(CN) - If a study involving mice is any indication, Zika infection may reduce fertility and lower the testosterone levels in men - marking another threat to families posed by the mosquito-borne virus.
Researchers studied mice infected with the virus, which displayed signs of lasting damage to key cells in the male reproductive system. In some cases, the damage caused shrunken testes, reduced fertility and lower levels of sex hormones, according to the team.
"While our study was in mice and with the caveat that we don't yet know whether Zika has the same effect in men it does suggest that men might face low testosterone levels and low sperm counts after Zika infection, affecting their fertility," said co-senior author Michael Diamond, whose findings were published in the journal Nature.
Previous studies have shown the virus can linger in semen for months, and the U.S. Centers for Disease Control and Prevention recommended that men who have traveled to areas with active transmission of Zika practice safe sex. However, it is still unknown whether the virus has an impact on men's reproductive systems.
To test whether prolonged exposure to Zika can harm testes, the team injected male mice with the virus, tracking its impact weekly.
After one week, the researchers found infection in the testes and sperm, and recovered evidence of viral genes in certain cells in the testes.
Symptoms grew more severe after three weeks, as the mice's testes had shrunk to 1/10 their normal size. The internal structure of the testes was completely destroyed, and their testicles did not heal by the end of the six-week study, even after the virus had cleared the bloodstream of the mice.
"We don't know for certain if the damage is irreversible, but I expect so, because the cells that hold the internal structure in place have been infected and destroyed," Diamond said.
The structure of the testes depends on a type of cell called Sertoli cells. Zika infects and kills Sertoli cells, which maintain the barrier between the bloodstream and the testes and nourish maturing sperm cells.
The testes, which produce sperm and testosterone, made less of each as the damage increased. Six weeks after infection, their sperm count was down tenfold and testosterone levels were just as low.
"This is the only virus I know of that causes such severe symptoms of infertility," co-senior author Kelle Moley said. "There are very few microbes that can cross the barrier that separates the testes from the bloodstream to infect the testes directly."
While no studies have been published connecting Zika infection to infertility in men, Moley noted that it can be a difficult to track symptoms in epidemiological surveys.
"People often don't find out that they're infertile until they try to have children, and that could be years or decades after infection," Moley said. "I think it is more likely doctors will start seeing men with symptoms of low testosterone, and they will work backward to make the connection to Zika."
Symptoms of low testosterone include erectile dysfunction, fatigue, a low sex drive and loss of muscle mass and body hair.
Diamond and Moley said human studies in areas with high rates of Zika infection will help determine the impact of the virus on men's reproductive systems.
"We don't know what proportion of infected men get persistently infected, or whether shorter-term infections also can have consequences for sperm count and fertility," Diamond said. "These are things we need to know."