Traveling horses are not a major source for the emergence of equine encephalitis viruses into new areas, according to Prof. Dr. Martin Pfeffer and Gerhard Dobler, MD, who looked at the role of animal trade and migration in spreading arboviruses into Europe.

"After an intensive search of the literature, we concluded that in most cases the viremia (levels of virus in the blood) in the horse is not high enough to support new arthropods to acquire the infection and thus to transmit it (to another animal)," said Pfeffer, a professor of epidemiology at the Veterinary School of the University of Leipzig in Germany.

The mosquito-borne West Nile virus and Eastern, Western, and Venezuelan equine encephalitis viruses are a group of viruses called arboviruses that are "among the most important viruses invading new areas in the world," Pfeffer said.

But the lifecycle is complex, requiring a virus, a vector species, and a reservoir host. In addition, geography and climate play important roles in the establishment of viruses in new areas. In essence, to successfully migrate to another area, there must be a "perfect storm"–all factors must be available at the same time, in the same place, and in sufficient quantity