Zika virus ravages the testes of male mice, sharply reducing sperm counts and fertility, says a study that raises a new specter about its threat to people.

Experiments found testes of infected mice shrank about 90 percent by weight, while their output of useful sperm fell by three-quarters on average, and often more. Now it's time to find out if Zika causes similar damage in men, experts said.

"We just don't know that yet," said Michael Diamond of the Washington University School of Medicine in St. Louis, an author of the study. The virus is known to infect a man's reproductive system and persist in sperm and semen.

Diamond said he suspects that in mice, the damage is permanent. But mice are not men, and experts unconnected with the study agreed that it can't be assumed that mouse results apply to people.

Shannan Rossi, who studies Zika in mice at the University of Texas, noted that researchers had suppressed the animals' immune system defense against the virus. That's a standard step in such experiments but it adds another level of difference from humans, she said.

Zika, which is transmitted by the bite of a tropical mosquito, is such a mild disease in people that most who get it don't even know they are sick. But it can cause serious birth defects if women are infected while pregnant, so health officials have been concerned mostly with helping women who are pregnant or about to become pregnant avoid the disease.

The mouse results, released Monday by the journal Nature, show the virus attacks the anatomical structure where sperm are made and reach maturity. Testosterone levels also fell.