  • Seasonal variation with field crop management; pesticides are used at time coinciding with specific insect’s life cycle (pupae, larvae, immature or adult) or animal invasions, ie field mice, gophers.
  • Horse owner’s seasonal practices for ecto- and or endoparasite management.


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Further Reading


Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Nagy L, Cortinovis C, Spicer L J, Perego M C & Caloni F (2018) Long-established and emerging pesticide poisoning in horses. Equine Vet Educ 31 (9), 496-500 VetMedResource.
  • Bates N (2017) Acute poisoning in horses: Part 1. Livestock 22 (2), 105-109.
  • Caloni F, Cortinovis C, Rivolta M & Davanzo F (2015) Suspected poisoning of domestic animals by pesticides. Sci Total Environ 1 (539), 331–336 PubMed.
  • Plumlee K H (2002) Toxicosis from agricultural chemicals. Clin Tech Equine Pract 1 (2), 94-97 ScienceDirect.
  • Plumlee K H (2001) Pesticide toxicosis in the horse. Vet Clin North Am Equine Pract 17 (3), 491-500 PubMed.


  • ASPCA Animal Poison Control Center. Tel: +1 (888) 426-4435. Website: www.aspca.org.
  • Veterinary Poisons Information Service (VPIS). Tel: +44 (0)2073 055 055. Website: www.vpisglobal.com.