INACTIVATION OF VIRUSES 401 



This, however, could hardly account for the reduction in the inactivation rate 

 of 80 % in 9 minutes that was recorded in one of the experiments presented. 

 Therefore, it must be concluded that Keogh's data show a deviation from the 

 course of a first-order reaction similar to that found in other viruses, the 

 explanation of which is not immediately clear. 



4. Myxovirus 



Lauffer and Wheatley (1949) studied inactivation of PR8 influenza A 

 virus and concluded that the course probably was that of a first-order reac- 

 tion. On this assumption the energy of activation was estimated at about 

 20,000 cal./mole. In the pH range of 5-6 to 8 increments in log inactivation 

 rate were slightly below those in log formaldehyde concentration; in the same 

 range the rate of inactivation was clearly pH-dependent, increasing with pH. 

 Below pH 5-6, which is close to the isoelectric point, the virus was increas- 

 ingly unstable and the presence of formaldehyde added little or nothing to the 

 rate of normal heat inactivation at the temperatures tested (25-40°C.). The 

 authors concluded that formaldehyde acted over the nonionized form of an 

 ionizable basic group and that inactivation was the result of a single event. 



Inactivation studies on influenza virus are complicated by the fact that 

 noninfective virus may still retain the capacity of interference. In order to 

 eliminate interference in infectivity assays, one has to work with compara- 

 tively high dilutions of the test material, thereby narrowing the range of 

 activity accessible for observations. If work over a wider range is called for, 

 and higher concentrations for that reason have to be included, it is important 

 that primarily negative results in infectivity tests be checked in subpassages. 

 In the case of interference, the virus content in primary cultures may stay 

 well below the level detectable by means of haemagglutination and the 

 presence of active virus may become manifest only after further passages. 

 With this fact in mind, Uhler and Gard (Gard, 1957) studied inactivation of 

 influenza A virus, using tissue cultures for assay of residual infectivity. Under 

 these conditions a systematic deviation from the first-order course was 

 observed. 



5. Poliovirus 



Salk et al. (1954) described poliovirus inactivation as a first-order reaction. 

 When, after initiation of commercial formol-vaccine production, it became 

 evident that deviations from a first-order course occurred, an explanation 

 was sought in the formation of aggregates and precipitates presumably 

 shielding the virus from the action of the chemical. As a means of ensuring 

 regularity of the process, filtration through Seitz pads was recommended 

 (U.S. Teclmical Committee, 1955, 1957). 



