Sitara Brooj Akbar (3rd year BSc Biomedical Science student)
Background to the disease:
Tick-borne encephalitis (TBE) virus, a fast-emerging health threat that is spreading in many parts of the world, has been recently detected in ticks in the United Kingdom. The growing number of TBE cases reported could be attributed to a combination of socio-economical, ecological and geographic factors as well as the increasing number of possible hosts (Kaiser, 2008).
TBE is an infection of the central nervous system (CNS) caused by tick-borne encephalitis virus, a single-stranded RNA virus that belongs to genus Flavivirus in the family Flaviviridae. Three virus sub-types also exist: European TBE virus (transmitted by Ixodes ricinus), Siberian TBE virus (transmitted by I.persulcatus), and Far-Eastern TBE virus (Riccardi et al., 2019).
The infection often manifests as either meningitis, encephalitis, or meningoencephalitis, and is an important cause of CNS infection that can result in long-term neurological problems and even death.
How is it Transmitted?
Ticks act as both the vector and virus reservoir for the diseases; small rodents are the primary amplifying host. Moreover, the virus can have a wide range of possible hosts including birds, horses and humans. The virus can be spread from animals to humans with ruminants and dogs providing the most common source of infection in humans; and can also be acquired by consuming unpasteurized dairy products, e.g. milk and cheese, from infected cows. TBE virus transmission has also been reported through laboratory exposure, slaughtering of animals and from person-to-person spread through blood transfusion, solid organ transplantation, or breastfeeding (ECDC, 2019).
What are the signs and symptoms of infection?
TBE has an average incubation period of 8 days (range, 4–28 days) after an infected tick bites and introduces the virus in a human. Non-specific symptoms are mild fever, stiff neck, discomfort, headache, nausea, vomiting and muscle ache, which may be present at primary manifestation of the disease and spontaneously resolve within 1 week. After another week the patient may start to develop neurological symptoms due to the virus infecting the brain and the CNS (encephalitis), the meninges (meningitis) or both (meningoencephalitis). On average, the mortality due to a TBE infection is between 1% to 2%, with deaths occurring around 7 days after the onset of neurological signs.
Disease severity and long term outcome varies by age (increasing over time with less severe infection in children), and TBE virus subtype. Thus, the European subtype is associated with milder infections, with a fatality ration of <2% as compared to the Far Eastern subtype, which has more severe results with a fatality ratio of 20%-40%; although disparity in the severity of the infection by virus subtypes might be attributed to patient’s factors, including access to suitable medical care or age-specific exposure.
In animals, the infection symptoms include lethargy, refusing to eat and development of respiratory signs. Neurological disorders might also develop with signs varying from tremors to seizures and death.
How TBE is diagnosed?
TBE diagnosis mainly relies on clinical suspicion based on typical signs and is confirmed by serological and molecular assays on serum for specific IgM and IgG antibodies. In more complicated cases, such as infection after having been vaccinated, testing of cerebrospinal fluid might be necessary in supporting the diagnosis (CDC, 2019).
TBE virus’s RNA can be also detected in serum samples using virus isolation or reverse transcription polymerase chain reaction. However, by the time the neurologic symptoms are recognized, the viral RNA is usually undetectable, thus these procedures cannot be used to rule out TBE diagnosis. Clinicians should contact their state or local health department for confirmation in case of a suspected case (NHS, 2019).
Are there any treatments available?
To date, no specific antiviral treatment is available for TBE. Therefore, supportive treatment as well as intensive care in a hospital setting is the main procedure for treatment. Because of limited options for treatment, vaccination against the disease is of crucial importance to prevent TBE-related mortality.
How can we prevent it?
Prevention includes non-specific methods such tick-bite prevention, tick checks, using biocides/repellents and wearing long sleeved clothes when outside, as well as specific prevention in the form of vaccination (The Encephalitis Society, 2019).
References
CDC (2019). Tick-borne Encephalitis (TBE) | CDC. Available at: https://www.cdc.gov/vhf/tbe/index.html [Accessed 31 Oct. 2019].
European Centre for Disease Prevention and Control. (2019). Factsheet about tick-borne encephalitis (TBE). Available at: https://www.ecdc.europa.eu/en/tick-borne-encephalitis/facts/factsheet [Accessed 31 Oct. 2019].
Kaiser, R. (2008). Tick-Borne Encephalitis. Infectious Disease Clinics of North America, 22(3), pp.561-575.
NHS. (2019). Tick-borne encephalitis (TBE). Available at: https://www.nhs.uk/conditions/tick-borne-encephalitis/ [Accessed 31 Oct. 2019].
Riccardi, N., Antonello, R., Luzzati, R., Zajkowska, J., Di Bella, S. and Giacobbe, D. (2019). Tick-borne encephalitis in Europe: a brief update on epidemiology, diagnosis, prevention, and treatment. European Journal of Internal Medicine, 62, pp.1-6.
The Encephalitis Society. (2019). What is encephalitis? Available at: https://www.encephalitis.info/what-is-encephalitis [Accessed 31 Oct. 2019].
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