Lathyrus hirsutus (Caley Pea) Intoxication in a Herd of Horses

BACKGROUND Caley Pea (Lathyrus hirsutus) is potentially toxic to horses, but large case series are not reported. OBJECTIVES To describe the clinical signs of horses intoxicated with Lathyrus hirsutus and speculate on the neuroanatomical lesion localization and pathogenesis based upon the observed clinical signs. ANIMALS Twenty-two of 25 horses ranging in age from 6 to 34 months were affected. Five affected horses were presented to the OSUCHVS for evaluation and treatment after having been attended at the ranch by a local veterinarian (ALA). An additional horse that had been euthanized was also presented for necropsy. METHODS A case series is presented. Diagnostic evaluation included: physical examination, complete blood count, serum biochemistry, CSF analysis, EMG, ERG, upper airway endoscopy, muscle biopsy, and serum vitamin E analysis. The grain ration consumed by the affected horses was analyzed for ionophores and cultured for fungi: the hay was examined for toxic plants. RESULTS Bermuda grass hay consumed by the horses contained large quantities of mature Lathyrus hirsutus. Acute clinical signs conform to earlier descriptions of Lathyrus hirsutus intoxication in cattle. Residual neurologic signs were characterized by incoordination in the rhythmicity of multiple gaits. Evidence of mild neurogenic muscle atrophy was recognized in 1 of 5 horses biopsied. CONCLUSIONS AND CLINICAL IMPORTANCE Caley Pea intoxication may occur within days of seed pod consumption. The neurologic signs are unique and suggest involvement of the upper motor neuron system and regions of the spinal cord influencing voluntary motor movement. Drought conditions during plant growth may increase the risk of toxicosis.