Outbreak of poisoning by sodium hydrogen methylarsonate (MSMA)-an arsenic-based herbicide-in horses in Brazil.
Authors: Pereira Gabriella Faria, Blimbliem Maria Clara Hornich, Machado Anna Laura Previato Rosa, Abdala Junara Bianca Rosa, Nogueira Geison Morel, Toma Hugo Shisei, de Carvalho Tatiane Furtado, Delfiol Diego José Zanzarini
Journal: Equine veterinary journal
Summary
# Arsenic Poisoning from Herbicide Overdose in Horses: Clinical Features and Diagnostic Approach Arsenic-based herbicides present an under-recognised hazard to grazing horses, despite their widespread agricultural use; this Brazilian outbreak provides valuable clinical and epidemiological data on sodium hydrogen methylarsonate (MSMA) toxicity following accidental pasture contamination. Researchers documented 31 horses exposed to a pasture sprayed at 7.9 kg/ha—approximately 16 times the recommended dose—and compared clinical findings with a control group receiving appropriate application rates. The outbreak resulted in a 45.2% morbidity rate, 19.4% mortality rate, and 42.9% case fatality rate among affected animals, with toxicological analysis confirming elevated arsenic concentrations in biological samples (kidney, liver, faeces) and environmental sources; the comparative group remained clinically unaffected with low arsenic levels. Arsenic toxicosis primarily affects the gastrointestinal, vascular, and renal systems, and diagnosis can be supported through anatomopathological examination, biochemical alterations, and detection of arsenic residues in organ and faecal samples. For practitioners, this research underscores the importance of investigating pasture herbicide applications in cases of unexplained gastrointestinal or systemic illness in grazing herds, and highlights that arsenic contamination—though rare—warrants inclusion in differential diagnoses when multiple horses present with acute clinical signs following recent land management activities.
Read the full abstract on PubMed
Practical Takeaways
- •Verify herbicide application rates on pastures where horses graze; even small overdoses of MSMA can cause serious poisoning with high mortality rates
- •Suspect arsenic toxicity in horses presenting with gastrointestinal, vascular, or renal signs after pasture herbicide application; confirm via arsenic levels in kidney, liver, or faecal samples
- •Ensure strict coordination between farm managers and veterinarians before any pesticide or herbicide application on horse pastures to prevent accidental overdosing
Key Findings
- •An overdose of MSMA herbicide (7.9 kg/ha versus recommended dose) resulted in 45.2% morbidity, 19.4% mortality, and 42.9% case fatality rate in 31 exposed horses
- •Arsenic concentrations in biological samples (kidney, liver, faeces) and environmental samples confirmed diagnosis in affected animals
- •Horses in the comparative group exposed to recommended dosage remained clinically normal with low arsenic concentrations
- •Arsenic toxicosis affects gastrointestinal, vascular, and renal systems with diagnosis supported by anatomopathological findings and biochemical alterations