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

Vasculitis (inflammation of blood vessels) caused by immune complexes occurs in horses. At first, abnormalities are seen as purplish red dots appearing on the skin. Depending on which blood vessels are involved, signs may appear on the legs, mouth, or lips. Drugs are a frequent cause of 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 characterized by swelling and abscesses. It is a serious complication of infection with Streptococcus equi bacteria. The bacteria can spread from one horse to another by inhalation or ingestion of infected fluids or cells. It can also spread through contamination of the horse's environment. The disease ruptures lymph nodes and damages the horse's blood vessels. It can be fatal. Infected horses may lack energy, be in pain when swallowing and lose their appetite, and develop a fever or cough. Bacteria cultured from the material in the abscesses can be identified for a definitive diagnosis.

Horses suspected of having the infection should be isolated from other horses, kept warm and dry, and encouraged to eat soft, palatable feed. Complete drainage of the abscesses is necessary along with regular flushes of the ruptured lymph nodes until healing occurs. The material from the abscesses is infectious, and contaminated objects—including boots, hands, tack, hay, stall, and soil—should be cleaned and disinfected or discarded. The treatment consists of a strict quarantine of any new animals on the premises for up to 6 weeks. Recovered horses should not be considered free of infection until bacterial cultures are negative.

Anterior Uveitis

Anterior uveitis in horses is also known as moon blindness or periodic ophthalmia see Eye Disorders of Horses: Equine Recurrent Uveitis (Periodic Ophthalmia, Moon Blindness). 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 whole-body corticosteroids and other drugs that suppress the immune system.

Last full review/revision July 2011 by Christine Andreoni; Kevin T. Schultz, DVM, PhD