Thermal Sensitivity of Polioviruses Isolated during Oral Vaccine Field Trial: Comparison with Monkey Neurovirulence

Summary 1. Virulent and attenuated strains of the 3 types of poliovirus have been shown to have different patterns of growth at high (39°C) temperatures when tested by means of agar overlay tissue culture technic. Attenuated strains (T marker) are inhibited and have a low EOP at this temperature, while virulent strains (T+) grow equally well at 35°, 37°, and 39°C. Some strains give an intermediate (I) pattern. 2. The thermal marker T-T+ was used to screen poliovirus strains isolated during a type I oral vaccine field trial, in an effort to determine: a) whether reversion to greater virulence had occurred as a result of human passage; and b) whether strains isolated from local flies were the vaccine virus which had been orally administered; or were naturally occurring wild strains. The results indicated that virus excreted by 2 of the 3 infected children consistently had thermal markers identical with that of the vaccine virus; one child, on the other hand, excreted virus which over a period of 29 days showed a shift to the thermal marker pattern of more virulent strains. Neurovirulence for monkeys was low for all strains isolated from children. One T strain isolated from flies had an EOP at 39°C in the same range as the LSc vaccine strain, while the 14 other fly strains tested all fell between this range and that of virulent strains. It was concluded that most of the fly strains recovered in the course of the field trial probably represented naturally occurring wild strains. 3. Comparison was made between the thermal (T-T+) marker, the EOP at low and high bicarbonate concentrations (d-d+ marker), and neurovirulence as tested by intracerebral inoculation of monkeys. The thermal marker correlated better with monkey virulence than did the d-d+ marker. 4. Use of the thermal in vitro marker should be of some practical value in determining degree of spread of attenuated poliovirus strains introduced into a community, and in assessing degree of stability of vaccine strains on human passage.