Regular ingestion of Eupatorium adenophorum [Ageratina adenophora (Spreng.)] or Crofton weed causes chronic pulmonary disease in horses mainly in Australia, New Zealand, and the Himalayas. The disease is characterized by pulmonary interstitial fibrosis, emphysema, alveolar epithelisation and reduced tolerance to exercise. Horses apparently are the only animals affected and there are numerous reports of farms losing all their horses. The disorder was produced experimentally in horse feeding trials, and it was shown that characteristic lesions occurred in the lungs. In studies with laboratory animals, mice were shown to be suitable test animals, but in this species lesions occur in the liver rather than the lungs. The hepatic injury in these animals is characterized by multiple areas of focal necrosis of the parenchyma associated with degeneration and loss of the epithelium lining the small bile ducts. The active principle 9-oxo-10,11 dehydroagerophorone responsible for these lesions in mice has been isolated from E. adenophorum. Although the compound has been shown to exhibit toxicity to larvae of invertebrate species, no mammalian toxicity studies have been previously reported involving the isolated toxin. The mechanism of the toxic effect of the compound as well as its possible relevance to the respiratory disease in the horse remain to be investigated.

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