Perilla mint (Perilla frutescens) is a potent respiratory toxin most commonly affecting horses and cattle.  You did not see this plant on thorough examination of pasture and areas around barn.

Toxin: Perilla ketone, a 3-substituted furan similar to 4-ipomeanol.

History: Seen sporadically usually in late summer or fall after grazing plant.

Clinical Signs:  Similar to 4-ipomeanol or 3-methyl indole, perilla ketone causes respiratory distress.  Clinical signs include difficulty breathing, especially when exhaling (affected animal may grunt when exhaling).  Nasal discharge, cyanosis, frothy hypersalivation, increased lung sounds.

Mechanism of action: Mixed function oxidases in the lung bioactivate the perilla ketone to reactive intermediates that bind to tissue macromolecules causing pulmonary cellular destruction.  Type I alveolar cells are preferentially destroyed leading to proliferation of Type II alveolar cells and ultimately pulmonary fibrosis. Pulmonary emphysema, edema and congestion can be seen on post-mortem exam.

Treatment and prevention: No specific antidote.  Once clinical signs are seen course of disease is difficult to reverse.  Supportive care may include administration of steroids, antihistamines and antibiotics.  Prevention involves eliminating plant in areas where animals graze.
 

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