Tick-Borne Encephalitis (TBE)

Definition, Etiology, PathogenesisTop

Tick-borne encephalitis (TBE) is a viral central nervous system (CNS) infection that usually follows a biphasic course.

1. Etiology: Tick-borne encephalitis virus (TBEV), a neurotropic virus that is a member of the Flaviviridae family. TBEV is divided into European, Siberian, and Far Eastern (also known as Russian spring-summer) subtypes. Two additional subtypes—Himalayan and Baikalian—are less well reported and described.

2. Reservoir and transmission: Small rodents and Ixodes ticks (transmitting the infection to next generations). The virus is usually transmitted via a bite of an infected tick, and rarely via oral route by consumption of unpasteurized milk from infected animals (this may cause small-scale epidemics). The incidence of tick-borne diseases varies geographically and follows seasonable patterns that depend on the activity of ticks, which feed on human and animal blood and are active from spring until late fall.

3. Epidemiology: The disease is found in endemic areas across large regions of Europe (central and eastern) and Asia (southern Russia, northern Kazakhstan, China, Mongolia, and Japan). Epidemiology is changing with climate changes, with TBE being reported, for example, in Scandinavia. Readers are encouraged to seek information regarding local epidemiology.

4. Risk factors: Longer stays or work in forests in endemic areas, consumption of unpasteurized milk from animals kept in endemic areas.

5. Incubation and contagious period: Incubation period is usually from 7 to 14 days (occasionally up to 4 weeks). There have been no documented cases of human-to-human transmission.

Clinical FeaturesTop

Most human infections are asymptomatic by a ratio of 1:100. When symptoms do occur, there is a high risk of CNS involvement, which may include meningitis, encephalitis, or myelopathy.

The European subtype causes acute disease and follows a biphasic course with 5 to 7 days of no symptoms in between the 2 phases. The Siberian and Far Eastern subtypes are more likely to be monophasic.

1. Prodromal phase: Influenza-like symptoms, nausea, vomiting, and diarrhea persist for up to 7 days and in the majority of patients are followed by spontaneous recovery. In some patients features of CNS involvement appear after a few days of remission.

2. CNS infection phase: Meningitis (the most common manifestation; usually mild), encephalitis, cerebellitis, or myelitis. Most patients recover completely. The Siberian subtype is associated with a higher risk of chronic neurologic symptoms.

DiagnosisTop

Features of viral infection found on cerebrospinal fluid examination (CSF) and presence of specific serum IgM (enzyme-linked immunosorbent assay [ELISA]). To confirm diagnosis in uncertain cases, measure specific IgM levels in CSF.

TreatmentTop

Antiviral treatment is not available. General recommendations and symptomatic treatment are the same as in other viral CNS infections.

PrognosisTop

In the majority of patients the recovery is complete. In patients with encephalitis and myelitis, sensory disturbances, paresis, and impairment of memory and concentration may persist for several months. Paresis is usually accompanied by muscle atrophy. The mortality rate is ~1% in the European type and it is higher in the Siberian and especially Far Eastern types (5%-10%; the disease is fatal predominantly in patients with paralysis of limbs and respiratory compromise).

PreventionTop

1. Nonspecific protection against ticks: see Lyme Borreliosis.

2. Vaccination (see Vaccines: Tick-Borne Encephalitis).