The more we learn about Zika, the worse it seems to get. Though the symptoms of infection are typically mild (fever, rash, red eyes, joint pain, and headache), what's going on underneath the surface is far more sinister. First, researchers found that infection during pregnancy can cause brain damage and birth defects in a developing fetus. Then we found out that Zika can be transmitted through sex, and can linger in semen months after a man was first infected.
Now new research shows that the virus can do some damage while it sticks around in the testes, at least in mice. Several weeks after infection with Zika, male mice had significant damage in their sperm-producing seminiferous tubules, says Michael Diamond, an infectious diseases specialist who did the research with other scientists at Washington University School of Medicine in St. Louis. The testes of the infected mice also shrank.
The virus infected both cells that would mature into sperm, and the cells that direct the sperm-making process, says Diamond. Sperm count was down in the mice with Zika, as well as the levels of important hormones including testosterone. With lower sperm count, infected male mice were less fertile than uninfected males. Fewer female mice got pregnant after mating with the males that had Zika.
Scientists will need to track sperm count over time in men who’ve been infected with Zika to confirm that the virus has the same effects in humans as in mice, Diamond says. Already, some men have had related symptoms, such as groin pain and blood in ejaculated semen. If scientists find that infected men have reduced sperm count and fertility, they will also want to find out how frequently that occurs, and if it could happen to infected men without other symptoms.
In the meantime, this research is more evidence that women aren’t the only ones with reason to worry about Zika. If the West Nile is any indicator — at first it was only known to cause fever, but then further research showed it affected the nervous system and caused other symptoms — being aggressive about a mosquito-borne ailment that we don't know very much about may pay dividends.