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Disorders Involving Immune Complexes (Type III Reactions) in Horses


Ian Rodney Tizard

, BVMS, BSc, PhD, DSc (Hons), DACVM, Department of Veterinary Pathobiology, College of Veterinary and Biomedical Sciences, Texas A&M University

Reviewed/Revised Mar 2019 | Modified Oct 2022

Immune complex disorders are among the most common immune-mediated diseases. The location in the body where the immune complexes (combinations of antibodies and antigens) are deposited determines the signs and the course of the disease.

Hypersensitivity Pneumonitis

This condition occurs when horses are exposed to large amounts of inhaled antigens, such as those found in dusty feeds or moldy hay. The lung tissues become inflamed, and signs of respiratory distress, such as difficulty breathing or rapid breathing, may be noticed about 4 to 6 hours after exposure to the antigen. The most effective treatment involves detecting and removing the source of the antigen. Your veterinarian may also recommend certain drugs, such as corticosteroids, to help control the allergic reaction.


Vasculitis (inflammation of blood vessels) caused by immune complexes occurs in horses. The disorder causes the skin to develop reddened areas that quickly erode into scabbed sores. Depending on which blood vessels are involved, the sores may appear on the legs, mouth, or lips. The limbs may also swell due to fluid accumulation under the skin. The administration of certain drugs may cause vasculitis. The disorder is diagnosed by performing tests on samples removed from the affected areas. Vasculitis is treated by stopping the offending drug (if implicated as the cause) or by giving drugs that suppress the immune system.

Purpura Hemorrhagica

Purpura hemorrhagica is an immune reaction that is a serious complication of infection with Streptococcus equi bacteria (often called strangles). Immune complexes form between antibodies and the bacteria. These complexes then lodge within blood vessels, causing inflammation (vasculitis). This causes abnormal bleeding and fluid accumulation under the skin, often in the limbs. In some horses, the immune complexes can block blood flow to muscles, the tissue just under the skin (subcutaneous tissue), regions of the digestive tract, and lungs.

Signs include:

  • sudden colic (abdominal pain)

  • fluid accumulation under the skin

  • firm swellings within the muscles and skin

  • lameness

The diagnosis is made based on the signs and blood tests. Successful treatment involves early detection of the condition, antibiotics, and medications to suppress the immune system. Without prompt and thorough treatment, the conditions can be fatal.

Anterior Uveitis

Anterior uveitis is inflammation of the uvea, which is located in the front portion of the eye. In horses, recurrent uveitis is also known as moon blindness or periodic ophthalmia. One cause of anterior uveitis is the action of antibody–antigen complexes on the iris, which causes inflammation of the eye. Once started, the inflammation may cause blindness if not halted. Fortunately, its progression can, in many cases, be slowed or stopped by fast, aggressive, and consistent care. Treatment of immune-mediated anterior uveitis may include topical and whole-body corticosteroids and other drugs that suppress the immune system.

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