Let's start with some definitions. Sarcoidosis means a generalized or systemic granulomatous disease. Granulomatous disease is a disease where there are collections of white blood cells to such a degree that normal function of the tissue is interfered with. So Equine Sarcoidosis is a systemic disease that effects not only the skin but many tissues by invading them with white blood cells. This is not to be confused with the similar sounding equine sarcoid which is a common problem in horses consisting of discreet nodules of connective tissue. Sarcoidosis often starts as hair loss, crusting, and irregular thickening of the skin but can progress to wasting and death if not treated. This article deals with the clinical signs, diagnosis, treatment, and prognosis of Equine Sarcoidosis.

The cause of equine sarcoidosis is unknown. However it resembles the human form which is thought to be a immunologic reaction to infection or allergen. It is not the infection causing the disease but the generalized overreaction of the immune system. This is supported by successful treatment with corticosteroids. In horses repeated attempts to identify an infectious agent have failed. There has been an association with exposure to the weed hairy vetch but not all cases have this history. Also mycobacterium is frequently associated with the problem in other species with similar diseases but again this is rare in horses.

More Information on Lack of detectable equine herpesviruses 1 and 2 in paraffin-embedded specimens of equine sarcoidosis. J Vet Intern Med. 2009 May-Jun;23(3):623-5. White SD, Foley JE, Spiegel IB, Ihrke PJ. Department of Medicine and Epidemiology, University of California, Davis, CA. Background: Equine sarcoidosis is a rare, multisystemic, noncaseating, granulomatous and lymphoplasmacytic disease of unknown etiology. A recent report described a horse with granulomatous skin disease displaying histologic, electron microscopic, and polymerase chain reaction (PCR) findings consistent with equine herpesvirus 2 (EHV-2). Objective: To investigate the presence of EHV-2 and equine herpesvirus 1 (EHV-1) in 8 horses with sarcoidosis. Animals: Eight horses with sarcoidosis, reported previously. Methods: Retrospective study. PCR assays of the tissues were performed to detect DNA associated with EHV-1 and EHV-2. For both herpesviruses the target was their respective glycoprotein B gene. Positive controls consisted of DNA from viral cultures of culturettes from naturally occurring respiratory infections of EHV-1 and EHV-2. Results: The PCR analyses for both equine herpesviruses' DNA were negative in all 8 horses. Conclusion: The failure to detect DNA from EHV-1 and EHV-2 in paraffin-embedded skin of these 8 horses does not discount EHV-1 or EHV-2 as causing some cases of ES, but lends support to the presumably multifactorial etiologic nature of the disease.