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Hives, also known as urticaria, are a common skin reaction in horses characterized by the sudden appearance of raised, swollen bumps (wheals) on the skin. These wheals can vary in size, and may be accompanied by itching or discomfort. [1][2]

Hives in horses can be triggered by various factors, such as allergic reactions to insect bites, foods, or environmental allergens. [3]

If a horse develops hives, the first step is to identify and remove the trigger. However, identifying the exact cause of hives is often difficult. Horses can be susceptible to a wide range of potential allergens, and can even develop hives due to underlying health conditions.

It’s important to consult a veterinarian for evaluation and diagnosis to help determine the underlying cause and appropriate treatment plan for hives. Additional diagnostic tests may be needed to identify the trigger if it’s not immediately apparent.

Hives in Horses

Hives (urticaria) are raised, round wheals on the skin that can vary in size, typically ranging between 0.5 to 8 inches (1.3 to 20.3 cm) in width. When pressed with a finger or another object, they leave a temporary indentation or pit. [2]

Hives in horses may appear throughout the entire body or be localized to specific areas such as the back or neck. These wheals may merge together, forming larger areas of swelling known as angioedema. [1][4]

Hives typically emerge shortly after exposure to the triggering factor, within minutes to hours. Some horses may experience itching and vigorously rub, scratch, or bite at the affected area, while others may show little to no signs of irritation. [1][5]

In more widespread and severe cases of hives, wheals may appear on the mucous membranes of the body, including the mouth, nose, eyes and rectum. Severe reactions may lead to secondary symptoms indicating a significant immune system response, such as: [1][2]

Causes

Hives (urticaria) are often associated with allergic reactions or other causative agents that horses are exposed to via inhalation, ingestion, or injection. The horse’s immune system over-reacts to the trigger, resulting in skin inflammation.

Most cases of hives in horses are considered a type I hypersensitivity reaction, but horses can also develop a type III or complex reaction. [3][4]

Type I Hypersensitivity

During this reaction, the horse’s immune system produces immunoglobulin E (IgE) antibodies, which bind to other immune cells (mast cells) in the skin. Mast cells play a key role in allergic reactions. [3][4]

The binding of IgE to mast cells triggers mast cell degranulation. In this process, mast cells release histamine and other inflammatory mediators into the surrounding tissue. [3][4]

Histamine causes blood vessels to widen (vasodilation) and become more permeable, allowing fluid to leak into surrounding tissues. This fluid leakage from the blood vessels in the skin results in the characteristic raised, swollen wheals. [3][4]

Type III Hypersensitivity (complex)

Type III hypersensitivity, also called immune complex hypersensitivity, is a different type of reaction that occurs when antigen-antibody complexes form in the bloodstream and deposit in tissues. This triggers a delayed inflammatory response. [3][6]

This reaction is less common in horses than Type I, and typically occurs several hours to days after exposure to the trigger. Complex hypersensitivity can lead to symptoms such as tissue damage, inflammation, and widespread (systemic) effects like fever or joint pain. The Type III reaction may occur concurrently with Type I hypersensitivity. [2]

Common Triggers of Hives

Hives are commonly linked to allergic reactions triggered by contact with allergens on the skin, through inhalation, ingestion, or injection. [3] Hives can also be related to physical injury, or appear as a side effect or adverse reaction to medication. Sometimes the cause is unknown. [3]

Food Allergies

Allergic or idiosyncratic reactions to feed can cause hives in horses, occurring seasonally or year-round based on specific triggers. An idiosyncratic reaction occurs when the horse’s response to a trigger doesn’t follow typical allergic patterns. [4]

A comprehensive feed history including supplements, treats, and pasture access, is vital for identifying potential triggers for recurrent urticaria.

Some common feeds that can induce hives in horses include wheat, oats, bran, soy, barley, potatoes and distillery byproducts.

Some additives or preservatives in feed, as well as dust from forages, may also trigger a reaction. [2]

Airborne Allergies

When horses inhale or come into contact with airborne allergens such as pollen, dust mites, or mold spores, their immune system may react abnormally, leading to the development of hives on the skin. [2][4][7]

Identifying specific airborne allergens via diagnostic tests like intradermal skin testing (IDT) can help horse owners and veterinarians implement targeted allergen avoidance measures and treatment plans to minimize allergic reactions and improve the horse’s quality of life. [8]

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Insects

When horses experience insect bites or stings, especially from allergenic insects such as midges (Culicoides spp.), black flies, and mosquitoes, an immune response may occur. However, insect hypersensitivity typically presents as crusty scaling on the skin rather than hives. [2][4]

Affected horses are itchy, and can develop lesions and hair loss as a result of excessive scratching. In certain areas, the prevalence of Insect Bite Hypersensitivity (IBH) in horses, also referred to as sweet itch, can reach rates as high as 60%. [9]

One study of insect hypersensitivity showed that horses relocated to areas with active insect populations developed large patches of hives that last for 2 to 4 weeks. These horses tested positive for traces of mosquitoes and Culicoides on skin tests. Lesions appeared on the neck and trunk sides, typical feeding areas for mosquitoes, suggesting a temporary allergic response to the insects. [4]

While it’s challenging to completely avoid insect exposure, management strategies can help minimize the risk. These include applying insect repellents and fly sprays, setting up fans and mosquito nets in barns, and outfitting horses with fly sheets.

