There are seven species of Swainsona in Australia that are capable of causing disease in horses and other livestock. Some of the species include the dwarf darling pea, grey swainsona, and the smooth darling pea. In central and western Australia the species most commonly found is the grey swainsona, Swainsona canescens. Sturt’s desert pea (Swainsoma formosa) is the state flower of South Australia.
In the US the swainsonine-containing plants of importance are Astragalus and Oxytropis species. Astragalus does grow in Australia but doesn’t produce sufficient toxin to be considered important. In the US these plants are called “locoweeds” and the poisoning is referred to as Locoism. Most poisonings occur when feed becomes limited during periods of drought or over-grazing. Horses the consume locoweeds develop a craving for them, even when offered normal feeds.
The toxic compounds are indolizidine alkaloids swainsonine and swainsonine N-oxide which are readily absorbed from the intestinal tract. They act by inhibiting the lysosomal enzyme, mannosidase, leading to the accumulation of a protein-carbohydrate in numerous tissues in the body, including the brain.
Signs of poisoning
Clinical signs often begin suddenly after 2 to 6 weeks of exposure to the plants. Weight loss occurs quickly and often progresses to emaciation. The weight loss is noted before the onset of neurologic signs. These include dementia with periods of obtundation, alternating with frenzied excitement when horses are disturbed. Variable visual impairment, head-nodding and dysphagia can also be present. Gait abnormalities may be severe and characterized by high-stepping in the forelimbs, toe-scuffing i the hindlimbs, stumbling and swaying. These signs are exacerbated by excitement, and mildly affected horses may seem normal until disturbed. In addition to the neurotoxic effects, locoweed ingestion by pregnant mares has been associated with abortions and limb deformities in foals.
Mildly affected horses recover within 1-2 weeks if promptly removed from the toxic plants. However, there is said to be no recovery from chronic poisoning, even following removal of the offending plants from the horse’s diet. Slightly affected animals remain unsafe for riding and show nervous signs when excited. Death has been noted after 10 weeks of plant ingestion.