A nurse extracts blood from a pregnant woman as part of a general routine check, which includes examination for mosquito-borne viruses like Zika, at the maternity ward of the Hospital Escuela in Tegucigalpa, Honduras, April 15, 2016. (REUTERS Photo)

WASHINGTON, July 2 (Xinhua) -- American researchers suggested that more women could be losing their pregnancies to the Zika virus without knowing they are infected.

The study, published on Monday in Nature Medicine, found that 26 percent of nonhuman primates infected with Zika during early stages of pregnancy experienced miscarriage or stillbirth even though the animals showed few signs of infection.

"These rates of fetal losses and stillbirths in Zika-infected pregnant monkeys were about four-fold higher than what is normally seen in unexposed monkey populations at these research centers," said Koen Van Rompay, the paper's corresponding author and core scientist at the California National Primate Research Center.

"Many of the fetal and placental tissues had evidence of Zika virus replication and also had pathological lesions, which further supports the role of Zika virus in this detrimental outcome."

Previous Zika research only measured miscarriages and stillbirths in women who displayed signs or symptoms of the virus.

"There are limitations to the human studies, which rely on symptomatic infections," said Dawn Dudley, lead author of the study and scientist in the University of Wisconsin-Madison's Department of Pathology and Laboratory Medicine.

"Women get enrolled in the studies because they have Zika symptoms, but we know that up to half of people who have Zika don't show any symptoms at all. So, the pregnancy studies are probably missing half of the people who have Zika."

The Zika virus is widely known for causing children to be born with a brain abnormality called microencephaly and other malformations.

Zika disease in human adults includes fever, rash, headache, joint and muscle pain, as well as red eyes; however, most are asymptomatic.

"For pregnant women who live in areas where Zika virus is prevalent, and who may experience spontaneous abortions, the possible link to Zika virus infection may be missed," said Lark Coffey, a scientist at University of California, Davis and the paper's co-author.

"Our data in monkeys indicate more research is needed so researchers can develop intervention strategies to protect pregnant women and their fetuses from Zika virus," said Coffey.