ZIKA infection could cause lasting infertility and lead to men's testicles shrinking, medical researchers warn.

Doctors warn that if the 'dramatic' findings, in mice, apply to humans it could lead to an epidemic of infertility caused by the disease.

It is not yet known whether the 90 per cent shrinkage in mice would apply to humans – but doctors believe at the very least the virus is likely to reduce sperm counts and testosterone levels in affected men.

ZIKA infection could cause lasting infertility and lead to men's testicles shrinking, medical researchers warn (stock image)

The virus is already known to leads to shrunken heads in babies whose mothers catch the infection, which is carried in tropical countries and has recently been found in the tourist hotspot of Florida.

The virus has the unusual ability to cross the barrier that separates the male reproductive organs from the blood stream.

Michael Diamond, of Washington University School of Medicine said: 'We undertook this study to understand the consequences of Zika virus infection in males.

'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 infertility.'

The virus was already known to persist in semen for months – but it was not known what impact this could have on an infected man.

Research published in Nature Genetics, Professor Diamond and colleagues infected mice with Zika.

After two weeks the testicles had shrunken significantly, their 'internal structure collapsing' with many dead or dying cells, the researchers said.

And after three weeks, the mices' testicles had shrunk to a tenth of their normal size.

Their testicles did not heal even after six weeks, when the virus had cleared from their bodies.

Doctors warn that if the 'dramatic' findings, in mice, apply to humans it could lead to an epidemic of infertility caused by the disease

Professor Diamond said: '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 Zika virus was found to attack Sertoli cells, which do not regenerate, and which nourish growing sperm cells.

Infected mice were four times less likely to get a female mouse pregnant, their sperm numbers fell tenfold, and their testosterone levels were very low.

Co-author Kelle Moley said: 'This is the only virus I know of that causes such severe symptoms of infertility. There are very few microbes that can cross the barrier that seprates the testes from the bloodstream to infect the testes directly.

Because the study of Zika is relatively new, no studies have been published linking infertility to men with Zika.

'People often don't find out they're infertile until they try to have children, and that could be years or decades after infection.

'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.'

It is not yet known whether the 90 per cent shrinkage in mice would apply to humans – but doctors believe at the very least the virus will reduce sperm counts and testosterone levels in affected men (file picture)

The effects of low testosterone, which can be diagnosed with a simple blood test, include low sex drive, erectile dysfunction, fatigue and loss of body hair and muscle.

'If testosterone levels drop in men like they did in the mice, I think we'll start to see men coming forward saying, 'I don't feel like myself,' and we'll find out about it that way.

'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.'

As of 26 October 2016, there have been 244 diagnosed cases of Zika caught by travellers returning home to the UK since 2015.

British experts not involved in the research said the findings may mean humans could be affected.

Dr Peter Barlow, British Society for Immunology spokesperson and Reader in Immunology & Infection at Edinburgh Napier University said: 'While it is currently unclear if Zika virus infection would cause reduced testes size and fertility in man, this study does raise concerns that Zika virus could potentially have direct effects on male fertility. Therefore, more work is needed to determine if these observations in mice would translate to men.'

Dr Derek Gatherer, Lecturer in the Division of Biomedical and Life Sciences, Lancaster University, 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.'

Prof Richard Sharpe, Honorary Professor, MRC Centre for Reproductive Health, and expert in male reproductive health, University of Edinburgh, said there were already anecdotal reports of testicular and groin pain in infected men, but some virus effects could be 'species specific'.