The Zika virus ravages the testes of male mice, sharply reducing sperm counts and fertility, according to a study which raises fresh questions over the infection's threat to people.
Experiments found the testes of infected mice shrank about 90% by weight, while their output of useful sperm fell by three-quarters on average, and often more.
Experts said it is now time to find out if Zika causes similar damage in men.
Michael Diamond of the Washington University School of Medicine in St Louis, a senior author of the study, said the virus is known to infect a man's reproductive system and persists in sperm and semen.
He suspects that in mice, the damage is permanent. But other e xpects said it cannot be assumed that the mouse results apply to people.
Shannan Rossi, who studies Zika in mice at the University of Texas Medical Branch in Galveston, noted that the researchers had suppressed the animals' immune system defence against the virus - a standard step in such experiments, but one which adds another level of difference from humans.
Zika, which is transmitted by the bite of a tropical mosquito, is such a mild disease in people that most who get it do not even know they are sick. However, 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.
Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland, said the study alerts researchers to look for effects in men.
"Don't jump to the conclusion right off that this is definitely what is happening to the human," he said.
However, he added that the mice finding is a "red flag you need to pursue".
The results appear in a paper released by the journal Nature. They show the virus attacks the anatomical structures where sperm are made and reach maturity. Testosterone levels also fell.
The infected mice were able to impregnate females at only about one-fourth the normal rate. And in females who got pregnant, the number of foetuses was less than half of normal.