Medications

Medications can trigger hives in horses, but the specific mechanisms by which some drugs cause these reactions are not always completely understood.

Examples of medications that may cause hives in horses include: [2][3][4][5]

Reactions may involve both immune responses and direct effects of the drug on the body’s cells or tissues, contributing to hives. [2]

Fortunately, diagnosing hives in horses on medication is usually straightforward because there’s often a clear link between the appearance of hives and the start of a new medication.

Diagnosing hives can be more challenging if the horse is allergic to dyes or preservatives in medication rather than the medication itself because these substances may be present in multiple medications.

Contact your veterinarian if you think your horse is having a reaction to medication. Do not stop administering medication as prescribed without veterinary guidance. Suddenly stopping medication can cause further complications.

 

Idiopathic Urticaria

Idiopathic urticaria refers to hives that occur without an identifiable cause. These hives may appear suddenly and without warning, and their triggers cannot be determined through standard diagnostic methods. [3]

Treatment involves symptom management, aiming to alleviate itching, pain, and discomfort with antihistamines and/or corticosteroids over several weeks. Gradually withdrawing medication assists in assessing treatment effectiveness and resolution of hives. [4]

Between 50% to 75% of urticaria cases in horses have unknown origins, posing significant challenges for diagnosis and treatment. [2][3]

Physical Urticaria

Physical urticaria is a a type of hives in horses triggered by physical stimuli rather than allergens or medications. This less common condition occurs when the horse’s skin reacts to physical forces such as pressure, friction, cold or heat. [3][4][5]

Dermatographism

Dermatographism in horses is characterized by the release of histamine from mast cells after pressure has been placed on the skin, leading to hives. Affected horses develop raised wheals a few minutes after pressure is applied by a blunt object. [1][10]

This condition is relatively common and may occur in response to various stimuli, including grooming or tack pressure. Cases of dermatographism often go untreated as it is usually difficult to determine the cause of the condition. [5]

In some cases, dermatographism resolves on its own after a few months. While antihistamines and corticosteroids are commonly used to manage dermatographism symptoms, their effectiveness varies among horses. [5]

Temperature

Horses living in cold climates or directly exposed to cold weather may develop cold urticaria. Diagnosis involves placing an ice cube on the skin for a few moments. When the area is warmed, a hive appears, confirming the diagnosis. [5]

Heat-induced (cholinergic) urticaria is triggered by an increase in body temperature, typically occurring after exercise or bathing in hot water. Applying a hand warmer or heat pack to the skin can trigger a hive to form, confirming the diagnosis. [5]

Exercise

Non-cholinergic exercise-induced urticaria is uniquely triggered by physical exertion such as exercise. Unlike heat-induced urticaria, which can be provoked by activities that increase body temperature like hot bathing, this form of urticaria occurs solely in response to exercise. [5]

Diagnosis

Diagnosing urticaria in horses is straightforward in many cases, especially when characteristic signs like raised wheals on the skin are present. However, it can become more complex to identify the underlying cause, as there are various potential triggers. [2]

Collecting a thorough horse history is essential, including recent changes in diet, environment, medications, or exposure to potential allergens. During the diagnostic process, your veterinarian may advise you to eliminate all non-essential medications, feeds, and supplements. [2]

It’s also important to rule out other conditions that can present with similar signs, such as skin infections (folliculitis) and autoimmune disorders other than allergies. [2]

Hypoallergenic Diet

A hypoallergenic diet is a key component in diagnosing food allergies by pinpointing specific food triggers that may cause allergic reactions in horses. [2]

This process is also known as a dietary elimination trial. Initially, the horse is put on a restricted diet where they are only fed a forage not previously part of its diet for at least 3-4 weeks. Following this trial period, individual components from the previous diet are gradually reintroduced every 7-10 days to identify any reactions.

This process may be repeated several times for confirmation. Once allergenic food sources are identified, they can be permanently removed from the horse’s diet to prevent future allergic reactions and effectively manage the condition.

Diet elimination trials are a long-term commitment and require diligent compliance to accurately identify allergens in the diet. Work with a veterinarian to create an effective diet elimination protocol for your horse.

