toxicity is a nervous system disease of horses resulting from chronic and cumulative exposure to the indospicine and/or 3-nitropropionic acid (3-NPA) toxins present in certain Indigofera
species. The Indigofera
genus is composed of over 750 species of brightly-colored, flowering, exotic plants found in tropical and subtropical regions worldwide. Indospicine (L-6-amidino-2-amino-hexanoic acid) is a non-proteinogenic amino acid found in more than 30 species within the Indigofera
genus. Indospicine has the potential to interfere with the horse's arginine metabolic pathways, as arginine is a precursor for the synthesis not only of proteins but also of nitric oxide, urea, polyamines, proline, glutamate, creatine and agmatine. Indospicine has been found to accumulate in the horse's tissues, upon continued ingestion of Indigofera
species, demonstrating a cumulative nature. Additionally, some Indigofera
species contain 3-NPA, a neurotoxin which is thought to be the toxin responsible for the neurological syndrome observed in horses as a result of eating the plant.
Sporadic outbreaks of equine poisoning cases have occurred in specific areas, caused by species of Indigofera
that contain high amounts (over 500 mg/kg DM) of the toxin indospicine. The concentration of indospicine in plants is also influenced by the growth stage, with the highest amounts found in the seeds.
In northern Brazil, I. lespedezioides
is responsible for cases of poisonings in horses that occur most commonly at the end of the dry season. I. lespedezioides
is found in the Americas and native to Mexico and South America. Affected horses had loss of appetite, lethargy, severe ataxia, weakness, stumbling, eye discharge, blindness, abortion in mares, and progressive weight loss. Horses are often seen dragging their hind limbs, resulting in excessive wear of the toes of their hooves. Death occurs 2-4 months after first onset of clinical signs.
In the southeastern United States, poisoning cases in horses have occurred since the 1970s from ingestion of I. hendecaphylla
and I. spicata
in parts of Florida. I. hendecaphylla
is native to regions in Africa, Comoros, Magagascar, Reunion, Asia to Papua New Guinea and the Philippines, but was introduced to Florida, French Polynesia and Australia, where it is now naturalized and considered an invasive weed in some areas. I. spicata
is native to Africa, Madagascar, Mauritis, and Yemen but was introduced to Australia, the Hawaiian islands, Japan, New Caledonia, Micronesia, and Cook Islands.
In Australia, outbreaks of poisoning cases have been associated with horses grazing I. linnaei
and I. spicata
; resulting in the development of a neurological syndrome known of Birdsville disease. I. linnaei
is native to Australia, India, Indochina through Malesia, Melanesia to New Guinea.