Is it Time to Consider Zika a Sexually-Transmitted Infection?

As new research on Zika comes to light, some investigators are suggesting that the virus should be labeled a sexually transmitted infection. Indeed, in a recent PLOS Pathogens article, investigators from Spain and the United States used a “novel mathematical approach for multi-route pathogens,” to show that “previous epidemic threshold estimates could under-estimate the risk of sustained sexual transmission by at least an order of magnitude.” They argue that the results of this approach, as well as “emerging clinical, epidemiological, and experimental evidence for an increased risk of sexual transmission,” strongly suggest that the Zika virus should be classified as a sexually transmitted infection.

Cases of sexual transmission and shedding of the virus in bodily fluids in humans have already been reported. Further studies in animals have shown high rates of this mode of transmission in mouse and macaque models. The totality of this research prompted the Centers for Disease Control and Prevention to recommend the use of condoms or the practice of abstinence in couples that could be affected, particularly couples in which the woman is of child-bearing age.

To better protect against the virus and establish effective protection and treatment plans, researchers need to determine if sexual transmission of the virus is sustained or sporadic. According to the investigators on the PLOS study, “Sustained sexual transmission would have the immediate effect of increasing the number of cases during local outbreaks, and a longer-term effect if it led to endemic maintenance of [the Zika virus] in regions that would otherwise experience only sporadic outbreaks.”

In an attempt to test for sustained sexual transmission of the virus, the investigators used a “modeling approach of asymmetric bond percolation on random sexual networks,” which uses prewritten determinants of a population’s percentage of individuals of a certain sex and sexual orientation. In addition, the investigators “characterize[d] the overall threat of [the Zika virus] as a [sexually transmitted infection] by measuring the potential prevalence of the epidemic and potential for sustained, endemic transmission.” The methods used in the study set up the conclusions to be most applicable in areas without the appropriate mosquito vector for transmission, but with the propensity for travelers to bring the virus back with them after visiting an endemic region.

In simplified terms, the results revealed that, because of the higher infectious period in males (>180 days) vs females (<20 days), a greater than average number of sexual contacts, as well as previous evidence that there is increased infectivity in males, men who have sex with men (MSM) run the risk of the virus infecting their community and becoming endemic. There is also the potential for spillover into the heterosexual community in the case of a bisexual man who was part of the MSM epidemic infected a heterosexual female (who in turn, infects a heterosexual male, and so on).

The authors concluded that “for pathogens, like [the Zika virus], which are both vectored by an insect and transmitted sexually (with sex-based asymmetry in transmission rate), conventional approaches will underestimate the risk associated with sexual transmission.” Therefore, increased levels of testing should be considered in all adults.