Definition, Etiology, PathogenesisTop
Tick-borne encephalitis is a viral central nervous system (CNS) infection following a biphasic course.
1. Etiology: Tick-borne encephalitis virus, a neurotropic virus that is a member of the Flaviviridae family.
2. Reservoir and transmission: Small rodents and 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).
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). The patient is not infectious for contacts.
The disease has a sudden onset and follows a biphasic course.
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.
Features of viral infection found on cerebrospinal fluid examination (CSF) (see Analysis of Cerebrospinal Fluid) and presence of specific serum IgM (enzyme-linked immunosorbent assay [ELISA]). To confirm the diagnosis in uncertain cases, measure specific IgM levels in CSF.
Antiviral treatment is not available. General recommendations and symptomatic treatment are the same as in other viral CNS infections.
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% (the disease is fatal predominantly in patients with paralysis of limbs and respiratory compromise).
1. Nonspecific protection against ticks: see Lyme Borreliosis.
2. Vaccination: see Immunoprophylaxis of Infectious Diseases in Adults.