VIRUS PARTICLES AND THEIR FUNCTIONAL ACTIVITY 351 



somewliat surprising average ratio of less than one particle (0.67) per pock by 

 titration on chick embryo chorioallantoic membranes. Also Dumbell et al. 

 (1957) estimated elementary body per infectious unit ratios for vaccinia and 

 variants of cowpox viruses. Particle counts in this instance were made by the 

 spray droplet technique. They observed average ratios of approximately 100 

 and 20 for the cowpox variants assayed for infectivity on chorioallantoic 

 membrane and in rabbits, respectively. Those for vaccinia virus were found 

 to approximate 18 particles per pock or ID50, indicating no detectable 

 difference in susceptibility between the two hosts used for assay. Again it 

 might be postulated that inherent differences in the virus-host systems 

 employed account for the disparate results obtained m the two laboratories. 

 However, Overman and Tamm's (1956b) finding that infectivity titer exceeds 

 total count suggests that technical difficulties may also have been responsible 

 in part. The latter workers made particle counts using impure preparations in 

 which debris may have obscured elementary bodies in the electron micro- 

 graph; thus their estimate of physical particle numbers may have been low. 



3. Myxoviruses 



Although this group includes the mumps, Newcastle disease, fowl plague, 

 and influenza viruses, only the results with influenza virus will be presented 

 briefly, since it has been the most extensively studied. Influenza virus, m 

 common with the other myxoviruses, possesses the property of hemagglutina- 

 tion as well as infectivity for developing chick embryos. Both properties are 

 associated with the virus particle yet can be studied independently. Thus it 

 has been possible to investigate simultaneously the quantitative relationship 

 between actual particle count and two functional activities of the virus 

 particles. 



Infectivity titers are generally expressed as the number of ID5Q per unit 

 volume for mice or eggs, principally the latter. Hemagglutinin titers are 

 usually determined by the pattern test and are expressed as the dilution of 

 virus present at the partial agglutination end point. The conditions for 

 carrying out this titration may vary from one laboratory to the next. Isaacs 

 (1957) described the partial agglutination end point as that pattern of 

 agglutinated cells intermediate between complete agglutination and absence 

 of agglutination and defines an agglutinating dose as, on the average, the 

 amount of virus present at the partial agglutination end point in a pattern 

 test using 0.25 ml. of a 1 % suspension of chick red cells (approximately 

 lO''-^^ cells). For greater precision a 50 % hemagglutination end point may be 

 deternuned by a photoelectric densitometer as described by Miller and 

 Stanley (1944), but the results are not readily compared with the pattern 

 test end point. 

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