Creeping indigo (Indigofera spicata
) is a highly palatable, perennial legume grown in tropical and subtropical regions worldwide. I. spicata
is one of several species of Indigofera
genus which contain chemicals that are toxic to horses, resulting in Indigofera toxicity
All parts of the plant contain high concentrations of the non-protein amino acid indospicine, in its free state, which is a specific antagonist of arginine and an inhibitor of protein synthesis. In addition to indospicine, the leaves and stems (but not the seeds) of plants of the I. spicata
contain another toxic component, known as 3-nitropropionic acid (3-NPA). 3-nitropropionic acid is a naturally occurring mycotoxin, which is an irreversible inhibitor of succinate dehydrogenase that produces adenosine triphosphate (ATP) depletion in cerebral cortical explants. It is associated with motor disorders in livestock and humans when ingested.
Clinical signs observed in horses appear similar to that seen associated with Birdsville disease in Australia, and I. hendecaphylla
poisoning observed in the United States. Often, without treatment, horses will die two to four months after they first develop clinical signs.