THE INFLUENCE OF COLD ON VIRUS INFECTIVITY 

 Dr. T. G. Metcalf 



Department of Bacteriology 



University of New Hampshire 



Durham, New Hampshire 



ABSTRACT 



This study was on the characteristics of influenza A2 strains which might play a 

 role in facilitating virus invasion of host cells, and the effect of cold upon the invasive 

 process. Strains of influenza A2 selected for study were isolated from fatal cases of 

 influenza in humans. The strains were examined for mouse toxicity following intra- 

 venous injection, cytotoxicity in HeLa and L- cells, mouse and chick embryo IDg^ values, 

 and neuraminidase activity. Virus enzyme action was singled out for special attention 

 on the basis of its constant association with A2 strains. The enzyme activity was de- 

 termined (1) by means of thiobarbituric acid analysis for free neuraminic acid, and 

 (2) by reduction of the hemagglutination titer of mucoid inhibitor. Substrates used 

 included neuraminmucoid from edible birds nest; neuraminlactose from bovine colostrum, 

 and ovomucin from hens eggs. The rate and extent of enzyme action exhibited by virus 

 showed a progressive decline as the temperature was lowered from 37° C to 4° C. 

 A comparison of enzyme activity, virus titer, and inhibitor concentration in chick em- 

 bryos at 37° C and 20° C showed corresponding decreases in enzyme activity and virus 

 titer while inhibitor remained virtually unchanged at lower temperatures. Infection 

 of mice was followed by extensive lung damage and death at 4° C. Considerably less 

 damage and fewer fatalities within the test period were found at 20° C. Contrary to 

 the chick embryo experience, the enzyme activity and virus titer showed increases at 

 the lower temperature. Cell monolayers of monkey or hamster kidney were used in 

 conjunction with fluorochrome analysis to follow the course of cell invasion by virus 

 at 37° C and 20° C. Virus was first demonstrated in the cytoplsism around the nuclear 

 membrane at 6 hours following incubation at 37° C. Virus presence in the cytoplasm 

 was shown for at least 72 hours. The course of invasion at 20° C was different. Virus 

 was first detected after 10 hours. It appeared in the cytoplasm, but failed to show a 

 significant increase in numbers. It was possible to show virus accumulating at the per- 

 iphery of allantoic membranes within 12 hours when embryos were incubated at 37° C. 

 No virus accumulation was observed in membranes from embryos incubated at 20° C. 

 Single-caged mice pre-exposed and maintained at 4° C showed massive virus invasion 

 of lung and bronchi, while group-caged mice showed only minimal virus invasion. 



The effect of cold upon the course of influenza virus infections 

 represents a special problem in host-parasite relationships. Is the 

 primary influence of temperature a factor altering the biology of a 

 host, or is it directed against the virus? Which is of greater impor- 



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