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Essentials of Diagnosis
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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
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General Considerations
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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
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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
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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