Get a clearer picture of what happens when a horse’s immune system malfunctions

Owners often describe their horses as reactive when they spook at leaves, a gust of wind, a mouse, or something invisible to everyone else. But in some cases horses could be deemed reactive when their immune systems go haywire, resulting in disease. Fortunately, auto-immune diseases occur infrequently, which means most horses avoid their serious effects.

Scientists define autoimmune diseases as those where the immune system targets the body’s own normal proteins, causing damage and/or organ dysfunction. In contrast they refer to secondary immune-mediated diseases as the body’s excessive inflammatory reactions to foreign materials such as drugs, supplements, viruses, bacterial toxins, or vaccines.

“Key examples of autoimmune diseases include various skin conditions, such as pemphigus foliaceus and immune-mediated drug reactions that occur when the immune system builds a response to a specific drug, and the antibodies produced against the drug end up causing damage to the animal,” explains Rosanna Marsella, DVM, Dipl. ACVD, a professor at the University of Florida’s College of Veterinary Medicine, in Gainesville.

In this article we’ll talk about some of the rare autoimmune diseases based on organ system, including skin, blood vessels, and the gastrointestinal tract. But be aware that immune-mediated diseases can also affect many organ systems, including the joints (immune-mediated arthritis), muscle (immune-mediated myositis), and eyes (recurrent uveitis).

The Skin: Immune-Mediated Dermatopathies

Among rare immune-mediated skin conditions, pemphigus foliaceous ranks as one of the most common primary autoimmune disorders in horses. This condition occurs when the horse’s body produces antibodies—molecules generated by the immune system typically in response to a bacterial or viral infection—that begin attacking normal proteins located in the skin.

“Sometimes a drug or vaccine can trigger the immune system, but other times the cause is idiopathic (unknown),” says Marsella.

The proteins the immune system attacks, called desmosomes, anchor individual cells together in the skin’s epidermal layer. When the horse’s errant antibodies destroy desmosomes, it compromises the epidermal layer, resulting in the formation of superficial pustules and blisters.

Horses with pemphigus foliaceous have pustules anywhere over the body but often appear in the periocular (around the eye) and muzzle areas. But the pustules often burst and are therefore transient, quickly replaced with crusting. Owners often report a repeating cycle of pustules to crusting lesions accompanied by dullness, lethargy, and anorexia in their horses during the worst periods. Severity varies on a case-by-case basis, with some horses covered in lesions and others with lesions limited to small areas, such as the coronary band or face. Many horses become pruritic (intensely itchy) and can injure themselves through scratching and rubbing