The mosquito-transmitted Zika virus that is responsible for some irreversible birth defects in babies could also be harmful to male fertility, a new study suggests. Infected mice had shrunken testicles and lower levels of testosterone. The virus hits male mice's reproductive system hard, said a study published Monday in Nature magazine. The lab tests run on mice infected with Zika have shown low sperm counts and testicles reduced 90 percent by weight. That's not yet a reason for men to panic, but already a matter for concern and further studies. "I think this is an important finding [in mice], but this has to be tested in humans… This virus is from a class of viruses that causes cell death in a lot of cells, so it's not hard to extrapolate that if you had a lot of virus in a tissue for a long period of time, then cells would die. That said, we absolutely don't know what the extent of this will be in humans… The only way to find out is to track men longitudinally," Dr Michael Diamond,