Tick-borne encephalitis (TBE) is a viral infectious disease involving the central nervous system. It may manifest as meningoencephalitis or a mild illness with fever. Over the past decades, TBE has become a growing public health concern in Europe and Asia and is the most important viral tick-borne disease in Central and Eastern Europe as well as Russia.
There are three forms of the disease related to the virus subtypes - namely, European, Far Eastern and Siberian.
TBE has never been endemic in the UK. There have been a handful of cases reported, all in people who have travelled to one of the 27 European countries where it is known to be endemic.
The countries with areas most affected by TBE are: Austria, Belarus, China, Croatia, the Czech Republic, Estonia, Germany, Hungary, Japan, Latvia, Lithuania, Poland, Siberia, Slovakia, Slovenia, Switzerland, Russia and Ukraine.
It is present at a lower frequency in Denmark, France, Italy, Liechtenstein, Norway, Finland and Sweden.
It is also probably present in Albania, Bosnia and Herzegovina, Bulgaria, Greece, Kazakhstan, Montenegro, Moldova and Romania and Serbia.
Up-to-date information on areas of risk is available on the travel websites in "Further reading & references", below.
TBE has become a growing concern over the past decades and incidence is increasing. Worldwide, there are estimated to be 10,000-15,000 new cases per year.It is probably under-reported, but is notifiable in 17 European countries. Incidence is thought to be increasing, as a result of:
- Increasing recreational activity in areas where ticks are prevalent.
- Improvements in diagnosis and case reporting.
- Climate changes.
TBE is most prevalent in the warmer months. Men are affected more frequently than women. Around 10-20% of cases are in children.
TBE is caused by tick-borne encephalitis virus, a member of the Flaviviridae family of viruses that can affect the nervous system.
- Ticks act as both the vector and reservoir for TBE virus.
- The virus is transmitted by 11 different species of ticks, most commonly:
- Ixodes ricinus (transmits most cases of European TBE).
- Ixodes persulcatus.
- The main hosts are small rodents, along with deer, sheep and goats. Migratory birds can also carry infected ticks.
- Human infection usually occurs as a result of tick bites. However, the virus can also be transmitted through drinking unpasteurised milk from infected animals.
- TBE cases occur during the highest period of tick activity, ie between April and November.
- Person-to-person transmission has not been reported.
In endemic areas, people with recreational or occupational exposure to rural or outdoor settings are potentially at risk:
- Forest workers
Only a small proportion of those infected develop clinical symptoms. Between 70-98% of infections are thought to be asymptomatic. The incubation period of TBE can be from 2 to 28 days (usually between 7 and 14 days) and is asymptomatic. Shorter incubation times have been reported after milk-related exposure (3-4 days).
- In most cases of the European subtype, there is a characteristic biphasic febrile illness, with an initial phase that lasts 2 to 4 days - the viraemic phase.
- It is nonspecific with symptoms that may include:
- Muscle aches
- Nausea and/or vomiting
- After 8 days of remission the second phase of the disease occurs and involves the central nervous system with symptoms of meningitis or encephalitis or meningoencephalitis. For example:
- Meningitis: high fever, headache, nausea, vomiting, photophobia, vertigo.
- Encephalitis: impaired consciousness (drowsiness to stupor or coma), personality and behavioural changes, concentration and cognitive disturbances, tongue fasciculations, tremor of extremities, focal or generalised seizures, delirium and psychosis.
- Meningoencephalomyelitis: muscle pains, flaccid paresis, respiratory paralysis.
- Rates of serious neurological deficit are estimated to be around 10% and death occurs in less than 2%.
- A small proportion have a monophasic illness, which usually has central nervous system involvement.
The course of infection with the Far Eastern variety clinically differs from the European form:
- The onset of illness is more often gradual than acute, with a prodromal phase including fever, headache, anorexia, nausea, vomiting and photophobia.
- These symptoms are followed by a stiff neck, sensory changes, visual disturbances and other neurological deficits.
- It is associated with higher rates of severe neurological sequelae and death (20-40% fatality rate).
There is little information available on presentation and course of the Siberian subtype. Fatality rates are around 2-3%.
During the first phase of the disease the most common laboratory abnormalities are as follows:
- 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:
- Mildly increased white blood cells in the blood and the cerebrospinal fluid (CSF).
- Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are normal in the majority but may be elevated.
- The virus is no longer detectable in the blood.
Diagnosis usually depends on detection of specific IgM and IgG antibodies in either blood or CSF, usually appearing during the second phase of the disease. Testing is done at the Rare and Imported Pathogens Laboratory at Public Health England (PHE) Porton.
There is no specific drug therapy for TBE.
- Meningitis, encephalitis or meningoencephalitis require hospital supportive care based on syndrome severity.
- Antipyretics, analgesia, antiemetics and maintenance of fluid-electrolyte balance may be required.
- Management of convulsions may be required.
- Intubation and ventilatory support may be necessary where there is neuromuscular paralysis and respiratory failure.
