Influence of early passages on the character of freshly isolated strains of Tahyna virus.

The plaque size, thermostability, virulence, and average lethal time of uncloned freshly isolated strains of Tahyna virus were studied in the course of 3 passages in suckling mice. All of the 10 strains were in the 1st passage thermostable (indices 4 degrees/50 degrees C less than 1.0 log10) and highly virulent (indices ic/sc less than 1.0 log10). They differed significantly in plaque size only. The effect of 3 passages in suckling mice was manifested by titre increase and plaque size reduction. The differences in thermostability and virulence were too small as to be distinctly significant, but at least they indicated the heterogeneity of the strains. The variability in behaviour of uncloned strains and significance of changes observed during passages are discussed.