Intoxication of cattle by Perilla frutescens (purple mint).

Perilla frutescens or purple mint has been associated with atypical interstitial pneumonia (AIP) for a quarter of a century. The amount and the stage of the plant required to produce AIP have been much debated. A field case in which catastrophic loses occurred in cattle ingesting hay containing purple mint showed that more than the green plants have the capability of producing atypical interstitial pneumonia. In this study, Perilla frutescens produced atypical interstitial pneumonia in three of five calves to which it was given. The amount required to produce the syndrome ranged from 2.3 to 15.5 kg of green seed stage mint and 11.8 kg of mint hay. The toxic syndromes were similar in signs, but quite different in duration. Necropsy examinations showed varied amounts of pulmonary emphysema and edema. Two of the three affected animals' lungs histologically displayed a marked proliferation of Type II pneumocytes. The flowering or seed parts of perilla mint were found by high pressure liquid chromatographic analysis to contain the highest concentration of perilla ketone, considered the most toxic agent involved. This stage of plant growth was also shown to be the most toxic in our calf feeding trial. Calves fed the flowering plant developed the toxic syndrome while those fed earlier plants (collected before seed stage) and late plants (collected after frost) remained asymptomatic. The time of year when perilla reaches the seed stage often corresponds to periods when pasture grass is scarce forcing cattle to consume plants not normally eaten when ample desirable forage is available.(ABSTRACT TRUNCATED AT 250 WORDS)