Tick-borne encephalitis

Tick-borne encephalitis (TBE), a zoonotic disease, occurring as a result of tick-borne encephalitis virus (TBEV) infection, is a recognised flavivirus encephalitis and an increasingly considered differential diagnosis globally ¹. 

Epidemiology 

Tick-borne encephalitis is endemic to Asian, eastern European, Baltic, and Nordic countries ². The number of confirmed tick-borne encephalitis cases in Europe specifically has increased by almost 400% in decades, with a reported annual incidence of 10,000-12,000 ^3,4. Infection rate peaks over the summer months, with higher risk associated with outdoor activity, particularly in woodland areas.

Clinical presentation 

Initial symptoms following infection with European tick-borne encephalitis virus subtypes are often biphasic in nature and will typically occur within 14 days. These include marked pyrexia, myalgia, fatigue and vomiting. Following a short asymptomatic period, the pyrexia returns with signs of central nervous system involvement. This may manifest as one or more of:

• meningitis

• encephalitis (~ 50% patients)

• myelitis

• radiculitis

Cranial nerve involvement has been reported ⁵. 

Less commonly, there is no asymptomatic period i.e. the infection is monophasic in presentation: this is frequently seen with the more aggressive Far Eastern subtype.

Pathology

Aetiology

Tick-borne encephalitis virus is in the mammalian tick-borne flavivirus group of Flaviviridae, which are single-stranded RNA viruses. There are three subtypes of tick-borne encephalitis viruses, the most virulent being the Far Eastern subtype ². 

Ticks of the Ixodidae family act as the vector transmitting tick-borne encephalitis virus to mammalian hosts ³. 

Radiographic features

Imaging is consistent with inflammation of the involved central nervous system anatomy, which is of broad aetiology. However, tick-borne encephalitis has a few anatomical predilections on MRI that may aid in developing a differential diagnosis, along with positive IgM and IgG serology ⁵: 

MRI

• T1: involvement of the meninges may be more pronounced in the cerebellar folia and the basal cisterns

• T2/FLAIR/DWI: ​focal or diffuse hyperintensities within the thalami, basal ganglia, cerebellum and anterior horns of the spinal cord

Of note, contrast medium enhancement is rare.

Treatment and prognosis

There is no treatment for tick-borne encephalitis and vaccination is encouraged. Although the risk of death is small, long term neurological sequelae - mainly gait disturbance - occur in approximately half of patients and may persist for several years ⁶.

Differential diagnosis

• flavivirus encephalitis of different flavivirus origin

• demyelinating disorders

  • multiple sclerosis

  • acute disseminated encephalomyelitis

• bacterial infection

• metabolic encephalopathy

• venous infarction

• stroke

• Creutzfeldt-Jakob disease

• Behçet disease