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 dairy products

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

• Therapy is largely supportive

• Prevention: avoid tick exposure, pasteurize dairy products, and vaccinate

General Considerations

• Occurs predominantly in the late spring through fall

• Usually a consequence of exposure to infected ticks

• Unpasteurized dairy products from viremic livestock are also a recognized form of transmission

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

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

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

• There are three main subtypes

  • European subtype, transmitted by Ixodes ricinus

  • Siberian subtypes, transmitted by Ixodes persulcatus

  • Far Eastern subtypes, transmitted by I persulcatus

• Dermacentor reticulatus is another vector

• Powassan virus

  • Only North American member of the tick-borne encephalitides, reported in northeastern and north central United States as well as Canada and Russia

  • Vector is several Ixodes species of ticks

  • Incubation period can range from 1 to 5 weeks

  • Most reported cases are neuroinvasive

Demographics

• Virus is endemic in certain parts of Europe and Asia

• The number of cases reported annually (thought to be a gross underestimate) 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 7–10 (reported extreme range: 4–28) 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

• Acute myositis is rare but can be associated with severe disease

• Alimentary infection

  • Occurs within 3–4 days of ingesting unpasteurized milk and dairy products

  • Is reported throughout Central Europe

  • Occurs within families or in small outbreaks (where the clinical attack rate is 14%)

  • Is more commonly (38.9% incidence) biphasic with frequent non-severe meningoencephalitis

  • Has a high probability of recovery

• The post-encephalitic syndrome

  • Characterized by

    • Headache

    • Difficulties concentrating

    • Balance disorders

    • Dysphasia

    • Hearing defects

    • Chronic fatigue

  • A progressive motor neuron disease and partial continuous epilepsy ...