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Arbovirus Encephalitis
Eastern Equine Encephalitis
| Modes of Transmission |
Begin life cycles as parasites of migratory wild birds and are transferred through mosquito bites to either horses or humans. |
| US Geographic Areas |
Atlantic and Gulf coasts, in New England, and around the Great Lakes. |
| Areas of the Brain |
Causes congestion in the brain and widespread changes in nerve cells. Lesions found in white and gray matter. Can affect all major parts of the brain.
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| After Effects |
After effects can include seizures, palsy, mental impairment, partial paralysis, and speech problems. Usually more severe in children. |
| Incidence and Mortality Rates |
The most serious of the US arboviruses and fortunately rare. Only 153 cases have been confirmed since 1964. Mortality rates are about 30% to 80%. Children are more likely to survive but also to suffer complications afterward. |
Western Equine Encephalitis
| Modes of Transmission |
Begin life cycles as parasites of migratory wild birds and are transferred through mosquito bites to either horses or humans. Snakes and rodents may also harbor the viruses. |
| US Geographic Areas |
Occurs throughout the country, but mostly in rural regions west of the Mississippi. |
| Areas of the Brain |
Causes less inflammation and fewer nerve cell changes than the Eastern variant.
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| After Effects |
After effects in infants and children can be the same as in Eastern equine encephalitis. |
| Incidence and Mortality Rates |
There were 639 confirmed cases between 1964 and 1998. Mortality rate is 3% to 4%; 30% survivors have complications afterward. Most severe in children. |
St. Louis Encephalitis
| Modes of Transmission |
Caused by an arbovirus known as a flavivirus. It is carried by mosquitoes. |
| US Geographic Areas |
It takes its name from an epidemic in St. Louis in which more than 1,000 people were infected, but outbreaks have continued in greater frequency and wider geographic areas. Outbreaks can occur in rural or urban areas. |
| Areas of the Brain |
Mild congestion in blood vessels with small areas of bleeding. Mild infiltration of infection into the meninges (the membrane around the brain). Infection accumulates in both gray and white matter. Thalamus and midbrain more likely to be affected. |
| After Effects |
After effects include headaches, insomnia, fatigue, and irritability that usually clear after several years. Some experience permanent neurological problems, including partial paralysis, problems with motor control and walking, and impaired speech. |
| Incidence and Mortality Rates |
It is the most common arbovirus encephalitis in the US, but the average number of cases per year is only 128. Mortality rate of between 2% and 20%, with about 5% of survivors suffering complications afterward. Most severe in people over age 60. Younger people usually experience mild, flu-like symptoms. |
West Nile Virus
| Modes of Transmission |
The virus circulates primarily between birds and mosquitoes and can be carried long distances by migrating birds. In a given geographic area, the appearance of the virus among birds and mosquitoes generally precedes infection in humans. The 1999 outbreak in New York City was preceded by illness and deaths among crows, which was unusual since the virus does not usually kill birds. |
| US Geographic Areas |
Until 1999, the West Nile virus was generally restricted to Africa, the Middle East, southwestern Asia, eastern Europe, and Australia. It emerged in the United States with the first outbreak in New York City in 1999. Mosquitoes infected with the West Nile virus have now been found across New York State, Connecticut, New Jersey, Massachusetts, Rhode Island, and New Hampshire. |
| Areas of the Brain |
May have a predeliction for the brain stem. It can also cause meningitis. |
| After Effects |
After effects include mild weakness and memory loss that usually clear up within a few weeks. Recovery is nearly always complete but less rapid in adults than in children. Low incidence of seizures. |
| Incidence and Mortality Rates |
In 1999, 59 people in New York City were hospitalized with the infection and seven died. The average age was 71. In 2000, the number of reported cases dropped significantly, with 12 reported cases, all in people over 40. Fatality rates of reported cases are 3% to 15%, mostly in the elderly. |
California Encephalitis (Including LaCross virus)
| Modes of Transmission |
Caused by an arbovirus known as a bunyavirus. Its original hosts are small animals; it is transmitted to humans by infected mosquitoes. |
| US Geographic Areas |
Occurs most frequently in the north central US and along the eastern seaboard. |
| After Effects |
In some children, emotional difficulties, learning problems, or seizures may occur. |
| Incidence and Mortality Rates |
It is a major cause of encephalitis in the US, but fortunately it still only causes 70 cases a year and is very mild. Mortality rates less than 1%. More common and severe in children under 16. |
Japanese B Encephalitis
| Modes of Transmission |
Transmitted to humans by infected mosquitoes. |
| Geographic Areas |
Most common viral encephalitis outside the US. It occurs in rural areas in east, south, and southwest Asia, especially China and Korea. |
| Incidence and Mortality Rates |
Occurs mainly in children and has a very high mortality rate. Fortunately, a vaccine is available that has greatly reduced the disease's incidence. |
Herpes Encephalitis
Epstein-Barr Encephalitis
| Areas of the Brain |
Most often affects frontal and temporal lobes. Unusual degree of swelling in the brain. |
| After Effects |
After effects may range from mild neurological impairment to paralysis. Even after apparent recovery, there may be learning disabilities, fine motor control deficits, or speech problems. |
| Mortality Rates |
With treatment, mortality rates are 15% in newborns and 20% in others. Untreated, mortality rates are 60% to 80%. |
Epstein-Barr Encephalitis
| Mortality Rates |
Usually mild. Mortality rate from encephalitis is 8%, and 12% suffer complications. |
Cytomegalovirus Encephalitis
| Mortality Rates |
Particularly dangerous in AIDS patients, in whom mortality and post-survival complication rates are more than 50%. In people without AIDS, the mortality rates are less than 5% and complication rates are less than 10%.
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Last Updated: August 8, 2001
Source: Well-Connected
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