A high degree of DNA sequence homology between the different THV isolates has been detected using DNA/DNA hybridization, heteroduplex mapping of the viral DNA molecules, and generation of intratypic recombinant viruses. This indicates the close genetic relationship between the individual THV strains. The seven isolates of THV are genetically grouped into five strains according to DNA fragmentation patterns. THV strains 1,2,3 and 5 comprise isolates 1,2, 3 and 7 respectively, whereas isolates 4, 5 and 6 together represent THV strain 4. However, minor differences between the individual members of THV strain 4 are detectable in respect of slight size variations of specific restriction fragments.
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