Tick-borne encephalitis natural history, complications and prognosis

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Ilan Dock, B.S.

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Natural history

• Two thirds of infected individuals are asymptomatic and will not display any clinical manifestations.
• Incubation period will last an average of 8 days. However incubation periods have been shown to range from 4-28 days.
• Characteristic biphasic course:

1. Early phase:

• • Disease will begin with the onset of nonspecific febrile illness accompanied by a headache, myalgia, and fatigue.
  • The early phase of the biphasic course will commonly occur over the course of several days.
  • Following common symptoms of the early infection phase, patient may display an afebrile and relatively asymptomatic period.
  • Nearly two-thirds of patients have been reported to have recovered without any further illness, following the completion of the first phase.

1. Late phase:

• • As the disease progresses the onset of a second phase may result in central nervous system involvement including aseptic meningitis, encephalitis, or myelitis.
  • Further findings within the progression of tick borne-encephilitis include meningeal signs, altered mental status, cognitive dysfunction, ataxia, rigidity, seizures, tremors, cranial nerve palsies, and limb paresis.

Complications

• The majority of complications associated with tick-borne encephilitis are commonly developed during the second phase. Complication may include:
  • Aseptic meningitis
  • Encephalitis
  • Myelitis
  • Meningeal signs
  • Altered mental status
  • Cognitive dysfunction
  • Ataxia
  • Rigidity
  • Seizures
  • Tremors
  • Cranial nerve palsies
  • Limb paresis

Prognosis

• The prognosis is usually good for two-thirds of individuals diagnosed with tick-borne encephilitis. These patient will remain asymptomatic for the duration of the infection.
• More severe cases have been associated with ages about 50 years as well as young children. Though severity in young children is less often reported than in elderly populations.
• Prognosis also depends on subtype. European subtypes are commonly associated with lesser to mild symptoms while Far Eastern subtypes are associated with more severe cases and a case fatality ratio of 20-40%.
• In humans, the disease is lethal in approximately 1.2% of cases and leaves 15-20% of its survivors with permanent neurological damage.