Venezuelan equine encephalomyelitis.

The emergence of epidemic VEE viruses has been reported ever since the virus was first described; this phenomenon is likely to continue to occur because of the high mutation rate of these RNA viruses. A vaccine that was first developed by the US Military for human use has proved helpful in curtailing the spread of VEE virus during epizootics of the disease in equids but not during human epidemics. It has not, however, eliminated the source of these highly pathogenic and transmissible viruses. Occurrences of VEE in equids in Mexico in recent years suggest that the present vaccine is not effective in interrupting transmission of new epizootic viruses arising from what were previously known as avirulent enzootic cycles. Future vaccines against VEE should be based on immunogens derived from enzootic viruses to interrupt VEE virus transmission at the source itself rather than waiting for virulent phenotypes of VEE virus to emerge.