'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,' author Dr Michael Diamond of Washington University School of Medicine, Missouri, told CNN.
Zika is mainly transmitted through mosquito bites, but live virus can remain and even proliferate in male testes for months, and can be transmitted sexually.
As mice are not normally susceptible to Zika virus, the scientists first suppressed their immune system and then infected male mice with either of two different strains of Zika virus, one of which was adapted to replicate more efficiently in mice.
The study found that Zika virus mainly destroys sperm precursor cells and Sertoli cells, which do not regenerate and which protect precursor sperm throughout development. The virus was detected in testes within a week of infection, and a decrease in testicle size was apparent after two weeks.
As the infection progressed, more damage to the testes tissue was caused by inflammation by the immune system. The mouse-adapted strain caused the most severe symptoms, where after three weeks testes had shrunk to one-tenth of their normal size.
After six weeks, even though the virus had cleared the bloodstream, motile sperm was reduced by around three-fold, and reproductive hormone levels had fallen.
'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,' Dr Diamond said to CNN. 'These are things we need to know.'
Experts in the field of male fertility who were not involved in this work have urged caution in interpreting these results. Dr Peter Barlow from British Society for Immunology, said: 'It is worth noting that when another strain of Zika virus was compared, one that did not replicate well in mice, the damage to the testes was not as serious. It is not currently known if all strains of Zika virus would have the same effects.'
Furthermore, Professor Richard Sharpe, an expert in male reproductive health from the University of Edinburgh, emphasised that 'viruses can show highly species-specific effects', therefore more work is required to determine whether human males are similarly affected.
This study is the first to suggest that Zika infection can lead to male infertility. It was published in Nature.