by Michelle Arnold, DVM - In the Southeastern US, a severe type of pneumonia can result from ingestion of the leaves and seeds of perilla mint (Perilla frutescens). This common weed is also known as purple mint, wild coleus, mint weed, or beefsteak plant. Perilla mint thrives in late summer, when pastures are frequently dry and dormant. The weed prefers shaded areas along creeks, the edges of the woods and in fence rows. Once it becomes established, perilla mint produces many seeds and large colonies can develop in succeeding years. 

Perilla frutescensPerilla mint has a distinctive mint aroma, dark green to purplish square stems and serrated leaves with a purple tint. Mature plants reach 3-3.5 feet tall and produce small, white to purple flowers with abundant seeds. Picture accessed from a Project at theUniversity of Pennsylvania School of Veterinary Medicine.

The time of year when perilla mint reaches the seed stage often corresponds to periods when pasture is scarce, enticing cattle to consume plants they normally avoid. The flowering or seed parts of perilla mint contain the highest concentration of perilla ketone, considered the most toxic agent involved. The perilla ketone is absorbed into the bloodstream and carried to the lungs where it damages the lung tissue. The early pre-seed stage of the weed is of relatively low toxicity while the flowering and green seed stage plant is most toxic, especially the seeds themselves. Dried hay is less of a risk than green plants but still can be lethal while frosted plants have relatively low toxicity. Ingestion of perilla mint causes “acute respiratory distress syndrome”, a sudden and dramatic onset of severe breathing difficulty. Affected animals are frequently found dead. Mature cattle are most often affected but it can occur in yearlings and calves. Treatment is of limited value and cattle with severe cases seldom survive. Prevention involves implementing effective weed control and offering supplemental forage or feed when pasture is limited so cattle are not forced to graze toxic plants.

Perilla ketone and other related furan compounds are activated in the lung and cause atypical interstitial pneumonia (AIP). This type of pneumonia is straightforward to diagnose at necropsy when the chest cavity is opened because the lungs are found fully expanded with rib indentations on them rather than collapsed as a normal lung. AIP affected lungs are heavy and have a firm, rubbery texture instead of the expected light and spongy lung tissue. These necropsy findings are confirmed at the cellular level with a very distinct pattern of damage to the lung cells recognizable with a microscope. The leaves and distinct square stems are easily identifiable in rumen contents.

Acute respiratory distress syndromeThe clinical signs of acute respiratory distress syndrome include a sudden onset of open-mouth breathing with the head and neck extended, nostrils dilated, a sway-back appearance, tongue protruding with foam coming from the mouth, an open-shouldered stance, and sometimes aggression. 

Breathing is shallow and rapid (35-75 breaths per minute) and may have a loud expiratory grunt. Temperature is typically normal but may be mildly elevated due to the severity of the condition. In extreme cases, air under the skin (subcutaneous crepitation) may be felt over the upper portions of the neck, shoulders and back. Mild exercise may cause the animal to collapse and die. Generally, there is an absence of coughing and no signs of infection, such as fever or depression. Severely affected animals usually die quickly, but animals that live 48 hours typically survive although may develop chronic lung problems or heart failure. The stress of handling cattle can cause prompt death so treatment, if attempted, must be handled very cautiously. A dart gun may be necessary to avoid moving the animal to a treatment facility. Treatments administered or recommended by a veterinarian may include diuretics, nonsteroidal anti-inflammatory medications and corticosteroids used in an extra-label manner.