New research in male mice has revealed that Zika virus infection can break down and severely damage the animals' testes. Whether these findings have any bearing on the potential impact of the virus on the reproductive health of infected men is unclear; however, the study findings suggest this is an important question to explore. Results from the study, conducted by researchers at the Washington University School of Medicine in St. Louis, appear in Nature online October 31. The National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH), supported the research.
Although 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 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 7. 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. 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 conclude.
Additional funding for the study was provided by the NIH's National Center for Advancing Translational Sciences and National Institute of General Medical Sciences.
Govero et al. Zika Virus Infection Damages the Testes in Mice. Nature DOI: 10.1038/nature20556 (2016)
NIAID Director Anthony S. Fauci, M.D., and Cristina Cassetti, Ph.D., of NIAID's Division of Microbiology and Infectious Diseases' Virology Branch, are available to comment on this research.
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