Researchers at the Wisconsin National Primate research Center (WNPRC) at the University of Wisconsin-Madison, perform an ultrasound on a pregnant rhesus macaque monkey infected with the Zika virus today in Madison. (Photo by Scott Olson/Getty Images)

New research on monkeys found some good news that could have implications for humans: One infection with the Zika virus protects against future infections.

But along with good news were some troubling findings. Researchers at the University of Wisconsin at Madison and Duke University found the virus persisted in the blood of pregnant monkeys for much longer — up to 70 days — compared to the 10 days it lasted in males and non-pregnant female monkeys.

The prolonged infection has implications for the Zika virus in pregnant women because the virus causes microcephaly, a severe birth defect characterized by abnormally small heads and often incomplete brain development, as well as a range of other neurological abnormalities. The research was published Tuesday in the journal Nature Communications.

Because so much remains unknown about the way Zika virus infections progress in people, researchers are using rhesus macaque monkeys as a model for studying the underlying mechanisms.

Researchers said it was important to show in a lab setting what people have expected in humans, that once infection by the virus is cleared from a person's blood — usually within about a week — he or she is protected from future infections by the virus, said David O'Connor, a University of Wisconsin pathology professor and one of the authors.

"If you're not pregnant and not at risk of becoming pregnant, even if you happen to get a Zika infection, you don't have to worry about getting it again for a really long time," O'Connor said.

But the persistence of the virus in pregnant monkeys for 30 to 70 days is cause for worry, he said. One possible explanation is that the immune system is weakened by pregnancy and can't get rid of the virus as fast.

Another hypothesis researchers are testing is that the fetus also becomes infected and produces virus that then goes back into the mother's bloodstream, he said.

"That's the one we internally favor and we're trying to test that now," he said.

That mother-fetus infection loop was proposed earlier this year by a Johns Hopkins University obstetrician in an analysis of a Zika infection in a pregnant woman. A D.C. woman still tested positive for Zika 10 weeks after she likely was infected during a trip to Guatemala — far beyond what scientists have thought should be the duration of infection. Rita Driggers, one of the study's lead authors and medical director of Sibley Memorial Hospital’s maternal-fetal medicine division, has suggested the fetus was also infected and repeatedly infecting the mother.

In the monkey study, researchers infected monkeys with the same strain of virus that is causing an epidemic in Brazil and more than 40 other countries. They injected the monkeys with the amount of virus typically found in the saliva of one mosquito bite.

Pregnancy lasts 165 days in monkeys, and two monkeys were infected during the first trimester. One had virus continually detected for about 70 days, and the second monkey had virus in her blood for 30 days.

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