The virus that causes VEE is transmitted primarily by mosquitoes that bite an infected animal and then bite and feed on another animal or human. The speed with which the disease spreads depends on the subtype of the VEE virus and the density of mosquito populations.
Enzootic subtypes of VEE generally do not spread to other localities. These subtypes are associated with the rodent-mosquito transmission cycle, and these forms of the virus can cause human illness but generally do not affect equine health.
- Culex (Melanoconion) vomerifer
- Culex (Melanoconion) pedroi
- Culex (Melanoconion) adamesi
Epizootic subtypes can spread rapidly through large populations. These forms of the virus are highly pathogenic to equines and can also affect human health. Equines, rather than rodents, are the primary animal species that carry and spread the disease, as infected equines develop an enormous quantity of virus in their circulatory system. When a vector insect feeds on these animals, it picks up the virus and transmits it to other animals or humans.