Purpura Hemorrhagica in Horses: Causes, Treatment & Prognosis

Purpura hemorrhagica (PH) is a severe immune-mediated condition in horses characterized by widespread inflammation of blood vessels, known as vasculitis. This condition often develops as a secondary complication of infections, particularly those caused by Streptococcus equi, the bacterium responsible for strangles.

Horses affected by PH typically exhibit extensive swelling (edema) in the legs, neck, head, and abdomen, along with small red or purple spots (petechiae) and larger bruises (ecchymoses) on the skin and mucous membranes. ^[1]

Purpura hemorrhagica results from an immune response where antibody-immune cell complexes form and get trapped in the walls of small blood vessels, causing the immune system to target the horse’s own blood vessels. This results in inflammation and damage, producing the abnormal vascular effects of PH.

The prognosis for PH in horses varies with severity and response to treatment. The risk of PH underscores the importance of robust biosecurity and routine health care in controlling the spread of infectious disease in equine facilities.

Purpura Hemorrhagica in Horses

Purpura hemorrhagica in horses is often triggered by an abnormal immune response following vaccination, other stressors, or infections, particularly Streptococcus equi (which causes strangles).

Strangles is a highly contagious respiratory disease in horses. While most horses recover without significant or long-term health effects, a small percentage develop PH weeks or even months after the initial infection. Although strangles is contagious, PH itself is not. ^[2]

Left untreated, PH can lead to severe complications, including tissue death (necrosis) and secondary infections, which can significantly jeopardize the horse’s health. Prompt diagnosis and treatment are crucial to managing the condition and preventing its progression. ^[1]

Symptoms

Purpura hemorrhagica is associated with multiple clinical signs in horses that reflect the severity of vascular inflammation and damage.

Key indicators include: ^[1][2][3][4]

• Edema: Significant swelling from fluid accumulation can occur in various parts of the body, including the legs, neck, chest, and face. This swelling is often painful, warm to the touch, and may lead to oozing or skin sloughing if severe.
• Stiffness and Reluctance to Move: Horses may exhibit stiffness and a reluctance to move due to pain or muscle inflammation associated with vasculitis.
• Petechiae and Ecchymoses: Small red or purple spots (petechiae) and larger bruises (ecchymoses) may appear on mucous membranes such as the gums, around the eyes and vulva, resulting from damage to small blood vessels.

Purpura hemorrhagica is characterized by widespread inflammation of blood vessels, which can impact multiple organs and body systems. Respiratory issues may arise if edema affects the respiratory tract, leading to breathing difficulties. ^[1]

Gastrointestinal symptoms, such as abdominal pain, colic, and diarrhea, are common if the intestines are involved. General systemic effects, including fever, lethargy, tachycardia (increased heart rate), and tachypnea (rapid breathing), reflect the extensive inflammatory response. ^[5]

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Causes

Purpura hemorrhagica occurs when the immune system targets the horse’s own tissues, causing inflammation and damage. It is usually associated with some form of infection, with Streptococcus equi, the bacterium responsible for strangles, being the most frequent trigger. ^[5]

Strangles primarily affects the respiratory system, presenting with symptoms such as: ^[6][7]

• Fever
• Nasal discharge
• Swollen lymph nodes that may abscess

While most horses recover from strangles with appropriate supportive treatment, a small percentage may develop PH as a secondary complication. ^[6][7]

Other infections that may trigger PH in horses include: ^[8][4]

• Equine influenza
• Equine viral arteritis (EVA)
• Equine herpesvirus type 1
• Rhodococcus equi
• Corynebacterium pseudotuberculosis

Immune Mediation of Purpura Hemorrhagica

During an infection, the immune system produces antibodies to fight the bacteria. These antibodies sometimes combine with bacterial components to form immune complexes, which normally neutralize and remove the invaders. ^[2][8][9]

However, these immune complexes can circulate through the bloodstream and become lodged in the walls of small blood vessels. The immune system may incorrectly identify these complexes as a threat and attack them, leading to inflammation (vasculitis) and damage to the blood vessel walls.

