In one of the first studies of its kind, a research team lead by virologists at Beth Israel Deaconess Medical Center in Boston, Massachusetts, has characterized the progression of two strains of Zika virus. The study, published online in Nature Medicine, revealed Zika’s rapid infection of the brain and nervous tissues, and provided evidence of a risk for person-to-person transmission.
The researchers infected 36 rhesus and cynomolgus macaques with strains of the Zika virus obtained from Puerto Rico and Thailand. Over the next four weeks, the scientists tested blood, tissues, cerebrospinal fluid (CSF), and mucosal secretions for the presence of Zika virus, and monitored the immune response during early infection. Their data shed new light on how Zika causes the devastating neurological complications seen in adults and unborn babies.
“We found, initially, that the virus replicated very rapidly and was cleared from the blood in most animals within 10 days,” said corresponding author James B. Whitney, PhD. “Nevertheless, we observed viral shedding in other bodily fluids, such as spinal fluid, saliva, urine, and semen, up to three weeks after the initial infection was already cleared.”
“Of particular concern, we saw extraordinarily high levels of Zika virus in the brain of some of the animals––the cerebellum, specifically––soon after infection,” he added. “Only one in five [human] adults has noticeable symptoms of infection. However, if our data translate to humans, there may be need for enhanced clinical vigilance for any persons presenting with unusual neurological symptoms, and they should be tested for Zika infection.”
As in humans, Zika infection in the experimental primates appeared relatively mild, producing fever and an increase in the blood cells associated with an immune response. All animals recovered without intervention. But while the virus was cleared from the blood within 10 days, the researchers observed Zika virus in urine as soon as two days after infection in some animals. By the third day after infection, Zika was detectable in the saliva of up to half of the animals, where it remained until the study ended at four weeks after infection.
Early in the infection, the researchers found high levels of Zika virus in the genital tracts of both sexes. Zika remained detectable in semen and in uterine tissues until the end of the study. The first sexually transmitted case of Zika in humans was documented in 2007, but the new findings suggest transmission may occur long after Zika symptoms–– if they appeared––have resolved. Because the researchers found high levels of the virus in semen and in the uterus, the findings may also highlight the sexual transmission of Zika.
“We found that male-to-female transmission may be easier, while female-to-male may be less likely,” Whitney said. “Nonetheless, the high levels of Zika we observed in the uterus underscore the danger to a developing fetus.”
The new study also highlights the need for the rapid development of vaccines and therapies against Zika. The World Health Organization declared the virus epidemic a global public health emergency in February 2016.
Source: EurekAlert; October 6, 2016.