“The presence of viruses in semen is probably more widespread than currently appreciated,” say researchers Alex Salam and Peter Horby.[*] In response to the recent discovery that semen could carry and transmit Zika virus, the two researchers conducted a literature review to learn whether other infections could be transmitted through sexual intercourse even though they had not been considered to be STIs.
Because the testes, vas deferens, and epididymis offer only imperfect blood barriers, it may just be that the male reproductive tract can be frequently seeded when a man experiences a viremia. The researchers further conjectured that viruses might be able to persist in the male reproductive tract due to a naturally restricted immune response, which is present so that sperm can survive there.
The researchers looked at viruses that cause viremia, finding 27 different ones that can be detected in human semen. (See accompanying table.) Some we know can be sexually transmissible, such as HIV. Others we have recently learned can be sexually transmissible even though they have not been considered STIs, such as the Zika and Ebola viruses. Evidence of sexual transmissibility has been identified for about half of the 27, from direct isolation in semen to epidemiologic or molecular evidence. But for the remaining half, data on sexual transmission are lacking. Yet, should they have transmissibility through genital fluids, these other viruses could potentially cause not only passed through sexual intercourse, but they additionally could pose a risk for associated embryonic infection, congenital disease, or miscarriage, suggest the researchers. And by causing local inflammation, these viruses may further increase a male’s acquisition of STIs from a partner.
The study did not look at viruses that are detectable in the semen of other mammals and that can cause disease in humans, such as Japanese encephalitis virus, foot and mouth disease virus, parainfluenza virus, and paravaccinia virus. They suggest that still more viruses could be found in semen, particularly those that have been detected in human testes and can cause orchitis: influenza virus, lymphocytic choriomeningitis virus, phlebotomus fever virus, cocksackie B virus, echovirus, dengue virus, systemic acute respiratory syndrome virus, parvovirus, smallpox virus, vaccinia virus, and rubella virus.