NEW YORK — Zika virus ravages the testes of male mice, sharply reducing sperm counts and fertility, according to a study that raises questions about its effects on men.

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, scientists said.

‘‘We just don’t know that yet,’’ said Michael Diamond of the Washington University School of Medicine in St. Louis, a senior author of the study. The virus is known to infect a man’s reproductive system and persist in sperm and semen, ‘‘so it’s in the right place,’’ he said.

Diamond said he suspects that in mice, the damage is permanent. The results were reported Monday in the journal Nature.

Researchers unconnected with the study agreed that it can’t 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 defense against the virus. That’s a standard step in such experiments but it 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 don’t even know they are sick.

But it can cause serious birth defects if women are infected while pregnant, so 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, Md., 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.

The mouse results show the virus attacks the anatomical structure 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 that got pregnant, the number of fetuses was less than half of normal.

In a separate development, Vietnam reported its first case of microcephaly likely linked to the Zika virus. A 4-month-old girl with an abnormally small head was born in central Vietnam to a mother confirmed to have had the virus when she was pregnant.