Purpura Hemorrhagica in Horses: Immune-Mediated Vasculitis After Infection

Purpura hemorrhagica is a medical emergency in horses. It is an immune-mediated vasculitis, which means the horse's immune system forms complexes that damage small blood vessels instead of protecting them. This vessel injury allows fluid and blood to leak into tissues, leading to dramatic swelling, bruising, and pinpoint hemorrhages called petechiae.

Most cases are linked to recent strangles infection or exposure caused by Streptococcus equi subspecies equi. In some horses, the condition appears after vaccination against strangles or after another strong antigenic trigger. Clinical signs often start within 2 to 4 weeks of that exposure, not necessarily while the original respiratory illness is still active.

Affected horses may develop swelling of the head, limbs, and trunk, along with red or purple spots on mucous membranes and the whites of the eyes. In more serious cases, the swelling becomes so severe that fluid oozes from the skin, tissue begins to slough, or breathing and swallowing become difficult. Some horses also develop muscle injury or infarctive myositis, which can worsen the outlook.

Because this condition can progress quickly, early veterinary care matters. Many horses recover with prompt treatment, but delayed care raises the risk of skin damage, muscle complications, secondary infection, and death.

Purpura hemorrhagica is usually triggered by an abnormal immune response after exposure to streptococcal antigens, especially from strangles. The horse's body produces antibodies, and in some individuals those antibodies form immune complexes that settle in blood vessel walls. This causes inflammation, leakage, and tissue injury rather than normal protection.

The most common association is recent infection with or exposure to Streptococcus equi. However, purpura hemorrhagica has also been reported after strangles vaccination, and less commonly after other antigenic events such as equine influenza or chronic draining wounds. Not every horse exposed to strangles develops this complication, which suggests that individual immune response plays a major role.

In practical terms, many horses have a recent history of fever, nasal discharge, swollen lymph nodes, known barn exposure to strangles, or vaccination in the previous month. Some horses no longer look actively sick from the original infection by the time purpura hemorrhagica appears. That delay can make the connection easy to miss.

Your vet may also think about related complications such as infarctive myositis, especially if the horse has painful muscle swelling, high muscle enzymes, or rapid muscle loss. Those cases can be more serious and may need more intensive monitoring.

Diagnosis starts with the history and physical exam. Your vet will ask about recent strangles, exposure to sick horses, vaccination history, fever, nasal discharge, lymph node swelling, and how quickly the edema appeared. The combination of pitting edema plus petechial or ecchymotic hemorrhages after recent streptococcal exposure is often highly suspicious.

Bloodwork is commonly used to assess severity and look for complications. A CBC and chemistry panel can help evaluate inflammation, protein loss, dehydration, organ effects, and whether muscle injury may be present. If the horse is painful or stiff, your vet may also check muscle enzymes such as CK and AST.

Testing for the underlying trigger is also important. Depending on the case, your vet may recommend PCR or culture for Streptococcus equi from the upper airway or guttural pouch, or serology to evaluate recent exposure. These tests help connect the vasculitis to a recent streptococcal event and can guide herd-level decisions if strangles is still circulating.

Other diseases can also cause edema, fever, or hemorrhages, so your vet may work through differentials such as equine viral arteritis, equine infectious anemia, anaplasmosis, severe urticaria, or other causes of vasculitis. In complicated cases, referral, ultrasound, endoscopy of the guttural pouches, or tissue sampling may be discussed.

Prevention focuses first on reducing strangles exposure, because that is the most common trigger. Good biosecurity matters: isolate new arrivals, avoid shared water sources and equipment when disease is suspected, and work with your vet on testing horses that may still be shedding Streptococcus equi, especially after a barn outbreak.

Vaccination decisions should be individualized. While strangles vaccination can be useful in some settings, horses with a history of severe vaccine reactions or recent high antibody levels may need a more cautious plan. Your vet may recommend risk-based vaccination timing and, in some cases, serology before vaccinating horses with a history that raises concern for adverse immune reactions.

Prompt diagnosis and management of strangles, chronic draining wounds, and other inflammatory triggers may also lower risk. If your horse has recently had strangles or a strong vaccine reaction, tell your vet before future vaccines or travel plans are scheduled.

There is no guaranteed way to prevent purpura hemorrhagica in every horse, because individual immune response plays a role. Still, careful biosecurity, thoughtful vaccine planning, and early veterinary attention for respiratory disease give your horse the best chance of avoiding this serious complication.