Egyed et al.
Analysing the genome of two tick-borne encephalitis viruses isolated in Hungary in 1952 and 2019
Ticks Tick Borne Dis. 2021;12(6):101806. doi: 10.1016/j.ttbdis.2021.101806

In 1952, a TBE virus strain has been isolated in northern Hungary from questing ticks. This strain, called KEM-1, was subsequently used continuously as a positive control in immunofluorescence and haemagglutinin assays for several decades. During these 67 years, the strain KEM-1 was passaged hundreds of times in tissue culture and in suckling mice. Strain KEM-1 was closely related to Finnish (Kumlinge) and Russian (Absettarov) strains. The authors consider that this strain may have been imported by migrating birds.

In 2019, a TBE strain was isolated by flagging in an area in central Hungary where a fatal human milk-borne TBE virus infection occurred. This strain was called KEM-195.

Both strains were sequenced and belong to the European subtype. Strain KEM-1 was very similar to other strains from Hungary (isolated in western Hungary), which had already been sequenced. Strain KEM-195 differed significantly in the polyprotein region from the initial laboratory strain KEM-1. Strain KEM-195 was closely related to Slovenian, Slovakian, Czech and Austrian strains.

During the 67 years and many laboratory passages, strain KEM-1 remained very stable with only a few silent point mutations. This is in accordance with results of other studies showing that TBE virus has a stable genome. The virus must achieve continuous persistence both in (mostly) mammalian hosts and in ticks, and it cannot afford frequent genetic changes.

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