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Tick-Borne Flaviviruses Use a Novel Mechanism to Evade Host Defenses

 
  October, 4 2005 15:14
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Researchers from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, have made the surprising discovery that flaviviruses, which cause such serious diseases as West Nile fever, yellow fever and forms of encephalitis, evade immune system defenses in different ways depending on whether they are transmitted by mosquitoes or ticks. This finding could lead to new approaches to developing vaccines and treatments against these illnesses.

Mosquito-borne flaviviruses include West Nile virus, yellow fever virus, dengue virus and Japanese encephalitis virus; the less-familiar tick-borne flaviviruses are just as serious, causing tick-borne encephalitis or hemorrhagic fevers. Currently, a Japanese encephalitis outbreak is raging in India and Nepal and has killed more than 1,000 people. In Europe and Southeast Asia, tick-borne encephalitis typically results in more than 10,000 patient visits to hospitals annually and has a fatality rate of up to 25 percent in some regions. Viruses that cause encephalitis lead to inflammation of the brain. Hemorrhagic fevers are viral infections that cause capillaries to burst, leading to unusual bleeding on or under the skin or in various organs.

The study released online in the Journal of Virology describes how a single virus protein — NS5 — from the tick-borne Langat flavivirus counteracts the natural ability of interferon to combat the virus. Langat virus was originally isolated in the 1950s in Malaysia and Thailand. Langat virus can infect people following a tick bite, but there are no cases of natural disease recorded. In the 1970s Langat was briefly used as a live vaccine against more virulent tick-borne encephalitis viruses in Russia but caused encephalitis complications in about 1 of every 10,000 people.

Interferon, the body’s first defense against many viruses, triggers a cascade of immune defenses. According to researchers at NIAID’s Rocky Mountain Laboratories (RML) in Hamilton, MT, NS5 blocks the body’s attempt to signal for immune defenses, preventing the immune system from both stopping the spread of virus and helping the body recover from infection.

Interferon is so critical for recovery from these infections that it is being tested in clinical trials to treat infection with various flaviviruses. But the treatment appears to fail in about half of cases. Dengue virus, West Nile virus and yellow fever virus have a protein called NS4B that prevents interferon from functioning properly. It was thought that the tick-borne flaviviruses would use the same protein, so the NS5 finding was unexpected.


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