Effect of temperature on the infectivity and propagation of western equine encephalitis virus

The propagation of WEE virus variants L.P. #7 and S.P. #6 infected chick embryo cells maintained as monolayer and in suspension was studied at 25° and 37°C. The results may be summarized as follows: 1. The lag-period of the growth cycle for L.P. #6 and S.P. #7 was found to be 10 hours when the infected cells were maintained as a monolayer or in suspension and incubated at 25°C. The lag period for S.P. #6 infected cells in suspension and as a monolayer incubated at 37° was found to be 2 hours while that for L.P. #7 was 1 ½ hours. 2. The maximum number of PFU in both variants present in the extracellular fluid was reached after 72 hours of incubation of infected monolayers at 25°C. With S.P, #6 infected cells in suspension the maximum titer of S.P. #6 PFU was attained after 48 hours of incubation at 25°C while with L.P. #7 infected cells the maximum PFU was reached after only 36 hours of incubation at 25°C. S.P. #6 infected cell in suspension, however, released a larger number of PFU than did L.P. #7 infected cells. The reverse was true with cells maintained as a monolayer. 3. The maximum number of PFU released into the extracellular fluid by S.P. #6 and L.P. #7 infected cells in suspension or maintained as a monolayer occurred after 10 hours of incubation at 37°C. The highest titer was produced by cell infected with the large plaque variant. 4. On the basis of infective center assay and counts of replicate non-infected monolayers the number of PFU released per infected cell was determined. An experiment was designed to study the cytopathic changes that develop in WEE virus infected chick embryo cells incubated at 25°C and 37°C. the results of this experiment may be summarized as follows: 1. Cytopathogenicity was first observed in L.P. #7 infected cells after 8 hours of incubation at 37°C. S.P. #6 infected cell did not show definite signs of cytopathic changes until after 10 hours of incubation at 37°C. 2. Infected cells incubated at 25°C developed observable cytopathogenicity after 24 hours of incubation. 3. Comparisons were made with the development of cytopathogenicity and the propagation of the two variants at the two experimental temperatures.