Printed on: September 16th, 2024

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Tick-borne encephalitis

Tick-borne encephalitis

This article was updated in May 2024.

Tick-borne encephalitis is a viral illness usually transmitted through the bite of an infected Ixodes tick. It is a flavivirus, related to strains that can cause yellow fever, dengue fever and Japanese encephalitis. There are now five subtypes:

• European (central, eastern and northern Europe).
• Siberian (Urals, Siberia, far-eastern Russia, some areas in NE Europe).
• Far Eastern (far-eastern Russia, China, Japan).
• Baikalian (East Siberia).
• Himalayan (Qinghai-Tibet plateau).

The one that we are most likely to be concerned with is the European subtype, which is associated with milder disease (Emerg Infect Dis 2020;26:90, Green Book, accessed June 2024).

Since 2019, there have been small numbers of confirmed cases of tick-borne encephalitis in the UK, with evidence that the virus is spreading in geographical distribution. The virus is endemic in rural and forested areas of Europe and Asia. The risk of infection in the UK remains very low, but is rising.

This information comes from the Travel Health Pro website, and the latest guidance from the UKHSA (Green Book Chapter 31; gov.uk – tick-borne encephalitis: epidemiology, diagnosis and prevention) and the European Centre for Disease Control, all accessed May 2024.  

In humans, tick-borne encephalitis can also be acquired by consumption of unpasteurised milk or milk products from infected animals, although this is very rare. It is not directly transmitted from person to person.

• Two-thirds of infections are asymptomatic.
• The incubation period is 2–28 days (average 7 days).
• If symptomatic, the illness is often biphasic:
  • Initial viraemic phase lasting around 5 days (fatigue, muscle aches, temperature and headache).
  • Asymptomatic interval (around 7 days, range 1–33 days).
  • Second symptomatic phase in 20–30% of those with the European subtype. This leads to severe neurological problems in 10% and overall mortality in 0.5–2%. The second stage usually starts with a sudden rise in temperature. The following features may also develop:

10y follow-up data of patients who develop meningoencephalomyelitis suggests that 80% have long-term issues, and this risk is greater in those aged >60y or with comorbidities, including immune-compromise.

Fortunately for us in the UK, the European subtype has the lowest rate of developing the second phase of disease, with the Far Eastern subtype having the highest rate. Mortality is around 1% for the European form, but 5–20% for the Far Eastern subtype.

Testing for infection is done by the UKHSA laboratory at Porton Down. If infection is suspected, we should speak to the on-call UKHSA clinician for advice prior to testing.

Tick-borne encephalitis is a notifiable disease and should be reported to the UK Health Security Agency (UKHSA) if suspected.

The reporting of tick-borne encephalitis across Europe is variable, meaning that it is difficult to quantify risk. In simple terms:

• Anyone who is planning to go walking, hiking, camping or working in endemic areas is at risk.
• Risks are highest in spring, summer and autumn (peak walking months, then!).

Publicity about UK cases, combined with the popularity of walking holidays, may prompt an increase in patients seeking advice.

Just as we do for patients going to malarial areas, we should give good advice about bite avoidance:

• Dawn-to-dusk protection.
• Long sleeves/trousers.
• Avoid heavily-infested areas of forests (not too sure how you find that out!).
• Adequate use of insect repellent.

After walking/hiking:

• Check the body for ticks.
• Remove ticks using fine-toothed tweezers/tick remover (see article on Lyme disease: NICE guideline to learn how to do this).

• Avoid unpasteurised dairy products in endemic areas.

Vaccination for travel or work within the UK is not currently recommended.

The Green Book recommends the vaccine for:

• Individuals who hike, camp, hunt or undertake fieldwork in spring or summer in endemic areas.
• Those living in, or moving into, endemic areas, particularly those who work in forestry, farming and the military.

The interactive map at Travel Health Pro will help identify areas of risk.

One licensed vaccine (Tico-Vac) is available in the UK, with a junior version for children. It contains the Neudorfl virus strain, which is effective against the European strain and probably against the Far Eastern strain. It is an inactivated vaccine that is cultured on chick fibroblasts.

Administration is in the upper arm or anterolateral thigh. It can be given at the same time as other travel vaccines, but a different limb should be used.

Adverse reactions are rare; the usual local reactions following injection may occur. Any suspected serious adverse reactions should be reported through the Yellow Card Scheme. 

A booster dose is recommended every 3 years after the initial 3-dose course if the individual continues to be at risk.

No, it doesn’t. There is currently no vaccine against Lyme disease, although research is ongoing.

	Tick-borne encephalitis Consider discussing and offering vaccination to individuals going on walking holidays in high-risk areas. Vaccination is not currently recommended for walking holidays in the UK.
	Useful resources: Websites (all resources are hyperlinked for ease of use in Red Whale Knowledge) Travel Health Pro – countries (interactive map of risk areas)