Some viruses are more likely to kill men than women. That’s the finding from a 2016 study in the journal Nature Communications. Researchers from the University of London looked at how oral Human Papilloma Virus (HPV) infections were more likely to turn into terminal throat cancer in “Japanese men than women, while it is equally likely in Caribbean women and men,” according to the study.
Viruses like HPV spread through replication, copying themselves inside a host body and passing those copies to new hosts via bodily fluids. Viruses do that in two ways: vertically, from mother to child via breastfeeding or live birth, or; horizontally, between sexual partners. Either transmission method makes the host sick, and “that’s not something a pathogen particularly sets out to do,” study co-author Vincent Jansen told New Scientist. “It’s shooting itself in the foot.” Meaning, if the host becomes too ill, their body may redirect all resources toward fighting the spread of the virus rather than simply passing it on.
That’s not what a virus wants. So they find ways around it — namely, identifying and treating host bodies differently. “Females, but not males, provide an additional route of transmission,” the study explains, noting that “women can pass infections to their children during pregnancy, birth and breastfeeding,” in addition to horizontal transmission through sexual intercourse, study authors Jansen and Francisco Úbeda told New Scientist. By simulating the spread of the Human T-cell Lymphotropic Virus Type 1 (HTLV-1) via mathematical models for both men and women, the researchers saw the virus displaying “an evolutionary pressure” on women “to be less harmful to them,” New Scientist reports. Here’s how the study summarizes it:
Natural selection favours pathogens causing differential mortality in men and women when they are horizontally and vertically transmitted. In particular, 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.
Basically, if the host has an opportunity to spread a virus in more than one way, it’s too valuable to attack full on.
An additional theory about the discrepancy of the spread is “because women breastfeed their babies more commonly and for longer in Japan — giving the virus more opportunity to enter another host” Jansen told New Scientist. Yet Sabra Klein of Johns Hopkins also told New Scientist that this assumption “ignore[s] other variables – such as ethnicity or culture – that could also be involved” in the virus’ spread.
More importantly, as New Scientist reports, “The study emphasises the need to conduct clinical trials in both sexes, rather than predominantly in men as is often the case, says David Duneau, an evolutionary biologist at the University of Toulouse, France. “The parasites themselves are behaving differently in males and females, so we need to know what they do in both sexes. The researchers agree with that, writing “one of the reasons why a sex-specific treatment has not been implemented is that the causes of sex-differences in virulence are not well understood” in the study.
The researchers also aren’t sure how the viruses know the sex of the host, but “there are all sorts of hormonal and other pathways that are slightly different between men and women,” Jansen says. Once they figure that out, they they could treat viral infections in the future by tricking it. “We could try to make the virus think it’s in a female body rather than a male body and therefore take a different course of action,” Jansen told New Scientist.
The findings may prove to be helpful for the spread of other viruses, as HTLV-1 is far from the only one to kill men more often than women, according to New Scientist:
Men infected with tuberculosis are 1.5 times more likely to die than women; men infected with human papillomavirus are five times more likely to develop cancer than women; and men infected with Epstein-Barr virus are at least twice as likely to develop Hodgkin’s lymphoma as women.