Zika virus danger: How semen spreads this invisible killer

Author: Li Shihua, Associate Researcher, Institute of Microbiology, Chinese Academy of Sciences. Review: Dai Lianpan, Researcher at the Institute of Microbiology, Chinese Academy of Sciences. Zhao Xin, Researcher at the Institute of Microbiology, Chinese Academy of Sciences. Supervisor: China Science and Technology Publishing House Co., Ltd., Beijing Zhongke Xinghe Cultural Media Co., Ltd. Produced by: Popular Science Department, China Association for Science and Technology. This article is a supported work of the Science Popularization China Creative Nurturing Program.

The Zika virus poses risks to male reproductive health, potentially harming fertility and surviving in semen. Awareness is vital. In the long-term struggle between humans and viruses, we know that they can be transmitted through air, water, contact, and mosquito bites. However, you may not be aware that some viruses can also "lurk" in the semen of men and can be transmitted to another person through the most intimate contact.

Between 2015 and 2016, the Zika virus caused a massive outbreak in the Americas, drawing global attention. It belongs to the Flavivirus family and, like the dengue virus and Japanese encephalitis virus, is primarily transmitted through mosquito bites. Its uniqueness lies in the fact that it is the first virus in the Flavivirus family known to be sexually transmitted. Research has indicated that the Zika virus can persist in semen for about 57 days, and in immunocompromised individuals, it can even remain dormant for up to 941 days. This means that even after an infected person has recovered, their semen may still serve as a "safe haven" for the virus.

So, how does the Zika virus stealthily infiltrate the male reproductive system? A research team led by Academician Gao Fu from the Institute of Microbiology, Chinese Academy of Sciences, in collaboration with Professor Li Xiangdong's team from China Agricultural University, discovered that the Zika virus can infect the testes and epididymis of mice, causing inflammation, tissue damage, and reduced levels of hormones such as testosterone, which may ultimately lead to infertility.

In the male reproductive system, there is a "blood-testis barrier," which functions similarly to a computer's firewall by preventing the invasion of viruses and harmful substances. However, the Zika virus can disrupt the tight junctions between cells or trigger inflammation, creating "gaps" in the barrier, thereby allowing it to infiltrate tissues such as the testes and epididymis. Once inside, it primarily targets spermatogenic cells, supporting cells, and peritubular myoid cells. These cells serve as both "factories" for viral replication and are crucial for sperm production. The Zika virus not only impairs the normal functions of these cells but may also lead to cell death, ultimately affecting fertility.

Why was the immune system unable to promptly eliminate the Zika virus? This is because the testicles are considered an "immune privileged area." Imagine your body as a busy city, with the immune system acting like the city's "police force," responsible for capturing the "bad guys," such as viruses and bacteria. However, there are special "VIP zones" in the city that the immune police cannot freely enter. The testicles are one such "immune privileged area." Here, in order to protect the sperm, immune activity is intentionally suppressed. But this also provides an opportunity for the virus, allowing the Zika virus to persist and evade immune surveillance for an extended period.

The symptoms caused by the Zika virus are usually mild, such as fever, rash, and joint pain, and most people can recover on their own within a week without the need for special treatment. However, its greatest danger lies in its effects on pregnant women and their fetuses: the virus may be transmitted through the placenta to the fetus, leading to microcephaly and other serious congenital defects. In 2016, the World Health Organization declared it a "Public Health Emergency of International Concern" for this reason.

Although the pandemic has subsided, the Zika virus has not completely disappeared. It continues to circulate in certain areas, and because it can remain dormant for a long time and be transmitted asymptomatically, it still poses a risk. A more concerning issue is that the virus may mutate during transmission, potentially increasing its infectiousness or evading immune monitoring. If the conditions are right, it could trigger another outbreak.

The threat of the Zika virus to male reproductive health reminds us that the battle against this virus is far from over. Scientists and public health experts must continue to closely monitor its developments. In the face of the virus, only vigilance, research, and public education can safeguard public health.