By Sarah Knapton, science editor
31 OCTOBER 2016 • 4:14PM
The Zika virus could cause infertility in men, a new study suggests.
Scientists in the US discovered that mice infected with Zika had shrunken testicles, low testosterone levels and low sperm counts.
Although the findings have not yet been replicated in humans, experts say that the virus may also have worrying conseqences for men who become infected.
The more we learn about the Zika virus, the more interesting and alarming it becomes.
Prof Allan Pacey University of Sheffield
Dr Michael Diamond, of the University of Washington, who co-authored the study, said: “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.
“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.”
The research is the first to link Zika to male infertility. Previously it was though that the virus, which is passed on through mosquito bites, was only dangerous for pregnant women, because it can lead to babies being born with shrunken heads and brain damage, a condition known as microcephaly. In rare cases it can also lead to Guillain-Barre syndrome which can cause paralysis and lead to death.
According to Public Health England (PHE), some 244 British people have contracted the virus since the current outbreak from travelling abroad, but they did not have records for how many were men.
The study authors say men may not realise they are infertile until many years after infection.
“This is the only virus I know of that causes such severe symptoms of infertility,” said co-author Dr Kelle Moley, professor of obstetrics and gynecology at the University of Washington.
“You might also ask, ‘Wouldn’t a man notice if his testicles shrank?’ Well, probably. But we don’t really know how the severity in men might compare with the severity in mice. I assume that something is happening to the testes of men, but whether it’s as dramatic as in the mice is hard to say.”
British experts said the findings coincided with reports that men infected with Zika suffered from pelvic pain and blood in their urine. The effects are similar to those seen following human infection with other sexually transmitted infections.
Dr Derek Gatherer, lecturer in the Division of Biomedical and Life Sciences at Lancaster University, said it was important to start monitoring the fertility of men who had been infected with Zika to see if patterns emerge.
“The question of whether or not male Zika patients develop subsequent fertility problems ought to be answerable by comparing the numbers of children born to that group, and their sperm counts, against a social and age matched Zika-negative group,” he said.
“It’s been known for a while that Zika virus in men can find its way into the reproductive organs and may then go on to be sexually transmitted, but this study in mice is the first suggestion that this passage through the reproductive tract may actually be damaging.
“Whether the results are relevant to human Zika infection remains to be seen. Nevertheless, some clinical reports of pain in the lower pelvis of Zika patients, and blood in their sperm would be consistent with a similar effect happening in humans.”
The virus is known to persist in men’s semen for months and PHE currently recommends that men use condoms for six months upon returning from a Zika-endemic region to avoid passing the disease to their partners.
However, the new research suggests that the virus can also damage the male reproductive system.
Scientists found that after three weeks the testicles of Zika-infected mice had shrunk to one-tenth of their normal size and their internal structure was completely destroyed. Levels of their sex hormones also dropped and their fertility was reduced.
By six weeks after infection, the number of motile sperm was down tenfold, and testosterone levels were similarly low.
When healthy females were mated with infected mice, the females were around four times less likely to become pregnant as those paired with uninfected males.
Prof Richard Sharpe, of the MRC Centre for Reproductive Health at the University of Edinburgh said: “The big question is whether the same course of events might happen in men infected with Zika, because if so, it could well result in permanent loss of or reduction in fertility
“For sure, any generalised infection of the testes bad enough to provoke an immune reaction within the testes is bad news for testis function and sperm generation. The answer is that we just don’t know.
“These new findings strongly suggest that testes function in human men infected with Zika should be closely investigated so that we can try and establish whether or not Zika poses a new threat to male reproductive function.”
In August, US scientists said that the disease had the potential to cause long-term memory problems, similar to Alzheimer’s disease.
Prof Allan Pacey, professor of Andrology at the University of Sheffield, added: “The more we learn about the Zika virus, the more interesting and alarming it becomes.”
Governments in South America are attempting to combat the outbreak by releasing genetically modified or infected mosquitoes into communities to bring down insect populations.
The research was published in the journal Nature.