Viruses may have evolved to be more deadly to men than women

Researchers believe this is because women provide more opportunities for transmission.

A study released today in Nature Communications has found that pathogens may adapt over time to have a less severe effect on women than men, in order to be passed on from mother to child. We talked to corresponding author Francisco Ubeda from Royal Holloway University, to find out more. 

ResearchGate: Could you explain the main findings of your study?

Francesco Ubeda: The main finding is that some pathogens – those transmitted not only between individuals, but also from mother to child – may adapt to cause less severe disease and lower frequency of death in women than in men. 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.

RG: Why is it that pathogens may cause less severe disease and mortality in women in comparison to men?

Ubeda: Pathogens that can be transmitted from mother to child "value" the life of women hosts relatively more than men hosts because women provide an additional route for their transmission.

RG: What caused you to look into this?

Ubeda: We were surprised that all potential explanations to the observed differences in virulence between men and women were centered on the patient, and that the pathogen had largely been ignored. We took the "pathogen's eye view" and researched whether natural selection would favor a different behavior in each sex.

RG: Could you briefly explain the method of your study?

Ubeda: We formulated an epidemiological mathematical model for transmission of pathogens between men and women. Then we analyzed the model to determine the strategy that would be favored in each sex. Finally, we applied our results to the analysis of the virus.

RG: Could you provide an example of a virus that has evolved to affect the sexes differently?

Ubeda: The case study we discussed in our paper is Human T-cell Lymphotropic Virus Type 1 (HTLV-1) which is focalized in Japan, the Caribbean and West Africa. Infected individuals can develop Adult T-cell Leukaemia (ATL), which is lethal, being the virus that causes leukaemia. HTLV-1 is passed on by sexual transmission and from mother to child during the lactating period. HTLV-1 is between two and 3.5 times more likely to cause leukaemia in Japanese men than women. In the Caribbean, however, the likelihood of HTLV-1 progressing to leukaemia is roughly equal in men and women. This could be because a higher proportion of Japanese women breastfeed their children, and for longer, when compared to women in the Caribbean. This provides the disease more of  chance to be passed on to children.

RG: You mention that men and women often also react to illness differently. Could you explain this in more detail?

Ubeda: Men and women react to infection differently, and this has been attributed to differences in the behavior of their respective immune systems. Hormonal differences and sex-specific genes are responsible for these differences.

RG: What are the next steps in this research?

Ubeda: Ideally, the next step would be researching what triggers the differential behavior of viruses in each sex.

Image courtesy of Sanofi Pasteur.