A US study on mice suggests the existence of significant gender differences in susceptibility to infection after repeated exercise stress. Specifically, it seems that, although males and females have similar sickness rates after exercise stress and exposure to infection, females are less likely to die as a result.

Previous human studies have provided support for the hypothesis that intense exercise increases the risk of upper respiratory tract infection while moderate activity reduces it. And the researchers have demonstrated in a previous animal study that fatiguing exercise increases susceptibility to respiratory infection after viral inoculation in male mice.

The current study was set up to test the effects of gender on susceptibility to infection. To this end, 86 male and 89 female mice were randomly assigned to exercise (treadmill running to fatigue on three consecutive days) or control groups, after which they were inoculated through the nose with a standard dose of herpes simplex virus type 1 (HSV-1 – a cause of respiratory infection in mice), then monitored for 21 days.

Key findings were as follows:

• Exercise stress resulted in greater morbidity than the control condition (66% v 48%) with no observed difference between the sexes;
• Male mice experienced significantly more deaths than females (28% v 16%), even though the female mice tended to have more severe symptoms;
• Run time to fatigue was significantly longer in females than in males.

The researchers have called for further research to tease out possible explanations for these effects, which may include differences in immune responses, possibly resulting from hormonal differences.

In the meantime they conclude that ‘females generally are less susceptible to the lethal effects of intranasal HSV-1 infection than males’.

Med Sci Sports Exerc, vol 36, no 8, pp 1290-1295