- Intravenous mannitol and/or corticosteroids are used where there is cerebral oedema and rising intracranial pressure, although the evidence base is poor.
- The majority of symptomatic cases in Europe experience central nervous system involvement.
- Long-term neurological sequelae are common - 40-50% of cases, usually paresis, ataxia or gait disturbance. In 10% of cases, neurological deficits are severe.
- Complications are usually more severe and can be more frequent in the Far Eastern form.
- A chronic progressive form has been identified in Siberia and Russia but is not thought to exist elsewhere.
- In general, mortality is around:
- 1-2% for the European form.
- 20-40% for the Far Eastern form.
- 2-3% for the Siberian form.
- As above, the risk of long-term morbidity is high.
- People aged over 60 years are most at risk of death.
- In Europe, the disease course is usually milder in children with a better outcome.
- Unpasteurised milk should not be drunk in areas at risk.
- TBE can be prevented by using insect repellents and protective clothing to prevent tick bites. Long sleeves and long trousers tucked into socks should be worn in tick-infested areas. Light-coloured clothing is advisable as it is easier to see the ticks against it.
- 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 as close to the skin attachment as possible, by steady pulling without jerking or twisting.
- Inactivated vaccines are available in the UK for the protection of those individuals at high risk of exposure to the virus, through travel or employment.
There is only one licensed vaccine available in the UK: TicoVac® or TicoVac Junior®, (known in some countries as FSME-IMMUN® or FSME-IMMUN Junior®).
The TBE vaccination is recommended for:
- Those travelling to warm, forested parts of the endemic areas, particularly in the spring and summer, where ticks are most prevalent.
- Individuals who hike, camp, hunt and undertake fieldwork in endemic forested areas.
- Those who will be going to reside in an area where TBE is endemic or epidemic, particularly those working in forestry, woodcutting, farming and the military.
- The first dose is given on day 0.
- The second dose is given one to three months after the first dose.
- The third dose is given five to twelve months after the second dose.
- For rapid short-term protection of children and adults, the second dose may be given two weeks after the first dose and gives at least 90% protection.
- A booster dose should be given every three years if there is continued risk.
- The dose is halved for children aged from 1 year to below 16 years. They should receive Junior TicoVac®.
- Protection rate is estimated at 90% after the second dose.
- Rates of seroconversion are high, close to 100% after the third vaccination.
- It can be given at the same time as other inactive vaccines. They should be given at different sites, with different syringes.
- It can be administered at the same time as live vaccines.
- A confirmed anaphylactic reaction to a previous dose of the TBE vaccine or one of its components.
- A confirmed anaphylactic reaction to egg.
- Pregnancy or breast-feeding are not contra-indications to this vaccine.
- Local reactions to the vaccine, such as swelling, pain and redness at the site of injection, can occur.
- Some people develop a fever, particularly after the first dose. This is most common within twelve hours of having the vaccine and usually settles within 24-48 hours.
- Following a successful nationwide vaccination campaign in Austria, the annual number of TBE cases fell to about 10% of those reported in the pre-vaccination era.It is estimated that 4,000 cases were prevented by vaccination between 2000 and 2011 in Austria.
Further reading and references
; National Travel Health Network and Centre (NaTHNaC)
; Why is tick-borne encephalitis increasing? A review of the key factors causing the increasing incidence of human TBE in Sweden. Parasit Vectors. 2012 Aug 315:184. doi: 10.1186/1756-3305-5-184.
; Tick-borne encephalitis - a notifiable disease, a review after one year: report of the 16th Annual Meeting of the International Scientific Working Group on Tick-Borne Encephalitis (ISW-TBE). Ticks Tick Borne Dis. 2014 Sep5(5):453-6. doi: 10.1016/j.ttbdis.2014.05.001. Epub 2014 May 27.
; Epidemiology of tick-borne encephalitis (TBE) in Europe and its prevention by available vaccines. Hum Vaccin Immunother. 2013 May9(5):1163-71. doi: 10.4161/hv.23802. Epub 2013 Feb 1.
; Public Health England
; Tick-borne encephalitis: A review of epidemiology, clinical characteristics, and management. World J Clin Cases. 2015 May 163(5):430-41. doi: 10.12998/wjcc.v3.i5.430.
; Effect of climate change on vector-borne disease risk in the UK. Lancet Infect Dis. 2015 Mar 20. pii: S1473-3099(15)70091-5. doi: 10.1016/S1473-3099(15)70091-5.
; Tick-borne encephalitis: Clinical findings and prognosis in adults. Wien Med Wochenschr. 2012 Jun162(11-12):239-43. doi: 10.1007/s10354-012-0105-0. Epub 2012 Jun 14.
; Public Health England (April 2013)
; NICE Evidence Services (UK access only)
; The current perspective on tick-borne encephalitis awareness and prevention in Vaccine. 2011 Jun 2029(28):4556-64. Epub 2011 May 5.
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