Although the vast majority of natural infections occur as a result of a bite from an infected mosquito, these viruses are also infectious as aerosols and, therefore, could possibly be utilized as a bioterrorism weapon. Any outbreak of viral encephalitides in nonendemic areas or without preceding equine cases should be highly suspect for a bioterrorism attack. Mortality rates could be higher following an aerosol bioterrorism attack due to a higher initial exposure and/or greater virulence through genetic manipulation. Venezuelan equine encephalitis would be the most likely candidate for weaponization due to its stability and ability to be highly infectious at a low dose. It also produces nearly 100% symptomatic infections. In addition, it has the potential to be altered genetically to produce a more virulent strain.
Viral Encephalitides Overview
Abnormalities in brain function presenting with altered mental status, personality or behavior changes, or focal motor, sensory, speech, or movement deficits due to central nervous system infection by a viral pathogen are described as viral encephalitis. Viral encephalitis is separate from postinfectious encephalitis, as the latter is a consequence of prior infection (including some viral infections) and the former is caused by viral invasion of the central nervous system.
There are many different viral etiologies of viral encephalitis. Some viral etiologies are transmitted by insect vectors (eg, West Nile virus), and others are transmitted directly from person to person (eg, enteroviruses, mumps, measles, herpes simplex virus [HSV], HIV). The most common causes of viral encephalitides in the United States are HSV, West Nile virus, and enteroviruses.
The incidence of some viral encephalitides has significantly decreased due to the use of vaccinations (such as for measles and mumps), whereas other etiologies, particularly those impacting immunocompromised hosts, have increased in incidence (such as those due to cytomegalovirus [CMV] and Epstein-Barr virus [EBV]). Arboviral etiologies vary depending on the season and on geography.
Mosquito-borne viral encephalitides include the following.
Alphaviruses of the Togaviridae family such as:
- Venezuelan equine encephalitis (VEE)
- Western equine encephalitis (WEE)
- Eastern equine encephalitis (EEE)
And members of the Bunyaviridae family: California encephalitis group, Oropouche fever.
Etiologies of viral encephalitis that can be transmitted from infected animals include the rabies virus.
Etiologies of viral encephalitis that can be transmitted directly from person to person include:
- HSV
- Enteroviruses
- Varicella-zoster virus (VZV) (see varicella, herpes zoster)
- EBV
- CMV
- HIV
- Human herpesvirus 6
- Human herpesvirus 7
- Measles virus
- Mumps virus
- Rubella virus
Patients may also experience other symptoms specific to the viral etiology causing the infection.
The mortality rate of naturally occurring viral encephalitides is greater than 1% overall but may be as high as 20% after progression to severe encephalitis. EEE causes the most severe arbovirus encephalitis and has a high mortality rate.
The incubation period of arboviral encephalitides is typically 1-6 days. Viral reservoirs include horses, bats, birds, rodents, donkeys, mules, and tropical mammals from which the virus is transmitted via mosquitoes to humans and equines. Person-to-person transmission may be possible but has not been proven.
Borna disease virus 1 (BoDV-1) is a cause of severe encephalitis, and cases of transmission via organ transplantation have been observed.
Arboviral encephalitides are endemic to tropical regions of Central and South America, especially in rural areas. Eastern equine, Western equine, St. Louis, and West Nile virus encephalitis are all endemic in North America. Pathogens such as HSV, VZV, and HIV are ubiquitous worldwide.
Veterinarians, laboratory workers, tourists, military personnel, ranch workers, horseback riders, and residents of tropical areas are at higher risk for contracting arboviral encephalitides. Children are more susceptible to moderate or severe forms of the disease.
Related topic: encephalitis
