This document describes the characteristics of viral encephalitides.
Although natural infections with the encephalitic alphaviruses are acquired by mosquito bite, the viruses are also highly infectious by aerosol. The collective characteristics of alphaviruses, especially the equine encephalomyelitis viruses, lend themselves very well to weaponization. This fact was recognized by the designers of offensive biological warfare programs that were initiated before or during World War II. Alphaviruses can be produced in large amounts in inexpensive and unsophisticated systems. They are relatively stable and highly infectious for humans as aerosols. Strains are available that produce either incapacitating or lethal infections. The existence of multiple serotypes of some of the viruses, as well as the inherent difficulties of inducing efficient mucosal immunity confound defensive vaccine development. The American equine encephalitides comprise three virus complexes: Venezuelan equine encephalitis (VEE), Western equine encephalitis (WEE), and eastern equine encephalitis (EEE). They are recognized for their potential for neuroinvasion and encephalitis in humans, sometimes in epidemic proportions. Depending on the virus, patients presenting with the general syndrome of alphavirus encephalitis have a varying combination of fever, headache, confusion, obtundation, dysphasia, seizures, paresis, ataxia, myoclonus, and/or cranial nerve palsies. In a potential biological warfare scenario, alphaviruses should be considered in the differential diagnosis whenever epidemic febrile illness occurs, especially if a number of patients progress to neurological disease. No specific therapy exists for infections caused by these viruses. Although some vaccines are useful in protecting at-risk individuals, they have certain disadvantages, and improved vaccines are under development. 177 references