Inactivation of foot-and-mouth disease virus vaccine strains by activation of virus-associated endonuclease.

A new inactivation process for foot-and-mouth disease virus (FMDV) has been developed. This process is based on the activation of the FMDV endonuclease by incubation of unfractionated viral suspension or purified virions at 37 degrees C in the presence of high concentrations of monovalent cations such as K+, Cs+ or NH4+ at pH 8.5. This procedure completely inactivated several FMDV vaccine strains yielding preparations having similar amounts of 140S particles to untreated controls. The inactivation followed first-order kinetics and the rate of inactivation was faster than that achieved with other agents, e.g. binary ethyleneimine. Testing in suckling mice or tissue culture revealed no residual infectivity after inactivation. Virus particles purified from inactivated preparations showed (i) the same sedimentation coefficient as non-inactivated preparations, (ii) electrophoretic patterns of their viral capsid proteins identical to those derived from non-inactivated preparations, and (iii) extensive degradation of the 35S viral RNA. This method is safer than inactivation with aziridines because only innocuous chemicals are used in the process.

Comparisons of cell-free and cell-associated Marek's disease vaccines in maternally immune chicks.

Cell-free and cell-associated Marek's disease vaccines prepared from the TK/A isolate of the herpesvirus of turkeys (HVT) were compared to evaluate their relative effectiveness in protecting chicks with homologous maternal antibody. The influence of early challenge on protection was also investigated. Although the higher susceptibility of cell-free HVT to neutralising antibody could be demonstrated in vitro, no significant difference between the two types of vaccine could be established in vivo using chicks with maternal antibody. Chicks exposed to challenge immediately or only a few hours after vaccination were not adequately protected. A higher level of protection was observed when challenge was separated from vaccination by an interval of several days. The importance of proper vaccination procedure, vaccine virus titre and adequate management of vaccinated stock, with particular reference to the risks of early challenge are discussed.

Serological relationship between a pathogenic strain of Marek's disease virus, its attenuated derivative and herpes virus of turkeys.

Precipitating antigens present in extracts of chick embryo cells infected with the HPRS-16 attenuated strain of Marek's disease virus (att-MDV) were separated by gel filtration on Sephadex G200 and some of their properties determined. The two main antigens detected with convalescent MD serum, referred to as 'B' and 'C' antigens, had mobilities of 0-55 and 0-25 respectively relative to phenol red on electrophoresis in 7-5% acrylamide gel. The B antigen was relatively stable and of low mol. wt. in comparison with the C antigen. B and C antigens were in some instances also detected in culture medium of infected cells, but were distinguishable from the A antigen, a major glycoprotein antigen released into the culture medium of cells infected with HPRS-16. The results of immunodiffusion studies suggest that B antigen is common to MDV and strains of herpes virus of turkeys(HVT) and that at least 2 antigens (including C) are MDV specific. The A antigen was also common to MDV and HVT strains. It was noted however that the capacity of HPRS-16/att to synthesize A antigen was considerably reduced in comparison with HPRS-16 and HVT strains, and in some preparations the A antigen could not be detected. Evidence was also obtained for the presence of HVT-specific antigens associated mainly with the cell fraction.