Organophosphate toxicity

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Organophosphates (OP) are an older pharmaceutical group of insecticides and acaricides used as flea and tick collars and washes which have been replaced by newer drugs such as carbamates, pyrethrins, pyrethroids, insect growth regulators and spot-on formulations[1].

Organophosphates are synthesized from phosphoric or phosphonic acid and vary in their toxicity, residue levels, and excretion. Compounds commonly observed with toxicity in dogs include include malathion, dichlorvos, chlorpyrifos and trichlorfon.

Organophosphate toxicity is less prevalent these days but still occasionally occurs[2].

Signs of OP poisoning are those of cholinergic overstimulation, which can be grouped under three categories: muscarinic, nicotinic, and central. Muscarinic signs, which are usually first to appear, include hypersalivation, miosis, frequent urination, diarrhea, vomiting, colic, and dyspnea due to increased bronchial secretions and bronchoconstriction. Nicotinic effects include muscle fasciculations and weakness. The central effects include nervousness, ataxia, apprehension, and seizures. CNS stimulation in dogs and cats usually progresses to convulsions[3].

Onset of signs after exposure are usually rapid (within minutes) but may be delayed for more than two days in some cases. Severity and course of intoxication is dosage and route related in most cases.

In acute poisoning, respiratory distress and collapse followed by sudden death due to respiratory muscle paralysis is infrequently observed.

A diagnosis is based on history of exposure and clinical signs. A differential diagnosis would include snake bite envenomation, tick paralysis and myasthenia gravis.

Treatment is usually symptomatic, with washing and treatment with atropine to control muscle fasciculation. Other anticholinergic drugs are also effective, including physostigmine and scopolamine[4].


  1. Beugnet F & Franc M (2012) Insecticide and acaricide molecules and/or combinations to prevent pet infestation by ectoparasites. Trends Parasitol 28(7):267-279
  2. Wang Y et al (2007) Pesticide poisoning in domestic animals and livestock in Austria: a 6 years retrospective study. Forensic Sci Int 169(2-3):157-160
  3. Merck Veterinary Manual
  4. Meshulam Y et al (2001) Prophylaxis against organophosphate poisoning by sustained release of scopolamine and physostigmine. J Appl Toxicol 21(1):S75-S78