Scientists have successfully infected a group of rhesus macaque monkeys with Zika, marking the first time that non-human primates have been shown to be susceptible to the mosquito-borne virus. That’s good news for researchers, as it potentially opens up a new animal model to study Zika. Scientists could use the monkeys to trace how the virus spreads and test new vaccines or treatments on the animals.

Specifically, scientists infected eight macaques with an Asian strain of the Zika virus, according to a new paper published in Nature Communications. This strain of the virus is very closely related to Zika strains currently spreading throughout Central and South America. All of the monkeys — two of which were pregnant — contracted Zika when exposed to the virus. The non-pregnant animals stayed infected for around 10 days, which is similar to the length of infection in humans. But the pregnant animals remained infected longer, for at least 57 days.

The researchers aren’t quite sure why the pregnant monkeys held the infection longer. It’s possible that it’s due to expectant mothers’ weakened immune systems, which makes it harder for the body to fight off the infection. The other option is there’s an infection loop between the mom and the fetus. That means that when a pregnant mother gets Zika, the developing fetus then gets infected and sheds the virus back into the mother’s bloodstream. If that’s the explanation, a prolonged infection in a pregnant woman could be a sign that the virus is harming the developing child.

"Measuring the viral load on a Zika-infected pregnant woman on a weekly or biweekly basis could provide an indication for the likely degree of damage to the fetus," says lead study author David O'Connor, a professor of pathology at the University of Wisconsin-Madison. "If a pregnant woman comes into a clinic with Zika virus, but a week later shows no more evidence of infection, that could be a good indication that the fetus is unlikely to be affected."

Researchers have long suspected that the Zika virus can harm developing fetuses. The Zika epidemic in Central and South America has coincided with a sharp increase in babies being born with abnormally small heads — a condition known as microcephaly. In April, the US Centers for Disease Control and Prevention confirmed that the virus is causing microcephaly in newborns.

The monkeys in the study appeared to build up a resistance to Zika after they had been infected. The virus was reintroduced to the animals 10 weeks following their first infection, but they were completely immune. The researchers say that this finding is important, because it may indicate that people who have already been infected with Zika once won’t get the virus again.

The Zika virus explained