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

• Therapy is largely supportive

• Prevention: avoid tick exposure, pasteurize milk, and vaccinate

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 hosts; 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 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 of tick-borne encephalitis viruses

• 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 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

• 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 are complications

  • Longstanding psychiatric complications are reported and include

    • Attention deficits

    • Slowness of thought and learning impairment

    • Depression

    • Lability

    • Mutism

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 ...