Because of the insidious nature of the disease, it is difficult to determine its origin or how widely it is distributed throughout the world. In April of the following year, an outbreak occurred in Missouri. The disease was rapidly eradicated from both states and has not subsequently been found in the U.S. horse population. Transmission CEM is commonly transmitted directly during natural sexual intercourse between undetected CEM-positive breeding mares and stallions. Transmission may also occur indirectly by artificial insemination or fomites, such as contaminated hands or instruments.
Outbreaks usually occur at breeding facilities following international horse shipments.
Undetected carrier mares and stallions are the source of infection for acute outbreaks of the disease. During the breeding season, a carrier stallion may infect several mares before the disease is suspected or diagnosed. Clinical Signs Initial exposure to the disease usually results in infertility. Abortions related to CEM are rare, however. Stallions exhibit no clinical signs but can carry the CEM bacteria on their external genitalia for years.