Purpura haemorrhagica is a rare complication of equine strangles and is caused by bleeding from capillaries which results in red spots on the skin and mucous membranes together with oedema (swelling) of the limbs and the head.
It is thought to be caused by an auto-immune reaction where antibodies against the S. equi M- or R-protein cross-react with proteins on endothelial cells.
This results in vasculitis, leading to subsequent severe peripheral edema in the legs and ventral abdomen, as well as petechiation or ecchymoses over the mucous membranes.
Other clinical signs include depression, anorexia, fever, elevated heart and respiratory rate, reluctance to move, drainage from lymph nodes, exudation of serum from the skin, colic, epistaxis and weight loss.
[3] Rarely, horses may also develop disseminated intravascular coagulation (DIC), leading to infarction of various organs,[4] or chronic myositis and muscle atrophy.