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Key Features

Essentials of Diagnosis

  • Flaviviral encephalitis found in Eastern, Central, and occasionally Northern Europe and Asia

  • Transmitted via ticks or ingestion of unpasteurized milk

  • Long-term neurologic sequelae occur in 2–25% of cases

General Considerations

  • Occurs predominantly in the late spring through fall

  • Usually a consequence of exposure to infected ticks

  • Unpasteurized milk from viremic livestock is also a recognized form of transmission

  • Transmission by transplantation of solid organs is reported leading to fatal outcomes

  • The principal reservoirs for tick-borne encephalitis (TBE) virus are ticks with small rodents as amplifying host; humans are an accidental host

  • Incubation period for tick-borne exposures is 7–14 days but only 3–4 days for milk ingestion

  • There are three subtypes

    • European subtype, transmitted by I ricinus

    • Siberian subtypes, transmitted by I persulcatus

    • Far Eastern subtypes, transmitted by I persulcatus

  • Powassan virus

    • Only North American member of the tick-borne encephalitis

    • Vector is several Ixodes species ticks

    • Incubation period can range from 1 to 5 weeks

Demographics

  • Virus is endemic in certain parts of Europe and Asia

  • The number of cases reported annually fluctuates significantly depending on surveillance, human activities, socioeconomic factors, ecology, and climate

Clinical Findings

Symptoms and Signs

  • Most cases are subclinical

  • Many cases resemble a flu-like syndrome with 2–10 days of fever (usually with malaise, headache, and myalgias)

  • In some cases, the disease is biphasic where the initial flu-like period is followed by a 1- to 21-day symptom-free interval followed by a second phase with fevers and neurologic symptoms

  • Neurologic manifestations range from febrile headache to aseptic meningitis and encephalitis with or without myelitis (preferentially of the cervical anterior horn) and spinal paralysis (usually flaccid)

  • A myeloradiculitic form can also develop but is less common

  • Peripheral facial palsies, sometimes bilateral, tend to occur infrequently late in the course of infection, usually after encephalitis and usually are associated with a favorable outcome within 30–90 days

  • Mortality in TBE is usually a consequence of brain edema or bulbar involvement

  • European subtype is usually milder with up to 2% mortality and 30% neuroinvasive disease

  • Siberian subtype is associated with 3% mortality and chronic, progressive disease

  • Far Eastern subtype is usually more severe with up to 40% mortality and higher likelihood of neurologic involvement

  • All three subtypes are more severe among the elderly and usually less severe among children

  • Coinfection with Borrelia burgdorferi (the agent of Lyme disease; transmitted by the same tick vector) may result in more severe disease

Differential Diagnosis

  • Other causes of aseptic meningitis such as enteroviral infections, poliomyelitis (no longer reported from Eastern Europe), herpes simplex encephalitis

  • Tularemia, the rickettsial diseases, babesiosis, Lyme disease, and other flaviviral infections

  • Coinfections are documented with Anaplasma, Babesia, and Borrelia

Diagnosis

Laboratory Findings

  • No ...

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