Indigo
Creeping indigo (Indigofera spicata) is a legume that was introduced into southern Florida, where it has become well established. It causes a fatal neurologic disease resulting from liver failure. The disease of horses known in Florida as ''grove poisoning,'' originally thought to be due to chemicals used in the citrus industry, is now known to be due to horses' eating creeping indigo. In Australia, a similar disease of horses and other livestock referred to as ''Birdsville disease'' is caused by Birdsville indigo (Indigofera dominii).
Creeping indigo is a prostrate plant of tropical and subtropical areas with many branched runners fanning out from a crown of a white tapering taproot that may be up to 3 ft (1 m) in length. The stems are usually pale green with alternate pinnate leaves and alternate ovate leaflets on a short petiole. The pink to dark red flowers are produced on short spikes from the leaf axils. The pointed seed pods are produced in downward-pointing clusters. The plant is a prolific seed producer and tends therefore to be capable of spreading readily.
The liver poison amino acid indospicine is present in various species of Indigofera. The leaves may contain from 0.1 to 0.5% of this toxin in their dry weight, and the seeds as much as 2.0%. Horses apparently find the plant highly palatable and seek it out. The toxin acts as a specific antagonist of the amino acid arginine and, therefore, is an inhibitor of protein synthesis. Horses fed sufficient peanut meal or cottonseed meal, both of which are rich in arginine, are protected from the effects of this toxin. Arginine constitutes 10 to 12% of the protein in these two protein supplements as compared to 4 to 7% in other horse feeds.
After consuming creeping indigo for several weeks, affected horses develop incoordination, difficulty in turning, and inability to walk in a straight line; they eventually collapse. They become severely depressed, lose weight, and have been reported to develop corneal opacity and respiratory difficulty. Affected pregnant mares may abort. Death results from liver failure. Animals eating meat from horses that have been poisoned by creeping indigo may suffer similar fatal poisoning.
There is limited data available, but new information suggests that liver failure may not be the actual cause of death following Creeping Indigo poisoning. Veterinarians in Florida, where Creeping Indigo is highly prevalent, have noted that horses exhibit symptoms including abnormal neurologic behavior, corneal edema, congested oral mucous membranes, and tongue and other oral ulcers. However, they report that there is a lack of clinical or post mortem evidence of liver disease or dysfunction following Creeping Indigo ingestion. There is limited data available, but new information suggests that liver failure may not be the actual cause of death
following Creeping Indigo poisoning. Veterinarians in Florida, where Creeping Indigo is highly prevalent, have noted that horses exhibit symptoms including abnormal neurologic behavior, corneal edema, congested oral mucous membranes, and tongue and other oral ulcers. However, they report that there is a lack of clinical or post mortem evidence of liver disease or dysfunction following Creeping Indigo ingestion.
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