Outbreak of poisoning by sodium hydrogen methylarsonate (MSMA)-an arsenic-based herbicide-in horses in Brazil.
Abstract: 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. Methods: 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. Conclusions: 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.
© 2025 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
Publication Date: 2025-12-08 PubMed ID: 41361847DOI: 10.1111/evj.70132Google Scholar: Lookup
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Summary
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Overview
- This study investigated an outbreak of arsenic poisoning in horses caused by overexposure to sodium hydrogen methylarsonate (MSMA), an arsenic-based herbicide, on pasture land in Brazil.
- It detailed the clinical symptoms, laboratory results, and pathological findings in affected horses and compared them to horses exposed to safe herbicide levels.
Background
- Arsenic poisoning in horses is rarely reported, despite arsenic-containing compounds being present in various chemicals like rodenticides and herbicides.
- Sodium hydrogen methylarsonate (MSMA) is an arsenic-based herbicide that can pose risks if overdosed.
- Because of the potential for accidental exposure, understanding poisoning cases helps in diagnosis and treatment.
Objectives
- To describe the epidemiological characteristics of the poisoning outbreak in a herd of 31 horses exposed to an overdose of MSMA on pasture.
- To characterize the clinical signs, biochemical and hematological changes, pathological findings, and toxicological confirmation of arsenic poisoning.
- To compare these findings with horses exposed to recommended doses of MSMA to assess differences.
Study Design and Methods
- Observational field study conducted on a 31-horse herd grazing on pasture treated with an excessively high dose of MSMA (7.9 kg/ha).
- Out of these, 16 horses underwent:
- Clinical examinations to document symptoms.
- Hematological and biochemical analyses to identify internal physiological changes.
- Anatomopathological (post-mortem) assessments on deceased or euthanized animals.
- Toxicological analyses to measure arsenic levels in biological samples (kidney, liver, feces) and environmental samples from pasture.
- A control group of horses grazing on pasture treated with recommended MSMA doses was also evaluated to provide comparative data.
Key Findings
- The poisoning outbreak showed:
- Morbidity rate: 45.2% (14 out of 31 horses showed clinical signs).
- Mortality rate: 19.4% (6 out of 31 horses died).
- Case fatality rate among sick horses: 42.9% (6 out of 14 clinically affected horses died).
- Toxicological testing confirmed elevated arsenic concentrations in affected horses’ biological samples and pasture, validating the diagnosis of arsenic poisoning.
- Clinical symptoms were consistent with arsenic toxicosis affecting primarily gastrointestinal, vascular, and renal systems.
- Supportive treatment was given to poisoned horses, leading to recovery in survivors once the diagnosis was confirmed.
- In contrast, horses in the control group remained clinically healthy and showed low arsenic levels.
Clinical and Pathological Insights
- Arsenic poisoning in horses presents with:
- Gastrointestinal disturbances (likely including diarrhea, colic, loss of appetite).
- Vascular damage that can contribute to circulatory problems.
- Renal impairment as evidenced by biochemical changes and pathology.
- Anatomopathological findings (necropsies) supported damage to internal organs consistent with arsenic’s toxic effects.
- Biochemical analyses helped detect organ dysfunction relevant to arsenic toxicity.
- Detection of arsenic in kidney, liver, and feces is critical for confirming exposure and poisoning.
Limitations
- The study was observational without controlled experimental conditions.
- Monitoring started after clinical cases appeared, causing delays that limited early clinical data capture.
- Prior therapeutic interventions were initiated before detailed data collection, influencing lab results.
- Because arsenic poisoning in horses is rare, there is limited reference data, complicating interpretation and prognosis.
Conclusions and Implications
- Overdosing of arsenic-based herbicides like MSMA can cause serious toxic outbreaks in horses with substantial morbidity and mortality.
- Effective diagnosis requires integrating clinical signs, laboratory testing, anatomic pathology, and arsenic detection in tissues and environmental samples.
- Prevention by adhering to recommended herbicide dosages and careful pasture management is critical.
- Veterinarians should be aware of arsenic toxicosis signs and diagnostic methods to manage and prevent outbreaks.
Cite This Article
APA
Pereira GF, Blimbliem MCH, Machado ALPR, Abdala JBR, Nogueira GM, Toma HS, de Carvalho TF, Delfiol DJZ.
(2025).
Outbreak of poisoning by sodium hydrogen methylarsonate (MSMA)-an arsenic-based herbicide-in horses in Brazil.
Equine Vet J.
https://doi.org/10.1111/evj.70132 Publication
Researcher Affiliations
- School of Veterinary Medicine and Animal Science, Federal University of Uberlândia, Uberlândia, Brazil.
- School of Veterinary Medicine and Animal Science, Federal University of Uberlândia, Uberlândia, Brazil.
- School of Veterinary Medicine and Animal Science, Federal University of Uberlândia, Uberlândia, Brazil.
- School of Veterinary Medicine and Animal Science, Federal University of Uberlândia, Uberlândia, Brazil.
- School of Veterinary Medicine and Animal Science, Federal University of Uberlândia, Uberlândia, Brazil.
- School of Veterinary Medicine and Animal Science, Federal University of Uberlândia, Uberlândia, Brazil.
- School of Veterinary Medicine and Animal Science, Federal University of Uberlândia, Uberlândia, Brazil.
- School of Veterinary Medicine and Animal Science, Federal University of Uberlândia, Uberlândia, Brazil.
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