Tick Borne Viral Encephalitis

Tick-borne encephalitis, (TBE) is a viral infectious disease involving the central nervous system. It may manifest as meningo-encephalitis or a mild illness with fever.
There are three forms of the disease; European, Far Eastern and Siberian. The European subtype has a milder course than the Far Eastern, which has a longer course and higher mortality rate.

TBE is caused by tick-borne encephalitis virus, a member of the family Flaviviridae.

Risk Factors

In endemic areas people with recreational or occupational exposure to rural or outdoor settings are potentially at risk:

• Farmers
• Hunters
• Campers
• Forest workers

Infection also may follow consumption of raw milk from goats, sheep, or cows.

• Ticks act as both the vector and reservoir for TBE virus.
• The main hosts are small rodents, with humans being accidental hosts.
• TBE cases occur during the highest period of tick activity i.e. between April and November. Sometimes, in warmer southern areas, this period may be extended.
• Person-to-person transmission has not been reported.

The incubation period of TBE is usually between 7 and 14 days and is asymptomatic. Shorter incubation times have been reported after milk-related exposure.

• A characteristic biphasic febrile illness follows, with an initial phase that lasts 2 to 4 days - the viraemic phase.
• It is non-specific with symptoms that may include:
  • Fever
  • Malaise
  • Anorexia
  • Muscle aches
  • Headache
  • Nausea and/or vomiting
• After 8 days of remission, the second phase of the disease occurs in 20 to 30% of patients and involves the central nervous system with symptoms of meningitis or encephalitis or meningoencephalitis.

TBE is more severe in adults than in children.

During the first phase of the disease, the most common laboratory abnormalities are:

• Leukopenia
• Thrombocytopenia
• Liver enzymes in the serum may also be mildly elevated
• Virus can be isolated from the blood

After the onset of neurological disease during the second phase:

• Increased white blood cells in the blood and the cerebrospinal fluid (CSF) are usually found.

Diagnosis usually depends on detection of specific IgM in either blood or CSF, usually appearing during the second phase of the disease.

There is no specific drug therapy for TBE.

• If bitten by a tick medical advice should be sought locally, as specific immunoglobulin may be available and advised within 48 hours. However its efficacy has been questioned. (Immunoglobulin is unlicensed in the UK but can be obtained on a named doctor/named patient basis where it is believed to be beneficial.)
• Meningitis, encephalitis, or meningoencephalitis require hospital supportive care based on syndrome severity.
• Anti-inflammatory drugs, such as corticosteroids, may be considered under specific circumstances for symptomatic relief.
• Intubation and ventilatory support may be necessary.

• Only the early phase is seen in approximately 66% of patients.
• The remaining third of patients experience either the typical biphasic course of the disease or a clinical illness that begins with the neurological symptoms.
• The recovery can be long and the incidence of sequelae may vary between 30 and 60%, with long-term or even permanent neurological symptoms.
• Neuropsychiatric sequelae have been reported in 10-20% of patients.⁸

TBE is rarely fatal.
In general, mortality is 1% to 5%, with deaths occurring 5 to 7 days after the onset of neurological signs.⁹

• TBE can be prevented by using insect repellents and protective clothing to prevent tick bites.
  • Those in tick areas should check their skin for attached ticks, which is easier to do with a partner.
  • Ticks should be removed as soon as possible with tweezers (or fingers covered by tissue paper if no tweezers are available) as close to the skin attachment as possible, by steady pulling without jerking or twisting.
  • Only one to two per cent of ticks are likely to be infected, although up to ten per cent may be.
• Inactivated vaccines are available in the UK for those considered at risk.¹⁰
  • Ideally immunisation should be completed at least a month before travel. It is considered to be effective against all strains of the disease.
  • Should be offered to pregnant women at risk, as it provides protection to unvaccinated newborns, who may have a particularly severe disease course.¹¹
  • Adverse reactions including tenderness and swelling at the injection site with regional lymph gland swelling have been reported, although experience with the vaccine is limited in the UK.