Scientists exploring how the Zika virus passes from pregnant monkeys to their fetuses believe the infection may be more dangerous to human pregnancies than previously believed.

"The results we're seeing in monkey pregnancies make us think that, as they grow, more human babies might develop Zika-related disease pathology than is currently appreciated," said lead researcher Ted Golos.

Golos is professor of comparative biosciences and obstetrics and gynecology at the University of Wisconsin-Madison.

The researchers came to their conclusions after infecting four pregnant macaque monkeys with levels of the virus roughly equivalent to what they'd get from a mosquito bite. Some monkeys were infected in the first trimester and others in the third trimester of the pregnancies.

The scientists found that the virus made its way to each monkey's fetus.

"That is a very high level -- 100 percent exposure -- of the virus to the fetus along with inflammation and tissue injury in an animal model that mirrors the infection in human pregnancies quite closely," Golos said in a university news release.

Three monkey fetuses had small heads, but not small enough to be diagnosed with microcephaly -- the most dramatic result of Zika infection in human newborns.

But, while the monkey fetuses didn't show signs of abnormal brain development, the researchers discovered unusual inflammation in the eyes, retinas and optic nerves of those infected during the first trimester of pregnancy.

"Our eyes are basically part of our central nervous system. The optic nerve grows right out from the fetal brain during pregnancy," said study co-author Kathleen Antony, a professor of maternal fetal medicine at UW-Madison.

"It makes some sense to see this damage in the monkeys and in human pregnancy problems such as chorioretinal atrophy or microphthalmia in which the whole eye or parts of the eye just don't grow to the expected size," she added.

The researchers said studying Zika infection in monkeys is a way to follow progression of the mosquito-transmitted infection in people, although animal studies often don't yield the same results in humans.

The U.S National Institutes of Health funded this research, which also involved scientists at Duke University and the University of California, Davis.

The study appears May 25 in PLOS Pathogens.