Encephalitis presents as diffuse or focal neuropsychological dysfunction. Although it primarily involves the brain, it often involves the meninges as well (meningoencephalitis). Portals of entry are virus specific. Many viruses are transmitted by humans, though most cases of HSE are thought to be reactivation of HSV lying dormant in the trigeminal ganglia. Mosquitoes or ticks inoculate arbovirus, and rabies virus is transferred via an infected animal bite or exposure to animal secretions. With some viruses, such as varicella-zoster virus (VZV) and cytomegalovirus (CMV), an immune-compromised state is usually necessary to develop clinically apparent encephalitis. In contrast to viruses that invade gray matter directly, acute disseminated encephalitis and postinfectious encephalomyelitis (PIE), most commonly due to measles infection and associated with Epstein-Barr virus (EBV) and CMV infections, are immune-mediated processes that result in multifocal demyelination of perivenous white matter.
Most people with viral encephalitis have either no symptoms or mild flu-like symptoms, such as the following: Headache, fever, aches in muscles or joints, fatigue or weakness. Treatment for mild cases, which may be mistaken as the flu, mainly consists of: bed rest, plenty of fluids, anti-inflammatory drugs (acetaminophen, ibuprofen and naproxen sodium) to relieve headaches and fever.
Determining the true incidence of encephalitis is impossible, because reporting policies are neither standardized nor rigorously enforced. Japanese virus encephalitis (JE), occurring principally in Japan, Southeast Asia, China, and India, is the most common viral encephalitis outside the United States. Individuals at the extremes of age are at highest risk, particularly for HSE. Neonatal HSE is a manifestation of disseminated infection type 1 or 2, whereas older infants, children, and adults are much more likely to have localizing CNS infection almost exclusively due to type 1, in a bimodal distribution of patients aged 5-30 years or older than 50 years.