Abstract
Fatality rates of infectious diseases are often higher in men than women. Although this difference is often attributed to a stronger immune response in women, we show that differences in the transmission routes that the sexes provide can result in evolution favouring pathogens with sex-specific virulence. Because women can transmit pathogens during pregnancy, birth or breast-feeding, pathogens adapt, evolving lower virulence in women. This can resolve the long-standing puzzle on progression from Human T-cell Lymphotropic Virus Type 1 (HTLV-1) infection to lethal Adult T-cell Leukaemia (ATL); a progression that is more likely in Japanese men than women, while it is equally likely in Caribbean women and men. We argue that breastfeeding, being more prolonged in Japan than in the Caribbean, may have driven the difference in virulence between the two populations. Our finding signifies the importance of investigating the differences in genetic expression profile of pathogens in males and females.
Highlights
Fatality rates of infectious diseases are often higher in men than women
There is a limited literature on epidemiological models exploring the evolution of virulence in pathogens that are vertically transmitted only[26], showing that natural selection disfavours pathogens that are vertically transmitted only if they exhibit any degree of virulence[26,27]
Pathogens are expected to evolve a degree of male virulence equal to that of pathogens in a population without vertical transmission and a degree of female virulence lower than that of pathogens in a population without vertical transmission (Fig. 3)
Summary
Fatality rates of infectious diseases are often higher in men than women. this difference is often attributed to a stronger immune response in women, we show that differences in the transmission routes that the sexes provide can result in evolution favouring pathogens with sex-specific virulence. We will explore which factors lead to selection for differential exploitation strategies in pathogens residing in female and male hosts, and focus on the evolutionary implications of the different routes of transmission provided by each sex to the pathogens that they host While both sexes are vectors of transmission to another individual in the same population (horizontal transmission), transmission from parent to offspring through pregnancy, birth or breast-feeding (maternal vertical transmission; vertical transmission) is limited to women. We explore whether this fundamental biological difference alone can drive the evolution of sex-specific virulence in the pathogen.We advance evolutionary theory by formulating and solving analytically an extension of the classical susceptibleinfected-recovered individuals (SIR) model[23] that incorporates vertical and horizontal transmission between male and female hosts. We apply our results to explain the observed differences in virulence between men and women infected with HTLV-1 in two endemic regions: Japan and the Caribbean
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