Stages of the Disease
Incubation period: The incubation period, in most cases, is between 7 and 14 days prior to the onset of the first stage – but may last from 2 to 28 days.
First stage: On average the first stage lasts 2 to 4 days (durations of 1 to 8 days have rarely been observed), and corresponds with the viremic phase. It is associated with uncharacteristic flu-like symptoms, with temperature rising to 38°C in most cases. Sometimes an exceptionally high initial temperatures may occur.
Asymptomatic interval: An asymptomatic interval lasting about 8 days (extreme values between 1 and 20 days have been observed) follows the first stage of TBE. During this period, patients are usually without symptoms.
Second stage: About 2 to 4 weeks following infection, one third of patients passes into the second stage of the disease, which is characterized by CNS involvement. The clinical picture is that of meningitis, encephalitis, meningoencephalomyelitis or meningoencephaloradiculitis. The mortality rate in adult patients in Europe is about 1% and increases to about 3% in patients with a severe course of TBE including meningoencephalitis, meningoencephalomyelitis, and dysfunction of the autonomic nervous system. The patients have temperatures that are often higher than temperatures associated with other forms of viral meningitis or meningoencephalitis.
The main symptoms of meningitis are a severe headache, nausea and retching, nuchal rigidity, and high fever . Special attention should be paid to the lack of meningeal symptoms in 10% of patients diagnosed with TBE. Lack of meningeal signs in the course of TBE does not exclude serious neurological complications.
Encephalitis is characterized by disturbances of consciousness ranging from somnolence to sopor and, in rare cases, coma. Other symptoms include restlessness, hyperkinesia of muscles of limbs and face, lingual tremor, convulsions, vertigo, and speech disorders.
When cranial nerves are involved, mainly ocular, facial, and pharyngeal muscles are affected. In some cases, neuropsychiatric symptoms prevail, and the patient is sent to a psychiatric ward.
Symptoms of polyradiculitis may occur five to ten days after the remission of fever; they are usually accompanied by a paresis of the shoulder girdle. Paralysis may progress up to two weeks, followed by a moderate tendency of improvement.
The greatest extent of neurological abnormalities of TBE is found in the meningoencephalomyelitic manifestation of the disease, which is primarily characterized by flaccid paresis of the extremities. Since TBE viruses have a particular predilection for anterior horn cells of the cervical spinal cord, paresis usually affects upper limbs, shoulder girdle and/or head levator muscles. Mono-, para- and tetra paresis may develop including paresis of the respiratory muscles. This clinical form of TBE closely resembles polio virus infection, however, compared with poliomyelitis, paresis in TBE tends to have a proximal distribution, and more often affects the upper than the lower extremities. If the lesion spreads to the lower portion of the brain stem, and particularly to the medulla oblongata, bulbar syndrome may develop with the risk of sudden death due to respiratory failure or circulation disturbances. However, bulbar syndrome can be observed in the meningoencephalitic form of TBE, without association of myelitis. On all occasions this is the sign of an adverse prognosis.
Symptoms of polyradiculitis may occur five to ten-days after the remission of fever; they are usually accompanied by a paresis of the shoulder girdle. Paralysis may progress up to two weeks, followed by a moderate tendency of improvement.
Cases with paresis due to myelitis only have a slight tendency to regression, and are generally followed by pronounced muscle atrophy. In a german study all of 15 patients with myelitis were left with residual paresis.
Post-encephalitic syndrome: A post-meningoencephalitic syndrome may occur after TBE. It impairs the quality of life, causes high costs for the health care system with long periods of hospitalization, inability to work, as well as long-lasting neurological symptoms and social distress of the patients. Although severe manifestations usually subside after 1 to 3 weeks, TBE may cause long-lasting, mainly cognitive CNS dysfunction and the convalescence period may be very long.
According to a literature review by M. Haglund and G. Günther in 2003, the incidence of sequelae following TBE varies between 35 and 58%. In Austria, 10–20% of patients with a severe course of TBE have been reported to develop long-term or permanent neuro-psychiatric sequelae – such as a severe headache, dizziness, lack of concentration, depression, disorders of the autonomic nervous system, hearing impairment and mood disorders. Next in frequency are residual pareses and atrophies in 3–11%. The pareses usually tend to remit, but in rare cases muscular atrophies may persist. In some patients a spasmophilic tendency has been observed for as long as four years following TBE infection.
The post-encephalitic syndrome causes major expenses both to the health care system and society as long-term working disability ensues. Moreover, the long-lasting sequelae have a substantial impact on the patients’quality of life.
TBE with hemorrhagic syndrome: 8 fatal cases of tick-borne encephalitis with an unusual hemorrhagic syndrome were identified in 1999 in the Novosibirsk Region. Hemorrhagic fever was associated with the Far-Eastern TBEV subtype.
Immune Response: A TBEV infection confers life-long protection and there is no known human case of symptomatic re-infection.