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Purpura Hemorrhagica in HorsesBy Dr. Bryan Waldridge · February 20, 2012

Purpura hemorrhagica (PH) is defined as a condition of hemorrhage and edema that is triggered by an allergic-type reaction. Most often, PH is associated with strangles (Streptococcus equi), but it can occur following infections with any streptococcal bacteria or virus (influenza). Occasionally, PH can be a consequence of vaccination in hypersensitive horses.

Regardless of the cause, PH is the result of the overproduction of certain antibodies against a pathogen. These antibodies deposit on blood vessel walls and activate a strong immune response. White blood cells accumulate and release enzymes that damage blood vessels. The blood vessels become leaky, which results in hemorrhage and loss of fluid into the tissues.

Clinical signs of PH may not occur until 2-3 weeks after the initial infection. Some horses may be subclinically infected (show no signs of a respiratory infection) and still develop PH, because they already had antibody protection against the initial pathogen and went on to overproduce antibody that resulted in PH. Horses affected with PH generally have mild to severe edema of their legs and bottom of the chest and abdomen. Occasionally, horses may have small hemorrhages observed on their gums or other mucous membranes. Some horses can be extremely sore and reluctant to move. Severe cases can ooze serum from the skin or slough areas of affected skin.

Treatment of PH involves aggressive anti-inflammatory therapy and corticosteroids. Antibiotics are indicated if the horse has other clinical signs of an active respiratory infection. Bandages, cold-water hosing, and hand walking or paddock turnout are helpful to reduce swelling. Some cases of PH can be extremely challenging to manage and may require weeks of therapy. It is very important to consult with your veterinarian whenever PH is suspected.

Few horses exposed to respiratory or other pathogens go on to develop PH. Horses that have had strangles or been exposed to infected horses are at the highest risk. It is possible to measure antibodies against S. equi M-protein, which causes PH with strangles. Horses that have recovered from strangles with high M-protein titers are more likely to have complications with PH if they are vaccinated against strangles while their antibody levels are still high.