Outbreak of poisoning by sodium hydrogen methylarsonate (MSMA)-an arsenic-based herbicide-in horses in Brazil.

Abstract

BACKGROUND: Arsenic poisoning in horses is rarely reported in the literature. However, arsenic compounds can be present in rodenticides, pesticides, and herbicides, representing a potential source of accidental exposure for horses. OBJECTIVE: To describe the epidemiological, clinical, and laboratory findings from a herd of 31 horses exposed to pasture treated with an overdose of sodium hydrogen methylarsonate, and to compare results in a group of horses exposed to the recommended dosage of the same product. STUDY DESIGN: Observational field study. METHODS: Thirty-one horses were evaluated after grazing on a pasture sprayed with 7.9 kg/ha of MSMA. Sixteen animals underwent clinical examination, haematological and biochemical analyses, anatomopathological evaluation, and toxicological analysis. Arsenic concentrations were determined in biological and environmental samples to confirm exposure. The findings were compared with data collected from a group of horses grazing a paddock treated with the recommended dose. RESULTS: The outbreak had a morbidity rate of 45.2% (14/31), a mortality rate of 19.4% (6/31), and a case fatality rate of 42.9% (6/14). Toxicological analysis showed increased arsenic concentrations in biological and environmental samples, confirming the diagnosis of poisoning. Supportive therapy was administered until the definite diagnosis and recovery of the surviving horses. Animals in the comparative group remained clinically normal and showed low arsenic concentrations. MAIN LIMITATIONS: The observational nature of the study, the delayed initiation of monitoring, and prior therapeutic interventions limited the scope of clinical data. Moreover, the rarity of arsenic poisoning in horses and the paucity of reference data restricted interpretation. CONCLUSIONS: Arsenic toxicosis in horses affects gastrointestinal, vascular, and renal systems. Diagnosis is supported by anatomopathological findings, biochemical alterations, and detection of arsenic in kidney, liver, and faecal samples.