Encephalitis, Tick-Borne

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

  • Most reported cases are neuroinvasive

• Alkhurma hemorrhagic fever

  • A flavivirus first uncovered in Jeddah, Saudi Arabia in 1995

  • Reemerging in the Middle East with occurrences in tourists to Egypt, Djibouti, and possibly India

  • Its extent of geographic distribution is currently unknown

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