This respiratory syndrome is caused by a protein in the lush pasture called tryptophan and typically affects only adult cows. The protein is converted by the bacteria in the rumen and produces a toxin known as 3-methyl-indole when the cattle are exposed to a new pasture with higher levels of tryptophan and their rumen bacteria are not gradually adapted to it. The toxin is circulated in the cow’s blood stream to the lung, where it causes severe cellular damage. The result is a severe, untreatable respiratory syndrome.