Attenuation of foot-and-mouth disease virus by chemical means

Susceptibility of various FMDV strains to urea

Resistance to urea in vitro at 37 degrees C varied for each FMDV strain analysed. The urea marker did not correlate with other markers such as resistance to acid, resistance to acidity or size of plaques under agar on BHK21/13 cells. The resistance to urea of subtypes A24 Cruzeiro, O1 Caseros and C3 Resende varied in accordance with their antigenic potency when administered to swine as a trivalent water-in-oil emulsion type vaccine.

Rabies viruses increase in virulence when propagated in neuroblastoma cell culture

Several strains of attenuated rabies virus lacking the capacity to kill adult mice acquired a high lethal potential for mice after one to five serial passages in murine or human neuroblastoma cells. The virulence acquired after passage in neuroblastoma cells is a stable genetic trait retained during subsequent passage of viruses in nonneuroblastoma cell systems.

Isolation of temperature-sensitive conditional lethal mutants of "fixed" rabies virus

In an attempt to induce temperature-sensitive (ts) conditional lethal mutants of rabies virus, stocks of a plaque-purified substrain of strain CVS fixed rabies virus were subjected to mutagenesis by HNO(2), 5-fluorouracil, or 5-azacytidine. It was necessary to prepare virus stocks from clones of mutagenized virus selected at random and to test subsequently each stock for possible ts characteristics by measuring its relative capacity for growth at permissive (33 C) and nonpermissive (40.5 C) temperatures. Five ts mutants were detected in tests of 161 clones of mutagenized virus. Each of the mutants exhibited a remarkably low incidence of reversion and little demonstrable "leakiness." One of the five ts mutants (ts2), which formed formed very small plaques, and another (ts1), which formed plaques of only slightly reduced size, were further characterized. Virus ts1 was more thermostable at 40.5 C than the parental virus, but the ts2 mutant was unchanged in this respect. The ts1 virus exhibited normal pathogenicity for mice, but ts2 virus caused a very irregular death pattern. Both deaths and survivors immune to rabies virus challenge were noted in all groups of mice inoculated with ts2 virus, regardless of the virus dose.