A cattle owner in eastern Texas noticed several of the 50 weaned calves in his coastal pasture had diarrhea and didn’t seem to want to eat. The pasture had been sprayed with monosodium methyl arsenate (MSMA) to kill weeds about 10 days prior and was fertilized at 300 lbs per acre three days before the calves were noted to be ill. Specimens were submitted to the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) in College Station for testing. A clinical chemistry profile indicated some renal damage [BUN, 61 mg/dL (normal range 10-25); creatinine, 3.30 mg/dL (normal range 0.5-1.7)]. The blood arsenic concentration for the same animal was 0.72 µg/mL (normally below 0.08; over 0.17 µg/mL considered toxic).
MSMA is an organic arsenical herbicide. It was widely used in the past, but chemicals less potentially toxic to mammals have been developed and have largely replaced it. Since 2009, MSMA has been permitted for use in very limited situations: golf courses, sod farms, highway right-of-way, and to limit weed growth in cotton fields. Clinical signs observed in these calves are typical of those exhibited by ruminants that have consumed treated forages, along with increasing lethargy and inappetence. The compound causes damage to the mucosa of the acid compartment as it is absorbed. Additionally, the renal tubular epithelium sustains critical damage during the excretion process. Animals that consume high enough doses die of renal failure. In horses, MSMA can cause colic in addition to kidney damage. MSMA is very water soluble, so it is excreted fairly rapidly. In cattle, if damage to the kidneys is limited, this will be a temporary setback; however, the degree of damage depends on how much of the toxin has been consumed and continues to be ingested. Removal from the pasture for several weeks is necessary to prevent further exposure. Thanks to its water solubility, the MSMA will be removed from the forage by drenching rain.
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