Purpura hemorrhagica can also be triggered by factors beyond infections; vaccinations or severe stress can provoke a similar immune response. The development of PH often occurs weeks or even months after the initial trigger, which can make diagnosis and treatment difficult. ^[2][3][8]

Vaccination

Vaccination against strangles is an important preventive measure that helps reduce both the incidence and severity of strangles infections, which are primary triggers for PH. The strangles vaccine is considered risk-based, meaning it is administered to horses that are at a higher risk of exposure to the disease. ^[10][11]

There are two types of strangles vaccines: killed and modified live. The injectable killed vaccine reduces strangles cases by about 50% but may cause soreness, abscesses, or, in rare cases, PH.

The modified live vaccine, given intranasally, offers stronger protection by targeting the tonsils. It requires careful administration, and some horses may experience mild respiratory symptoms afterward. ^[10]

In most cases, vaccinated horses who contract strangles experience a milder form of the disease, which reduces the likelihood of a severe immune reaction leading to PH. ^[11] Rarely, PH occurs as a reaction to the strangles vaccine. ^[12]

Despite this risk, the benefits of vaccination in preventing strangles and its complications generally outweigh the potential for vaccine-associated PH. Careful assessment and monitoring during vaccination are essential, especially for horses with a history of immune-mediated conditions.

Diagnosis

Diagnosing PH in horses involves assessing clinical signs, conducting laboratory tests, and reviewing the horse’s medical history.

Since PH typically develops as a secondary complication 2 – 4 weeks after the initial infection, it can be difficult to directly link symptoms to the underlying cause. Additionally, PH is challenging to diagnose due to its varied clinical signs, which may overlap with those of other conditions. ^[9][13]

Endoscopy can aid in identifying infection sites by allowing a detailed examination of the guttural pouches for signs of infection, such as pus or swollen lymph nodes. Samples collected during the procedure are used in further testing to confirm the diagnosis.

Hematology

Blood tests are useful for diagnosing PH as they reveal signs of systemic inflammation. Key indicators include elevated neutrophil and white blood cell counts (leukocytosis). Additionally, anemia and decreased platelet counts are common. In severe cases, there may be a notable drop in both red blood cells and platelets. ^[1][5]

An elevated Streptococcus equi titer, indicating recent exposure to the bacteria, suggests a higher risk of PH. This, along with other markers of chronic inflammation, such as increased globulins and low serum iron, helps support the diagnosis. ^[1]

Biochemistry

Biochemistry tests of blood samples are essential for diagnosing and managing PH in horses.

Biochemical assays used in cases of PH include: ^[1][5]

• Total Protein: Measures the overall amount of protein in the blood, helping to assess general health and inflammation levels.
• Fibrinogen: Evaluates clotting ability and the presence of inflammation by measuring a protein involved in blood clotting.
• Creatine Kinase (CK), AST, LDH: Detect muscle damage. Elevated levels of these enzymes suggest muscle injury, which is common in PH.
• Creatinine and BUN: Check kidney function. High levels of creatinine and BUN can indicate kidney problems, which may occur if PH affects the kidneys.

Differential Diagnosis

Differential diagnosis for PH involves distinguishing it from several other conditions with overlapping symptoms.

Key conditions to consider include: ^[1][14][15]

• Vasculitis: General vasculitis, characterized by inflammation of blood vessels, can be caused by various infectious agents or immune-mediated diseases, and presents with symptoms similar to PH.
• Equine Viral Arteritis (EVA): EVA can cause systemic inflammation in horses, including swelling and fever, which may mimic the symptoms of PH.
• Hypoalbuminemia: This condition, resulting from protein-losing enteropathy or nephropathy, can cause edema and systemic signs similar to those seen in PH.
• Immune-Mediated Diseases: Rare conditions such as immune-mediated thrombocytopenia or systemic lupus erythematosus can present with symptoms like bleeding and swelling, overlapping with PH.

