Man flu could be real, after all. It seems that viruses may actually be evolving to infect men more severely than they infect women. A new study, published in Nature Communications, suggests that there is an evolutionary pressure on the organisms to cause weaker symptoms in women, as it makes them more likely to be spread.
The theory goes that it is advantageous for a virus to infect, but not necessarily harm, a woman because she has a greater chance of passing the virus on, from mother to child. For men, however, that pressure is not generally there as there are fewer opportunities for the virus to be passed on, so the virus can go all out on the male host, and as a result cause greater harm.
“Viruses may be evolving to be less dangerous to women, looking to preserve the female population” explained Royal Holloway’s Dr Francisco Úbeda. “The reason why these illnesses are less virulent in women is that the virus wants to be passed from mother to child, either through breastfeeding, or just through giving birth.”
It has long been observed that men tend to suffer worse infections from diseases than women. For example, men infected with the human papillomavirus are a shocking five times more likely than women to develop cancer as a result. This has usually been explained away by the subtle differences in the immune systems between the two sexes, but it seems that there could be more going on.
To examine whether or not there would be an evolutionary pressure for a virus to do less harm to women than men, the researchers used mathematical modeling. They found that there was indeed selective pressure for this to occur, and for the organisms to produce a lower mortality rate in women if the virus can be passed from mother to child.
“Survival of the fittest is relevant to all organisms, not just animals and humans,” continued Dr Úbeda. “It's entirely probable that this sex-specific virulent behavior is happening to many other pathogens causing diseases. It's an excellent example of what evolutionary analysis can do for medicine.”
Fascinatingly, this push for viruses that are spread between mother and child could be what is driving the observed differences in the lethality of Human T-cell Lymphotropic Virus Type 1 (HTLV-1) in different parts of the world. In Japan, for example, HTLV-1 is up to 3.5 times more likely to progress to become the lethal Adult T-cell Leukaemia (ATL) in men than women, whereas, in the Caribbean, the likelihood of the virus progressing to ATL is equal between men and women. This could be down to the fact that in Japan, women breastfeed their children for a much longer period of time than they do in the Caribbean.
This latest research could have profound influences for how doctors treat illness and disease in the future, tailoring the response to the sex and location.