Up to 2 out of 100 people infected with TBE die.
TBE in Adults
- The majority of tick-borne encephalitis virus (TBEV) infections are asymptomatic, but it remains difficult to ascertain the proportion who are asymptomatic due to underdiagnosis.
- It is estimated that 1 in 3 of infected persons develop symptoms for TBE.
- The incubation period of TBE is usually 7–14 days (range: 2–28 days), with a shorter incubation period of 3–4 days reported for alimentary transmission.
- Case-fatality rates of greater than 20% are reported, although the severity of the disease appears to be associated with the viral subtype involved:
- ≥20% for the Far-Eastern subtype
- 6–8% for the Siberian subtype
- 1–2% for the European subtype
- In ≤40% of encephalitic cases, the disease results in permanent central nervous system (CNS) sequelae, including various neuropsychiatric and cognitive complaints characteristic of postencephalitic syndrome.
- In fatal cases, characteristic neuropathological changes include polioencephalomyelitis.
For the EU TBEV subtype there are two distinct phases:
The initial disease phase may last 1-8 days and may include:
The initial phase is followed by an asymptomatic interval that usually lasts for about 1 week (range: 1–21 days).
Second disease phase-symptoms can occur in 20-30% of patients and can include:
- Meningitis typically manifests with high fever, headache, nausea and vomiting; many patients have photophobia, and some vertigo.
- Encephalitis may be manifested by impaired consciousness ranging from somnolence to stupor and, in rare cases, coma.
- Patients aged over ~40 years are more likely to develop the encephalitic form of the disease. Risk factors include age, comorbidities and immunosuppression.
- In patients aged over 60 years, TBE increasingly takes a severe course, leading to paralysis and the highest case-fatality rate.