Accurate diagnosis requires careful evaluation of clinical signs, laboratory tests, and, if necessary, advanced diagnostic procedures to differentiate PH from these conditions.

Treatment

Treatment PH in horses focuses on:

• Controlling inflammation
• Supporting overall health
• Preventing complications
• Addressing the underlying cause

Maintaining a clean, dry environment is best practice to reduce the risk of infection, particularly in areas where the horse may have open sores or swelling.

Providing soft, deep bedding helps reduce pressure on swollen limbs, and the horse should be encouraged to rest as much as possible. Regular gentle exercise, if recommended by a veterinarian, can help improve circulation and reduce edema and further complications. ^[1]

Corticosteroids

Corticosteroids are a key component in treatment protocols for PH due to their ability to reduce severe inflammation and modulate the immune response. These medications, such as dexamethasone or prednisolone, work by suppressing the overactive immune response that leads to vasculitis and tissue damage in PH. ^{[1][2][5][13]}

By alleviating symptoms such as swelling and pain, corticosteroids help prevent further progression of the disease and support recovery. However, their use requires careful monitoring due to potential side effects. Dosages may need to be adjusted during the recovery period. ^[16]

Antibiotics

Antibiotics, such as penicillin, may be helpful in managing PH when it is secondary to a bacterial infection like Streptococcus equi. Their primary role is to target and eliminate the underlying infection that may have triggered the immune-mediated response leading to PH. ^[1][5][17]

Additionally, antibiotics help prevent secondary infections that could arise from a compromised immune system or environmental exposure, reducing the risk of further complications and supporting overall recovery.

Prognosis

The prognosis for PH varies based on severity. Horses with mild symptoms and early intervention generally have a favorable prognosis. With appropriate treatment, including anti-inflammatory medications and supportive care, these horses can recover fully over time. ^[1]

In contrast, severe cases involving significant edema, tissue necrosis, or multiple organ systems have a more guarded or grave prognosis, with a higher risk of mortality and increased treatment challenges.

Complications such as secondary infections or organ failure can further impact recovery. Early, comprehensive medical intervention provides the best circumstance for a positive outcome.

Prevention

Since PH is most commonly associated with infectious diseases, the most effective prevention is maintaining robust hygiene and management practices to minimize the risk of contagion within the farm or facility. In addition, staying up to date with routine vaccinations, deworming, and veterinary care are cornerstones of controlling infectious illness in horses.

Biosecurity

Biosecurity is fundamental to managing strangles and other highly contagious diseases. Strangles spreads easily through direct contact with nasal secretions or lymph node discharges from infected horses, as well as through contaminated objects or surfaces (fomites). ^[18]

Implementing strict hygiene and biosecurity measures, such as regular disinfection of equipment and feeding areas, and providing clean water, helps minimize infection risks. In environments with dense horse populations or frequent movement, like boarding stables or training facilities, the risk of transmission is higher.

Effective biosecurity practices, including isolating infected horses and controlling horse movement, are essential to prevent outbreaks and safeguard the health of both individual horses and the entire equine community. In addition, it’s important to comply with quarantine protocols when transporting horses to and from events, and when introducing new horses to the herd.

Prompt Treatment of Infections

Early and effective treatment of infections, especially respiratory infections like strangles, can help prevent complications that might lead to PH. Immediate veterinary care is essential to manage infections and reduce the risk of immune-mediated responses.

Summary

Purpura hemorrhagica is an immune-mediated vasculitis that affects horses, most often triggered by infections like strangles (Streptococcus equi).

• Key symptoms include edema, petechiae, and stiffness. Untreated, PH can lead to severe complications.
• Diagnosis involves clinical signs and lab tests, with a history of recent infections being crucial for identification.
• Treatment includes corticosteroids to reduce inflammation and antibiotics if bacterial infection is present.
• Prevention of PH in horses relies on biosecurity and prompt infection treatment. Prognosis varies, with mild cases being manageable but severe cases posing significant risks.