 

Serology

Serology, the study of blood serum, aids diagnosis by identifying specific antibodies or antigens linked to urticaria. Techniques like enzyme-linked immunosorbent assays (ELISA) and radio-allergosorbent tests (RAST) detect IgE proteins, indicating allergic reactions. [2]

However, serology tests are not standardized and can be overly sensitive, increasing the likelihood of false positive results. Additionally, detecting IgE proteins in the serum does not necessarily mean that they are significant enough to cause symptoms. [3]

Intradermal Testing (IDT)

Intradermal testing (IDT) is used to find specific allergens that trigger reactions in humans, dogs, cats, and horses. It involves injecting tiny amounts of suspected allergens under the skin and observing the skin’s response. [8]

During the test, horses with recurrent urticaria may have immediate (within 15 minutes) or delayed (4-6 hours) reactions to common allergens. However, horses without allergies might also react to these allergens, complicating the interpretation of results. [2]

Differential Diagnosis

When diagnosing skin conditions in horses, it’s important to consider various possibilities beyond urticaria. Some conditions that may present similar clinical signs include: [11] [12] [13]

Additionally, there are several other conditions that may cause generalized swelling under the skin, which could be mistaken for hives. These include, but are not limited to: [2]

Treatment

Treatment of hives in horses typically involves two phases: initial symptomatic relief and standard long-term management.

Initially, treatment focuses on alleviating symptoms, which involves administering antihistamines like oral cetirizine or hydroxyzine to reduce itching. Additionally, corticosteroids such as oral prednisolone or dexamethasone can help reduce inflammation. [2]

During treatment, your veterinarian will advise you to discontinue any medications or supplements that are not crucial for your horse’s health or directly related to treating hives. Removing non-essential substances from the diet helps to simplify treatment and minimize factors that could impede recovery. [2]

Long-Term Management

Antihistamines (hydroxyzine, cetirizine, etc.) are frequently administered to horses to relieve hives symptoms by counteracting the effects of histamine, a chemical released during allergic reactions that causes itching, swelling, and inflammation. [2][3]

While these drugs can help prevent wheal formation, their effectiveness in treating existing lesions may be limited, as their main function is to block histamine effects before they trigger a reaction.

Prevention

Preventing hives in horses typically involves identifying and avoiding triggers. This includes minimizing exposure to known allergens such as specific foods or ingredients, medications, or environmental factors like insects or extreme temperatures. [3]

Preventing hives can be challenging as identifying specific triggers is often complex and may require thorough testing and observation. Some triggers, such as insect bites or environmental allergens, can be difficult to avoid entirely. [9]

While preventive measures can help reduce the frequency or severity of hives, they may not completely eliminate the risk, especially in cases where multiple triggers are involved or when horses have underlying health conditions predisposing them to allergic reactions.

Prognosis

The prognosis for urticaria in horses is generally favorable but can vary. In most cases, hives resolve spontaneously once the triggering factor is removed or treated. However, some horses may regress spontaneously or require long-term management. [1][2]

If the hives are recurrent or chronic, further investigation may be needed to identify and address any underlying factors. With proper management and treatment, many horses with urticaria have complete resolution of symptoms.

Summary

Hives, or urticaria, is a skin condition characterized by raised, swollen wheals on the horse’s skin that pit with pressure.

  • Hives in horses can be triggered by various factors including allergic reactions (insect bites, foods, medications) or physical stimuli like cold, heat or exercise
  • Treatment focuses on symptomatic relief with anti-inflammatory medications, and management involves identifying and avoiding potential triggers
  • Most cases of urticaria in horses can be treated effectively, but recurrence is possible, requiring long-term management

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References

  1. White, S.D. Hives (Urticaria) in Horses – Horse Owners.
  2. Marsella, R. et al. Urticaria/angiedema in Horses (Equis).
  3. Sauvé, F. Can Equine Urticaria Be Cured?. The Canadian Veterinary Journal. 2020.View Summary
  4. Fadok, V.A. Of Horses and Men: Urticaria. Veterinary Dermatology. 1990.View Summary
  5. Funiciello, B. and Coppack, R. Physical Urticaria in Horses (Equis).
  6. Usman, N. and Annamaraju, P. Type III Hypersensitivity Reaction. In: StatPearls. StatPearls Publishing. 2024.
  7. Moriello, K.A. Allergies in Horses – Horse Owners. Accessed Apr. 17, 2024.
  8. Jose-Cunilleras, E. et al. Intradermal Testing in Healthy Horses and Horses with Chronic Obstructive Pulmonary Disease, Recurrent Urticaria, or Allergic Dermatitis. Journal of the American Veterinary Medical Association. 2001.View Summary
  9. Cox, A. and Stewart, A.J. Insect Bite Hypersensitivity in Horses: Causes, Diagnosis, Scoring and New Therapies. Animals (Basel). 2023.View Summary
  10. Van Den Brom-Spierenburg, A.J. et al. Dermatographism in a Horse, Responsive to Cetirizine Treatment. Equine Veterinary Education. 2019.
  11. Paterson, S. et al. Dermatophytosis in Horses (Equis). Accessed Apr. 17, 2024.
  12. Logas, D. et al. Dermatophilosis in Horses (Equis). Accessed Apr. 17, 2024.
  13. Marsella, R. and Paterson, S. Amyloidosis in Horses (Equis). Accessed Apr. 17, 2024.