West Nile virus
Encephalitis is a rare disease, extremely uncommon in the U.S. even for people in the risk groups discussed below. Many people fall into the following categories; very few of them will ever contract encephalitis.
Encephalitis can occur at any age; increased age-associated risks depend on the type of encephalitis virus. Newborn infants are particularly at risk for herpes virus. For arboviruses, infants are most vulnerable to Western equine encephalitis. Older children and teenagers are more susceptible to Eastern equine and La Crosse encephalitis. Older and elderly adults are at higher risk for Eastern equine, St. Louis, and West Nile encephalitis.
Patients whose immune systems are compromised by conditions such as HIV-AIDS, cancer therapies, or organ transplantation are more susceptible than other individuals to any form of encephalitis. Of particular concern are varicella and cytomegalovirus encephalitis which tend to be more common and deadly in these patients than in the normal population.
U.S. 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 or farming areas, but they can also occur in cities. While some forms of arbovirus encephalitis are limited to specific geographical regions, the West Nile virus has become endemic throughout the mainland United States. [See Common Forms of Mosquito-Borne Encephalitis table for more detailed regional information.]
Seasonal Risks. Transmission of arboviruses correlates with the mosquito season and is highest during the months of July through September (late summer through early fall). The ideal conditions for mosquito breeding are a wet spring followed by a hot, dry summer.
Few people in the world have not been infected with at least one of the herpes viruses. Most of these viruses are easily transmitted in body fluids, including from saliva or droplets after people exhale or sneeze. Infants can contract herpes simplex virus from an infected mother during delivery, which can have very serious consequences. [For more information, see In-Depth Report #52: Herpes simplex.]
Bleck TP. Arthropod-borne viruses affecting the central nervous system. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 406.
Bleck TP. Cytomegalovirus, Epstein-Barr virus, and slow virus infections of the central nervous system. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 393.
Katz TM, Miller JH, Hebert AA. Insect repellents: historical perspectives and new developments. J Am Acad Dermatol. 2008 May;58(5):865-71. Epub 2008 Feb 13.
Lindquist L, Vapalahti O. Tick-borne encephalitis. Lancet. 2008 May 31;371(9627):1861-71.
Loeb M, Hanna S, Nicolle L, Eyles J, Elliott S, Rathbone M, et al. Prognosis after West Nile virus infection. Ann Intern Med. 2008 Aug 19;149(4):232-41.
Modlin JF. Enterovirus infections. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 402.
Nath A, Berger JR. Acute viral encephalitis. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 439.
Tunkel AR, Glaser CA, Bloch KC, Sejvar JJ, Marra CM, Roos KL, et al. The management of encephalitis: clinical practice guidelines by the Infectious Diseases Society of America. Clin Infect Dis. 2008 Aug 1;47(3):303-27.
Voelker R. Effects of West nile virus may persist. JAMA. 2008 May 14;299(18):2135-6.
Whitley RJ. Herpes simplex virus infections. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 397.
UMMC is a member of the University of Maryland Medical System,
22 S. Greene Street, Baltimore, MD 21201. TDD: 1-800-735-2258 or 1.866.408.6885