GRAYANOTOXICITY IN SMALL RUMINANTS

摘要

Small ruminants commonly encounter and ingest materials not meant for consumption, including toxic plants. This may occur due to intentional (such as discarded lawn clippings) or unintentional feeding of these plants to the animals. Although many toxic plants are unpalatable, there have been reports of animals readily consuming them, even when other feed sources are available.1"7 Some reports, however, have occurred during changes in weather patterns, when normal feed sources are compromised.8"9Among the plants that are toxic to small ruminants are those belonging to the heath family (Ericaceae), including Rhododendron or Azalea (Rhododendron spp.), Mountain Laurel (Kalmia spp.), Japanese Pieris (Pieris spp.J, Fetter-Bush (Leucothoe spp.), andMaleberry (Lyonia spp.). These plants contain andromedotoxins, also referred to as grayanotoxins, which act directly on voltage-gated sodium channels. The toxin is able to bind to intracellular receptor sites in such a way that it alters the configuration of the channel itself. This, in turn, prevents sodium channel inactivation, locking the cell in a depolarized, activated state.10 The primary effects are on the heart, nervous system, and gastrointestinal tract. All parts of the plant including the nectar contain the toxin and animals eating approximately 0.2% of their body weight of leaves are likely to develop signs of poisoning. Greater than 25 isomers of these toxins have been isolated, but grayanotoxin I and III appear to be the primary isomers associated with clinical toxicities.