Signs and Symptoms
The incubation period of TBE is usually between 7 and 14 days and is asymptomatic. Shorter incubation times have been reported after milk-borne exposure.
In contrast to Far-eastern TBE, European TBE is more severe in adults than in children where meningitis is more frequently observed.
In approximately two-thirds of patients infected with the European TBE virus, only an early (viremic) phase is experienced; symptoms are nonspecific and may include fever, malaise, anorexia, muscle aches, headache, nausea, and/or vomiting. After about 8 days of remission, a second phase of disease occurs in 20% to 30% of patients. These patients may experience a clinical illness that involves the central nervous system with symptoms of meningitis (e.g., fever, headache, and a stiff neck), encephalitis (e.g., drowsiness, confusion, sensory disturbances, and/or motor abnormalities such as paralysis), or meningoencephalitis.
The convalescent or recovery period can be long and the incidence of sequelae may vary between 30% and 60%, with long-term or even permanent neurologic symptoms. Neuropsychiatric sequelae have been reported in 10-20% of patients.
The range of clinical manifestations can be observed following infection by any of the TBE virus subtypes. Biphasic symptomatology (fever then neurological disorders) is frequent after infection with the European or Western TBE subtype. Infections by the Far-eastern TBE subtype are generally more severe and the case-fatality rate is higher. The Siberian subtype in children could be responsible for chronic encephalitis.
In general, mortality is rare, about 1% to 2%, with deaths occurring 5 to 7 days after the onset of neurologic signs in European TBE. During Far-eastern TBE, signs and symptoms are more severe and mortality is higher (5-20%).