Encephalitis: Viral

Description

Risk Factors

It is important to understand that encephalitis is a rare disease. It is extremely uncommon in the US, even for people in the risk groups discussed below. Many people fall into the following categories and very few of them will ever contract encephalitis.

Age

Encephalitis can occur at any age, but the highest risk is in children under four years of age, with a peak from three to six months, and among adults, especially those over age 60. The age risks also depend on the way the virus itself and they way it is transmitted. As examples, both eastern and western equine encephalitis disproportionately affect infants. The eastern variant also tends to affect children and the elderly. La Crosse encephalitis nearly always infects children (not infants or adults). St. Louis encephalitis usually occurs in adults over 40 years old.

Immunocompromised Patients

Patients whose immune systems are compromised by conditions such as AIDS or HIV, cancer therapies, or organ transplantation are more susceptible than other individuals to any form of encephalitis. Of particular concern are varicella and cytomegalovirus encephalitis because they tend to be more common and deadly in these patients than in the normal population.

Risk Factors for Arboviruses

US Geographic Regions. The primary risk factor for arbovirus encephalitis is living in areas of possible exposure to virus-carrying mosquitoes. Most viral outbreaks occur in rural areas, but they can also occur in cities. People all over the country can be exposed to western equine encephalitis, although it is more likely to occur in the west. In addition, people in the Midwest are at risk for La Crosse and St. Louis encephalitis. Mosquitoes infected with the West Nile virus have now been found up and down the eastern seaboard and the Midwest and are spreading.

Seasonal Risks. The chances for transmission of arboviruses are highest during the months of July through September. The ideal conditions for the mosquitoes that harbor the virus for St. Louis encephalitis are a relatively mild winter, wet spring, and hot, dry summer. It should be noted, however, that arboviruses such as West Nile are cropping up early in the east even after the drought-stricken spring of 2002. Mosquitoes can carry the virus over a winter and transmit it the next year.

Risk Factors for Herpes Viruses

It is a very rare individual anywhere in the world who has not been infected with at least one of the herpes viruses. Most of these viruses are easily transmitted in body fluids, including saliva or in droplets after people exhale or sneeze. Of note: infants can contract herpes simplex virus from an infected mother during delivery, which can have very serious consequences. [For detailed information, see the Well-Connected Report Herpes Simplex.]