Replication of two porcine parvovirus isolates at non-permissive temperatures

Previous studies have shown that replication in vitro of the porcine parvovirus (PPV) isolate, KBSH, was restricted at 39 degrees C but not at 37 degrees C. In contrast, replication of the Kresse isolate was restricted at 37 degrees C but not at 39 degrees C. In this study, Kresse and KBSH isolates were passaged up to ten times in swine testicle (ST) cells at non-permissive temperatures, and at subsequent passage viral protein synthesis, viral DNA synthesis, and progeny virus were evaluated. KBSH became adapted for replication at 39 degrees C upon serial passages, displaying an appreciable increase in viral progeny, viral polypeptides, and viral DNA concentration. This finding was also observed with Kresse virus isolate continuously passaged at 37 degrees C. Neither isolate became adapted for replication at 32 degrees C. In an attempt to examine the effect of in vitro passage at non-permissive temperatures on pathogenicity in swine, KBSH passaged 10 times either at 37 degrees C or 39 degrees C was inoculated into swine fetuses. Two of four fetuses inoculated with 39 degrees C-passaged KBSH were dead and hemorrhagic or mummified. All four fetuses inoculated with 39 degrees C-KBSH contained viral antigen and viral DNA. In contrast, fetuses inoculated with 37 degrees C-passaged KBSH, or with cell culture fluid were normal in appearance. Viral antigen and viral DNA were not demonstrated in fetuses inoculated with 37 degrees C-KBSH or cell culture fluids. These findings suggest the possibility that the ability to replicate at 39 degrees C is associated with virulence in swine fetuses.