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Toxicity of methomyl following intravenous administration in the horse
R I Krieger1, P South, A Mendez Trigo
1Department of Entomology, University of California, Riverside 92521, USA.
Abstract:
Methomyl (S-methyl-N-((methylcarbamoyl)oxy) thioacetimidate) toxicity was studied in horses using i.v. dosages from 0.01 to 3.0 mg/kg. Doses of 1-3 mg methomyl/kg produced increased GI motility and respirations, facial fasiculations, salivation, lacrimation and convulsions. The only effect at 0.5 mg/kg was increased GI sounds in 1/4 horses. This extensively used crop insecticide had been associated with episodes of morbidity and mortality and led to speculation that it was extremely potent to the horse. This was not supported by these clinical studies.
Insights
Methomyl insecticide toxicity in horses was evaluated. High doses caused adverse effects, but clinical studies did not support claims of extreme potency in horses.
Area of Science:
- Veterinary Toxicology
- Equine Pharmacology
Background:
- Methomyl is a widely used carbamate insecticide.
- Concerns exist regarding its potential high toxicity in horses.
- Previous reports suggested significant equine morbidity and mortality linked to methomyl exposure.
Purpose of the Study:
- To investigate the clinical toxicity of methomyl in horses.
- To determine the dose-response relationship of methomyl in equine subjects.
- To evaluate the validity of claims regarding methomyl's extreme potency in horses.
Main Methods:
- Intravenous administration of methomyl at varying dosages (0.01 to 3.0 mg/kg).
- Clinical observation and recording of physiological and neurological responses.
- Dose-escalation study design to assess toxicity thresholds.
Main Results:
- Doses of 1-3 mg/kg methomyl induced significant clinical signs, including increased gastrointestinal motility, respiratory rate changes, facial muscle twitching, salivation, lacrimation, and convulsions.
- A lower dose of 0.5 mg/kg resulted in only transiently increased gastrointestinal sounds in one out of four horses.
- No severe adverse events or fatalities were observed at any tested dose.
Conclusions:
- While high doses of methomyl can elicit toxic effects in horses, the clinical findings do not substantiate the notion that this insecticide is extremely potent in equine species.
- The observed effects at higher doses are consistent with known mechanisms of carbamate toxicity.
- Further research may be warranted to establish safe exposure limits and understand field-related incidents.