Perilla mint (Perilla frutescens) is sometimes called mint weed or beef steak plant. It has square stems and grows 2-4 feet tall. The leaves of perilla mint are heart-shaped, 2-5 inches long and 2-4 inches wide. The green leaves have a hint of purple to the underside. The odor is distinctive, but is not like that of mint used in iced tea.
The risk of lung damage to cattle increases as this common weed reaches the seed stage in late summer in pastures. Perilla is often found in shaded areas, but may be in full sun around barns and corrals and along fences.
"Although the lungs are affected, poisoning occurs when cattle eat several pounds of perilla mint," the veterinarian says, adding, "The toxicant is absorbed from the digestive tract and reaches the lungs through the bloodstream." Nicholson points out that sheep, goats and horses are susceptible, but these animals seldom eat a toxic amount. "For that matter," he remarks, "cattle may ignore the plant year after year."
Signs of illness develop within 24 hours. Severely poisoned animals have loud, open-mouth breathing. A grunt may be heard as the animal labors to exhale air from its distended, emphysematous lungs. Air from a ruptured lung may appear under the skin over the neck and back. Mortality can be as high as 35% of the affected animals. Animals that survive for 48 hours are expected to recover.
Nicholson adds that pulmonary emphysema in cattle also can be caused by ingestion of mold-damaged sweet potatoes.
The animal doctor says the disease also may follow a sudden change in diet from poor quality grass to an immature, high protein summer forage. In that situation, metabolism of the amino acid tryptophan within the digestive system produces the toxin. Fortunately, this doesn't happen often.
"There is really no effective treatment," Nicholson says, explaining, "Severely affected cattle are oxygen deprived and cannot tolerate any exertion." Since the signs are similar to severe pneumonia, which could be treated, it is important that a diagnosis be made. Necropsy of a dead animal by a veterinarian can identify the cause of death.
For additional information about cattle health, contact an extension agent in your parish LSU AgCenter office.
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