Characterization of the sickness response in young and aging rats following E. coli infection.

To more fully characterize the sickness response in young (3 mo) and older (24 mo) rats, we measured core body temperature (CBT), activity level, and body weight changes for 7 days following a peripheral immune challenge with Escherichia coli. CBT increases were delayed and blunted during the 12h following infection in older rats. Indeed, in aging subjects the initial response was hypothermia, but this was followed by a significant and prolonged elevation in CBT lasting 3 days. Young rats, in contrast, generated a rapid and robust CBT elevation lasting just over a day. Activity level was significantly reduced only on the day of E. coli administration in both young and older rats. Body weight loss was equivalent in both age groups one day after E. coli administration, although there was a trend for older rats to continue losing more weight across the next 6 days than in young rats. This is the first study to examine CBTs in young and older rats for a protracted amount of time, thereby revealing that aging rats do have an exaggerated, albeit delayed, fever which is in keeping with other exaggerated sickness behavioral responses observed in aging rodents.