Thermal inactivation studies with variola virus.

In recent years, considerable interest has been manifested in the thermal inactivation of animal viruses (Friedman and De Berry, 1959; LehmannGrube and Syverton, 1959; Youngner, 1957; Woese, 1960). Apart from the practical aspects, studies of the temperature sensitivity of virus particles offer a natural step in the analysis of their physicochemical properties (Gard and Maal0e, 1959) and, as such, may enhance our knowledge of the component parts of virus, namely, protein and nucleic acid. The reaction characterizing the thermal inactivation of animal viruses usually follows first-order kinetics, but in some cases it has been found that the inactivation process proceeds in a two-component fashion (Bachrach et al., 1957; Kaplan, 1958). The characteristic features *of animal virus thermal inactivations has been summarized by Woese (1960); the kinetics of the process at low temperatures involve one component; at high temperatures two components are manifested, each of first-order, with differentvalues for the heat of activation of the processes. Studies with vaccinia virus, however, indicate that the shape of the inactivation curve at high temperatures may be of either one or two components, depending on the nature of the virus preparation or condition of virus storage prior to heating (Woodroofe, 1960). It appears, therefore, that the complexity of thermal inactivation of animal viruses is compounded further by certain pretreatment conditions of the virus. Although variola virus is antigenically related to vaccinia virus, and may be assayed by a comparable, quantitative method (the inoculation of the chorioallantoic membrane of embryonated eggs), little is known of the reaction of variola virus to heat inactivation. This report describes an attempt to characterize the thermal inactivation of variola virus and to determine the influence of certain factors on the reaction, i.e., suspending media, dilution of virus, and condition of virus storage. MATERIALS AND METHODS