Encephalitis Causes

• Herpes simplex: This virus causes cold sores and lesions of the genitals. It is transmitted directly through human contact. Newborns can also get the virus by passing through an infected birth canal. Once inside the body, the virus travels through nerve fibers and can cause an infection of the brain. The virus may also undergo a period of latency in which it is inactive. At a later time, emotional or physical stress can reactivate the virus to cause an infection of the brain.
  
  
• Arbovirus: Hosts are animals such as birds, pigs, chipmunks, and squirrels that carry the virus. Mosquitoes (known as vectors or ways of transmitting the virus) feed on these animals and become infected. The virus grows and cycles between the hosts and the vectors. Humans become infected through mosquito bites. Once inside the body, the virus replicates and travels in the bloodstream. If there is a large enough amount of the virus, the brain can become infected. The majority of cases occur between June and September when the mosquitoes are most active. In warmer climates, the disease can occur year-round.
  
  
  • West Nile virus (WNV): This virus was first isolated from an adult woman with a fever in the West Nile District of Uganda in 1937. The nature of the virus was studied in Egypt in the 1950s. In 1957, as a result of an outbreak in Israel in the elderly, the WNV became recognized as a cause for severe inflammation of the spinal cord and brain in humans. In the early 1960s, it was first noted that horses were becoming ill in Egypt and France. This virus then emerged in North America in 1999, with encephalitis reported in humans and horses.
    
    
    • In 1999, 62 cases of severe disease, including 7 deaths, occurred in the New York area. In 2001, 21 cases were reported, including 2 deaths in the New York area. In 2002, 24 deaths have been reported (as of August 28, 2002).
      
      
    • The virus cycles between the Culex mosquito and hosts such as birds, horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits. The mosquito feeds on the infected hosts, carries the virus in its salivary glands, and then passes it on to humans or other animals during a blood meal. It usually takes 3-15 days from the time of infection to the onset of disease symptoms.
      
      
    • West Nile encephalitis is NOT transmitted from person to person (such as through touching or kissing or from a health care worker caring for a sick person) nor can it be passed from animal to human.
      
      
    • The chance that you will become severely ill from a mosquito bite is extremely small. According to the Centers for Disease Control and Prevention (CDC), even in areas where mosquitoes are reported to carry the virus, much less than 1% are infected. Furthermore, less than 1% of the people who get bitten and become infected will become severely ill. Therefore, the majority of cases are mild, and people can fully recover.
      
      
    • Prognosis is usually guarded in the extremes of age (infants and elderly). Death as a result of West Nile encephalitis ranges from 3-15% and are highest among the elderly. At the present time, there is no documented evidence to suggest that a pregnancy is at risk due to WNV infection. It is assumed that if a person contracts WNV, he or she will develop a natural immunity that is lifelong. However, it may wane in later years.
      
      
    • Since the virus first appeared in New York, researchers began looking for a vaccine. According to the Proceedings of the National Academy of Sciences, US government scientists have now developed a vaccine that protects mice from the West Nile Virus infection. Tests showed that mice injected with this vaccine were protected from subsequent exposures to the New York strain of West Nile virus. Researchers were expected to begin testing the vaccine in monkeys in March 2002, with testing in humans likely to take place in late 2002.
      
      

• La Crosse encephalitis: The first case occurred in La Crosse, Wisconsin, in 1963. Since then, the largest number of cases has been identified in woodland areas of the Midwestern and Mid-Atlantic United States. This virus is the most common cause of mosquito-borne encephalitis in children younger than 16 years. Each year, about 75 cases are reported to the Centers for Disease Control and Prevention (CDC). The virus cycles between the daytime-biting treehole mosquito (Aedes triseriatus) and hosts such as chipmunks and squirrels. The La Crosse encephalitis virus can cause adverse effects on IQ and school performance. About 1% of people with this infection die.

• St. Louis encephalitis: Since 1964, an average of 128 cases are reported per year. Outbreaks can occur throughout most of the United States, although large urban epidemics occurred in the Midwestern and Southeastern regions. The last major epidemic of St. Louis encephalitis occurred in the Midwest from 1974-1977. There were 2,500 cases in 35 states reported to the CDC. Most recently, there were 20 reported cases in New Orleans in 1999. The virus cycles between birds and the Culex mosquitoes breeding in stagnant water. It grows in both the mosquito and the bird but does not make either one sick. Only the infected mosquito can transmit the disease to humans during the blood meal. The virus cannot be transmitted from person-to-person through kissing or touching nor can it be transmitted from the infected bird. The disease tends to affect mostly adults and is generally milder in children.

• Eastern equine encephalitis (EEE): According to the CDC, there have been 153 confirmed cases in the United States since 1964. This virus is found along the East and Gulf coasts. The virus causes severe disease in horses, puppies, and birds such as pheasants, quails, and ostriches. In humans, flulike symptoms develop 4-10 days after the bite of an infected mosquito. Usually, human illnesses are preceded by those in horses. EEE can cause death in 50-75% of all cases. Those who recover may suffer severe permanent brain damage such as mental retardation, seizures, paralysis, and behavior abnormalities.

• Western equine encephalitis (WEE): This virus was isolated from the brain of a horse with encephalitis in California in 1930. Since 1964, there have been 639 confirmed cases. Today, it remains an important cause of encephalitis in the western part of the United States and Canada. In 1994, there were 2 confirmed and several suspected cases of WEE reported in Wyoming. In 1997, 35 strains of WEE virus were isolated from mosquitoes collected in Scotts Bluff County, Nebraska. The WEE virus cycles between certain types of birds (small, mostly songbirds) and the Culex tarsalis mosquitoes, a species associated with irrigated agriculture and stream drainage. The virus has also been found in several other mammals. Horses and humans become sick through mosquito bites. Infants are particularly affected and can have permanent problems such as seizure disorders and developmental delay.

• Venezuelan equine encephalitis (VEE): This virus is found in Central and South America and is a rare cause of encephalitis in the Southwestern part of the United States. It is an important cause of encephalitis in horses and humans. From 1969-1971 an outbreak from South America to Texas killed over 200,000 horses. In 1995, there were an estimated 90,000 human infections with VEE in Columbia and Venezuela. The virus cycles between forest-dwelling rodents and mosquito vectors, especially the species Culex. VEE infection in humans is much less severe than that of WEE and EEE. While adults tend to develop a flulike illness, children tend to develop overt encephalitis. Deaths are rare in humans but are common in horses. There is an effective vaccine for horses.

• Japanese encephalitis: This virus is responsible for 50,000 cases and 15,000 deaths per year. Most of China, Southeast Asia, and the Indian subcontinent are affected. The geographic distribution is expanding. Rarely, cases may appear in United States civilians and military personnel traveling to and living in Asia. Children and young adults are mostly affected. Older adults are affected when there are epidemics in new locations. The virus cycles between domestic pigs, wild birds, and the Culex tritaeniorhynchus mosquitoes, which breed in rice fields. The disease is not transmitted through human contact, pigs, or birds. Only the mosquitoes can transmit the disease during feedings.

Summary of Viral Encephalitis. Sources: Tierney, et al.; CDC: cdc.gov