Some pathogens may adapt to cause less-severe disease and lower frequency of death in women than in men according to a study published in Nature Communications this week. Women can pass pathogens to their children during pregnancy, birth and breastfeeding, in addition to passing them to other individuals in the population the same way as men do. The research shows that the additional opportunities of transmission provided by women compared to men can exert sufficient evolutionary pressure on pathogens to drive the evolution of sex-specific virulence.
Researchers have assumed that differences in the severity of certain pathogen-borne diseases in men and women are due to stronger immune responses in women. Francisco Ubeda and Vincent Jansen provide an alternative explanation by showing that when pathogens can spread through additional transmission paths from women compared to men, adapting to cause less-severe disease in women is a successful evolutionary strategy for pathogens. The authors argue that their findings may explain variations in the progression of human T-lymphotropic virus 1 (HTLV-1) infection to adult T-cell leukaemia (ATL, a type of blood cancer) in men and women in different populations. For example, there is no difference in the frequency of progression of HTLV-1 infection to ATL between the two sexes in the Caribbean. However, progression of HTLV-1 infection to ATL is more frequent in men than women in Japan, where a higher proportion of mothers breastfeed their children, and do so for a more extended period, compared to women in the Caribbean.
The authors suggest that it makes evolutionary sense for the pathogen to cause less-severe disease in females if they provide more opportunities for transmission than males, making them a more-valuable host.