"We monitored a group of pregnant women with a confirmed diagnosis of Zika and tested their urine over a period of several months at intervals of about a week. In some of these women, the viral load in their urine disappeared and later returned," said Mauricio Lacerda Nogueira, a professor at Medical School of Sao Jose do Rio Preto (FAMERP) in Brazil.
The patients included 13 women in different stages of pregnancy who were receiving treatment at the Children's and Maternity Hospital (HCM) of Sao Jose do Rio Preto.
According to Nogueira, the virus was detected in one patient's urine for as long as seven months.
In five women, the test was again positive for Zika virus after their viral load had disappeared in previous tests. In all cases, the virus vanished from the women shortly after they gave birth.
"These results suggest the virus continues replicating during pregnancy, in the fetus or the placenta, which must serve as a reservoir for the pathogen," Nogueira said.
"However, viral load in the mother's fluids is intermittent and very low, almost at the detection threshold," he said.
If the result of a molecular test is negative, it should ideally be repeated at least twice at intervals of no less than a week.
Three of the women monitored for the study had babies with complications that were probably caused by Zika: two displayed hearing loss and one was born with a brain cyst.
Researchers at the Massachusetts Institute of Technology (MIT) in the US developed a rapid test that identifies Zika virus in blood samples as well as all four serotypes of dengue virus during the acute phase of infection.
The method is low-cost and entails no risk of a cross- reaction, unlike other tests, according to the study published in the journal Science Translational Medicine.
It consists of a dipstick strip with antibodies that change colour in the presence of a viral protein known as NS1.
"We validated it in tests on blood samples from patients with a confirmed diagnosis of both Zika and dengue attending the Sao Jose do Rio Preto general hospital for treatment," Nogueira said.