Viral encephalitis

Viral encephalitis is inflammation of the brain parenchyma, called encephalitis, by a virus. The different forms of viral encephalitis are called viral encephalitides. It is the most common type of encephalitis and often occurs with viral meningitis. Encephalitic viruses first cause infection and replicate outside of the central nervous system (CNS), most reaching the CNS through the circulatory system and a minority from nerve endings toward the CNS. Once in the brain, the virus and the host's inflammatory response disrupt neural function, leading to illness and complications, many of which frequently are neurological in nature, such as impaired motor skills and altered behavior.

Viral encephalitis can be diagnosed based on the individual's symptoms, personal history, such as travel history, and different clinical tests such as histology, medical imaging, and lumbar punctures. A differential diagnosis can also be done to rule out other causes of the encephalitis. Many encephalitic viruses often have characteristic symptoms of infection, helping to aid diagnosis. Treatment is usually supportive in nature while also providing antiviral drug therapy. The primary exception to this is herpes simplex encephalitis, which is treatable with acyclovir. Prognosis is good for most individuals who are infected by an encephalitic virus but is poor among those who develop severe symptoms, including viral encephalitis. Long-term complications of viral encephalitis typically relate to neurological damage, such as experiencing seizures, memory loss, and intellectual impairment.

Encephalitic viruses are typically transmitted either from person-to-person or are arthropod-borne viruses, called arboviruses. The young and the elderly are at the highest risk of viral encephalitis. Many cases of viral encephalitis are not identified either because of lack of testing or mild illness, and serological surveys indicate that asymptomatic infections are common. Various ways of preventing viral encephalitis exist, such as vaccines that are either in standard vaccination programs or which are recommended when living in or visiting certain regions, and various measures aimed at preventing mosquito, sandfly, and tick bites in order to prevent arbovirus infection.

Many viruses are capable of causing encephalitis during infection, including:

Encephalitic viruses vary in their manner of transmission. Some are transmitted from person-to-person, whereas others are transmitted by animals, especially bites from arthropods such as mosquitos, sandflies, and ticks, such viruses being called arboviruses. An example of person-to-person transmission is the herpes simplex virus, which is transmitted by means of intimate physical contact. An example of arboviral transmission is the West Nile virus, which usually is incidentally transmitted to people from the bites of Culex mosquitos, especially Culex pipiens.

Viruses that cause viral encephalitis first infect the body and replicate outside of the central nervous system (CNS). Thereafter, most reach the spinal cord and brain via the circulatory system. Exceptions to this include herpesviruses and the rabies virus, which travel from nerve endings to the CNS. Once in the brain, the virus and the host's inflammatory response disrupt neural cell function, including causing fluid buildup in the brain, vascular congestion, and bleeding. Widespread presence of white blood cells and microglia in the CNS is common as a response to CNS infection. For some forms of viral encephalitis, such as Eastern equine encephalitis and Japanese encephalitis, there may be a significant amount of necrosis of nerve cells. Following encephalitis caused by arboviruses, calcification may occur in the CNS, especially among children. Herpes simplex encephalitis tends to produce necrotic lesions in the CNS.

If viral encephalitis is suspected, then questions can be asked about the individual's history and physical examination can be performed. Important aspects of one's history include immune status, exposure to animals, including insects, travel history, vaccination history, geography, and time of year. Symptoms usually occur acutely, and the most common symptoms of infection are fever, headache, altered mental status, sensitivity to light, stiff neck and back, vomiting, confusion, and, in severe cases, seizures, paralysis, and coma. Neuropsychiatric features such as behavioral changes, hallucinations, or cognitive decline are frequent. Severe symptoms are most common among infants and the elderly. Most infections are asymptomatic, lacking symptoms, whereas most symptomatic cases are mild illnesses.

Virus-specific symptoms may also exist or tests may indicate one virus. Specific examples include: