TEMPERATURE AND VIRAL INFECTION 



The effect of environmental temperature on viral infection has 

 appeared to be of some practical importance in Australia in the 

 evolution of myxomatosis in wild rabbits. Marshall (1959) noted that 

 there was repeated suggestion that myxomatosis spreading naturally 

 through wild rabbits was more lethal in winter than in summer, and 

 Mykytowycz (19 56) observed that rabbits experimentally infected with 

 an attenuated strain and housed in unheated quarters had a higher 

 mortality rate in winter than in summer. To test the possibility that 

 these observations were related to ambient temperature, Marshall 

 exposed inoculated rabbits to fluctuating cold (-1° C to +1° C for 16 

 hours and 15° C for 8 hours each day) and compared the results with 

 those at normal room temperatures (20° C to 22° C) and at elevated 

 temperatures (37°C to 39^0 for 16 hours and 26° C for 8 hours each 

 day). These fluctuating temperatures were chosen to simulate day 

 and night fluctuations of winter and summer. He found that ambient 

 temperature had little effect on infections with a highly virulent 

 strain of myxoma virus or with the quite virulent rabbit pox. But if 

 rabbits were inoculated with an attenuated strain of myxoma virus 

 that at 22° C caused death of about 60 per cent of rabbits, then ex- 

 posure to cold increased the mortality to over 90 per cent and ex- 

 posure to heat reduced mortality to about 30 per cent (Fig. 3), Myxo- 

 matosis in a rabbit is a verydistinctivedisease, and comparison of 

 the signs of disease in the rabbits at the various temperatures, as 

 well as differences in the levels of virus in the blood, were fairly 

 convincing indications that the differences in mortality rate were 

 due to alterations of the extent and severity of infection instead of to 

 some other effect of the temperatures. 



Somewhat similarly, Sulkin (1945) has shown that after inoculation 

 wath influenza A virus, mice maintained at 15,5° C have significantly 

 more pulmonary consolidation and mice at 35° C less consolidation 

 than do mice held at 21° C to 25° C, And I judge from the abstracts 

 of Drs. Metcalf and Marcus that they will be providing additional 

 data pertinent to this question. 



Can Exposure to Cold Activate a Latent Infection? 



Latent here means an inapparent infection that exhibits chronicity 

 and some degree of host-virus equilibrium. Although this is a par- 



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