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