With few exceptions, the medically important arboviruses are transmitted by either mosquitoes or by ixodid ticks. Humans are in most cases a dead end host for the virus, and are therefore not necessary for the continuing survival of the virus in nature. In the temperate Palaearctic region, there are two types of encephalitides transmitted among wild animals by Ixodes ticks. The central European or western tickborne encephalitis (TBE) is transmitted by I. ricinus and occurs in scattered foci in Europe and western Russia. The eastern form is called Russian spring-summer encephalitis (RSSE), is transmitted by I.
persulcatus and occurs from eastern Europe to the Far East and into northeastern China. Most infected people get a subclinical infection. In typical clinical cases, the disease has a bi-phasic course. After an incubation period of about 10 days (after the tick bite) influenza-like symptoms appear, which last for a few days. But after about a week with few or no symptoms the patient's
condition worsens. Signs and symptoms of meningoencephalomyelitis may develop, including fever, neck rigidity, and headache, sometimes with paralyses of one or more extremities. The second phase of the disease usually lasts for 2-3 weeks. Fatigue and remaining signs of paralysis may persist for a year or more. The RSSE type is, in general, more severe than the TBE type. Also, the symptoms are more severe in the elderly than in children. Effective vaccines are available.
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