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The Zika virus attacks cells in mouse testes crucial for sperm and sex hormone generation and hampers reproduction, according to new research that raises the possibility that the virus could affect fertility in men.

There are major caveats to the research, which was published Monday in the journal Nature. The study was conducted in mice, and many findings from mouse studies do not hold up in people. The researchers also used a very powerful dose of Zika when infecting the mice.

And even if the same outcomes bear out in people, researchers have no idea what percentage of men who contract Zika would be affected or just how damaging the infection could be. Many men, for example, can see a drop in their sperm count without having a harder time conceiving a child.


But the findings were enough for the paper’s authors to call for further study of the issue in men who have contracted Zika to determine whether the virus affects the male reproductive tract over time.

“This is what we see in mice,” said Dr. Michael Diamond, a viral immunologist at Washington University School of Medicine and senior author of the paper. “How much of this applies to humans? That’s the key question that needs to be addressed in longitudinal studies.”


Nikos Vasilakis, a Zika expert at the University of Texas Medical Branch in Galveston who was not involved with the new work, praised the research and said it warranted additional studies in monkeys and possibly finding a group of infected men to study.

“The key is, would that translate as well in humans?” Vasilakis said about the study’s findings.

Little was known about Zika until doctors in northeastern Brazil witnessed a spike in microcephaly, or underdeveloped brains and heads, in newborns last year. Since then, researchers have discovered that the mosquito-borne virus can cause that and other congenital defects when it infects a pregnant woman and her fetus, as well as apparent neurological conditions in rare cases in adults.

Beyond that, though, Zika has been thought to be harmless in most people. The majority don’t even show symptoms, and those who do experience a few days of rash and fever.

But Zika has some puzzling characteristics that have led scientists to question how else it may affect people. In this case, Diamond and his team decided to look at parts of the male reproductive tract because the virus can also be sexually transmitted — unlike other viruses spread by mosquitoes — and because scientists had seen lasting infections in both human semen and in mouse testes.

The researchers found that Zika targeted two specific cell types in mice: spermatogonia, which make sperm, and Sertoli cells, which are involved in helping sperm develop and sheltering them from the blood and immune system, building what’s called the blood-testis barrier. Overall, compared with uninfected mice, infected mice saw drops in the number of those cells; in the levels of two sex hormones, testosterone and inhibin B; and in the number of sperm.

When the mice mated, the infected males had a harder time getting females pregnant and produced fewer viable fetuses than uninfected males. The virus also seemed to shrink the infected mice’s testes and damage tissue in the epididymis, a small tube that transports sperm.

There is no vaccine or drug for Zika at the moment, but if public health experts find that the virus can undermine sperm production and function in men, it could expand the market for treatments.

If a man contracted Zika and was able to take an antiviral treatment quickly, for example, it could block the infection before it did much damage to the reproductive tract.

“It would mean we were much more aggressive about treatment,” Diamond said.

As of now, it’s not clear if any groups are specifically studying if Zika has an impact on male fertility. The Centers for Disease Control and Prevention has a number of studies underway looking at the virus’s persistence in semen, which could reveal how it affects sperm health, an agency spokesman said.