COLD AND VIRUS INFECTIVITY 



hours cjdioplasmic inclusions were observed. From 72 to 96 hours 

 the infected cell showed extensive cytopathogenic change. The spe- 

 cificity of the stain was controlled by the use of RN'ase which made 

 it possible to distinguish the presence of nonspecific staining. 



The sequence of events in the invasion of cell monolayers did not 

 differ noticeably at 37° C or 20° C. The presence of virus, its intra- 

 cytoplasmic growth, cytopathology, and cell destruction, could not 

 be differentiated with certainty at the two temperatures. 



Examination of allantoic membranes was accomplished by means 

 of paraffin sections prepared after the method of Coons and Sainte- 

 Marie (1960). Three to five micron sections were cut on a rotary 

 microtome and stained with acridine orange. Virus growth at 37° C 

 was shown to occur predominantly in the entodermal cells lining the 

 allantoic cavity, although some virus staining material was found in 

 the ectoderm. Infected membranes kept at 20° C showed practically 

 no virus staining material in the entodermal cells. 



When sections of mouse lung were examined, the extent of virus 

 invasion was shown. A section of non- infected mouse lung failed to 

 show virus- staining material. Sections of lungs from mice infected 

 at 20° C showed virus-staining material gathered around the bronchi 

 as well as scattered throughout the lung. Sections from infected 

 mice of the 4° C group showed a more massive accumulation of 

 virus- staining material. Greater concentrations of virus appeared 

 immediately adjacent to the bronchi. 



The results of the fluorochrome examinations furnished visual 

 evidence that the neuraminidase- active strains of influenza used in 

 the study found their way into host cells of chick embryos, monkey 

 kidney monolayers and mice. The initial stage of cell invasion was 

 not affected by the degree of enzyme activity associated with the A2 

 strains. Thus, strains of high enzyme activity could not be shown to 

 possess a penetration advantage over strains of low enzyme activity. 



359 



