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)

Members of the Flaviviridae family such as:

• St. Louis encephalitis
• Japanese encephalitis
• West Nile virus
• Zika virus
• Dengue fever

And members of the Bunyaviridae family: California encephalitis group.

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

Symptoms of the viral encephalitides range from none to mild to severe and may include 24-72 hours of spiking fever, chills, fatigue, myalgias (especially in the low back), headache, and photophobia. Patients often also present with abnormalities in brain function, altered mental status, personality changes, behavioral changes, sensory deficits, motor deficits, and speech disorders. Other manifestations include hemiparesis, flaccid paralysis, seizures, and paresthesias.

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