Although the Zika virus is largely transmitted to people via mosquito bites, the virus can also be sexually transmitted from person to person and through blood transfusion.
Although it can be found in the semen of infected men, the impact of Zika on the human male reproductive system is largely unknown.
To investigate further, the scientists from Washington University School of Medicine in St. Louis infected male mice with a mouse-adapted African or Asian Zika virus strain, or with the related dengue virus.
Every seven days, the researchers stained and visually examined samples of mouse testes to look for damage and tested cells from those organs for evidence of the virus.
Although the closely related dengue virus did not appear to infect the testes of mice, the researchers found that cells in the testes showed signs of Zika infection by day seven.
After 14 days, the testes visibly shrank in size. As Zika infection progressed, the seminiferous tubules where sperm is formed began to break down.
Additionally, the researchers found that inflammatory cells mounted a response, which added to the damage caused by the virus. After 21 days, the testes of mice infected by the African strain of Zika had shrunk substantially.
By 42 days after infection, damage from the virus had cut the test animals’ average motile sperm count by roughly three-fold, with some mice showing very low sperm counts.
Furthermore, their levels of testosterone and inhibin b, hormones vital to regulating sperm production and testes function, also fell, showed the study published online in the journal Nature.
Low sperm and hormone levels were associated with decreased fertility rates.
The scientists call the results “concerning,” although it remains unclear what these findings in mice may mean for humans.
Longitudinal studies of sperm function and viability in men who have experienced Zika infection are needed, the researchers concluded.