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 wastes.

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]