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Infection can be transmitted between horses in any of the following ways:

  • direct transmission during mating;
  • direct or indirect transmission during teasing;
  • artificially inseminating mares with semen from infected stallions or which has been contaminated during semen collection or processing. The virus can survive in chilled and frozen semen and is not affected by the antibiotics added;
  • contact with aborted fetuses or other products of parturition;
  • via the respiratory route (eg via droplets from coughing and snorting).

The shedder stallion is a very important source of the virus. On infection, the virus localises in his accessory sex glands and will be shed in his semen for several weeks, months or years - possibly even for life. The fertility of shedder stallions is not affected and they show no clinical signs but they can infect mares during mating, or through insemination with their semen. These mares may, in turn, infect other horses via the respiratory route.