Dozens of Viruses Can Be Detected in Human Semen

Since researchers found that the Zika virus was present in semen and can be passed through sex, health officials have warned of sexual transmission of the virus. Now in a recently published paper, researchers found evidence that more than two dozen additional viruses can be found in human semen, though many questions remain on how this may impact public health.

In 2015, a study in the journal Emerging Infectious Diseases, published by the Centers for Disease Control and Prevention (CDC), detailed preliminary findings from an outbreak in French Polynesia on the presence of Zika virus in semen, suggesting that the flavivirus could be sexually transmitted. Although the Zika virus has primarily been thought of as a mosquito-borne disease spread by the Aedes aegypti mosquito, health officials investigated other transmission pathways of the virus in the wake of the Zika virus outbreak in South America and the subsequent rise in the number of infants born with microcephaly.

In 2016, prompted by a case of sexual transmission of the virus reported in Texas, the CDC released findings confirming that Zika can be transmitted from a male to his partner via semen. That year, the United States saw 46 cases of sexually transmitted Zika virus, and researchers have continued to investigate how long the virus can remain in semen and vaginal fluids. Zika virus RNA is typically undetectable in most men by about 3 months after infection but has been found in semen for up to 188 days following the start of illness.

In a new study, recently published in Emerging Infectious Diseases, a pair of researchers investigating the persistence of viruses in seminal fluids conducted a review of 3818 articles found in a PubMed search. Noting that Zika virus RNA is often found in semen following infection, the authors acknowledged gaps in knowledge on the presence of other viruses in genital fluids. It is likely, they wrote, that many other viruses capable of causing viremia can be detected in semen. In addition, they noted that because the testes have a limited immune response to allow for the survival of sperm, viruses may remain present in the male reproductive tract even if they can’t replicate.

Although the search did not yield as much data on sexual transmission, the authors found that 27 viruses that lead to viremia have been detected in human semen. These 27 viruses come from a range of families, indicating that presence in semen is not dependent on viruses having similar antigens, immune invasion mechanisms, or the capacity for replication in the male reproductive tract.

The viruses that cause acute infection that were detected in semen were: Zika, Ebola, Lassa fever, Rift Valley fever, and chikungunya. In their literature search, the researchers also found articles discussing the presence of virus in semen for other viruses, including, Japanese encephalitis, foot and mouth disease, parainfluenza, influenza, smallpox, and rubella viruses.

“Given these findings, the following questions need to be addressed: which viruses are shed and remain viable in semen, for how long, and at what concentrations?” write the authors. “The answers to these questions have implications for risks for sexual transmission and, therefore, embryonic infection, congenital disease, miscarriage, and effects on epidemiologic and transmission models. The presence of virus in the male reproductive tract may increase the risk for acquisition of sexually transmitted infections and may reduce male fertility through spermatogonial stem cell infection or local inflammation.”

The article—along with the recent Zika outbreaks—sheds light on the many unknowns remaining, such as to what extent virus replication occurs within spermatozoa, and why a virus may persist in semen after systemic clearance of a viral infection. Because of such unknowns, and because people with Zika can pass the virus on to their sexual partners even when they are asymptomatic, the CDC recommends that sexual partners use barrier methods or abstain from sex to avoid sexual exposure to Zika.