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Background: Coital exanthema is a venereally transmitted disease of horses caused by Equine Herpesvirus 3 (EHV-3).  It is not currently a reportable disease in the U.S. or Canada.  Lesions are usually limited to the penile skin of stallions and the vulva/perineum of mares.  The virus initially causes vesicular lesions that heal spontaneously and leave whitish plaque-like scars. 

Clinical Signs: An incubation of 5 to 9 days is typical; the disease starts with development of fluid filled vesicles and progresses to crater-like ulcerative lesions on the epithelial surface of the penis and skin of the perineum or vulva.  Systemic signs are rare.  Reactivation of latency can occur under condition of stress.  This herpesvirus has not been associated with abortion. 

Transmission: Equine herpesvirus-3 is highly contagious and may be passed between horses by nose-to-nose contact and by contaminated fomites, in addition to breeding by natural cover or artificial insemination (AI).  To avoid transmission to susceptible horses, infected stallions or mares should ideally not be used for natural breeding or semen collection until lesions are fully healed.  Lesions are considered to be healed when the crater-like lesions are filled in, resulting in smooth whitish scars, and there are no signs of acute inflammation or discharge. 

Diagnostic Testing: Presumptive diagnosis is based on the presence of typical pox-type lesions.  A diagnosis of EHV-3 may be confirmed by virus isolation, by PCR from active lesions, by negative contrast electron microscopy and possibly by testing paired serum samples with the objective of demonstrating a four-fold or greater rise in neutralizing antibody titers to EHV.  Suitable specimens for virus detection can be swabs taken from some of the lesions or fragments of tissue from ruptured vesicles. 

Specific Control Measures: To avoid horizontal transmission of the virus, breeding of affected stallions, including semen collection for AI, should not occur until the lesions have completely resolved. Lesions are considered to be healed when the crater-like lesions are filled in, resulting in smooth whitish scars, and there are no signs of acute inflammation or lesion discharge.

Biosecurity Guidelines: Dedicated artificial vaginas, barrier procedures and gloves used in semen collection should be successful in preventing horizontal transmission and contamination of breeding equipment in latent carriers or stallions discovered to have lesions after collection.  Additionally, changing sleeves between palpations, and covering ultrasound probes with fresh sleeves, will help to reduce the chance of inadvertent transmission between mares if lesions are not noticed.  Stallions requiring tease mares for phantom collection of semen should be restrained to avoid nose-to-nose or nose-to-vulva contact with theses mares.  False mounting tease mares should be avoided.  Tease mares should be visually inspected for EHV-3 lesions prior to each use.  The virus is easily destroyed by common disinfectants, heat, sunlight and drying.