There are still some questions regarding how vesicular stomatitis is transmitted and why it only occurs sporadically in the U.S.
The disease is distributed only in North, Central, and South America, with a greater incidence in warmer regions. Due to the seasonal occurrence of VS during summer through early fall, it is believed that insects such as biting flies and midges contribute to maintaining the lifecycle of the virus. Black flies, sand flies, and midges are known to transmit the virus, but there may be other insect vectors that have not yet been identified.
VS also can be passed from horse to horse by contact with saliva or fluid from ruptured blisters.
Physical contact between animals, or contact with buckets, equipment, housing, trailers, feed, bedding, shared water troughs or other items used by an infected horse can provide a ready means of spread.