Zika virus, transmitted by mosquitos and sexual contact, causes fevers, rashes, headaches, and, most concerning, microcephaly in babies born to infected mothers. The current outbreak of Zika in the Americas has left us struggling to find ways to contain the virus. Unfortunately, the actual dynamics of infection, replication, and shedding of this virus are poorly understood. A recent paper published in Nature Medicine identifies some of the ways in which Zika interacts with its hosts.

The authors of this study infected 28 rhesus macaques and eight cynomolgus macaques using two different strains of the Zika virus, tracking the dynamics of the virus within blood, tissues, cerebrospinal fluid, and mucosal secretions. They found that both strains of Zika peaked in the blood within the first week of infection, with no significant differences between males and females.

The study also showed that all animals had significantly elevated body temperatures a day after infection and that they remained significantly elevated for the entire first week of infection.

Within one day of contracting Zika, all primate subjects showed increased white blood cell counts, which is a sign of a response to an infection. But these returned to normal by the end of the second day of infection, which isn't necessarily abnormal for other viruses. Lymphocytes, B cell, T cells, and natural killer cells, all subtypes of white blood cells, were particularly prone to decline after the first day of infection.

By contrast, virus-specific T cells, which are a part of the adaptive immune response in mammals, developed much later and didn’t reach peak levels until 28 days after initial infection.

The authors were particularly interested in Zika’s actions within the central nervous system, since it is known to cause microcephaly. They found the virus appeared in the cerebrospinal fluid of half the infected animals between days seven and 21 of infection.

Since Zika is sexually transmitted, they also looked at mucosal fluids. They found that the virus was present in the secretions and mucous of almost half the infected primates by the second day of infection. In terms of sexual fluids, they found notable levels of Zika in seminal fluids, but the levels in vaginal secretions were very low. There was, however, a high concentration of virus in these primates’ uteruses. This accumulation in the uterine tissues makes Zika particularly tricky for women of childbearing age.

This study showed that there was a rapid innate and adaptive immune response to Zika exposure. The data also suggested that the saliva and male ejaculate were particularly virus-laden and could serve as persistent viral reservoirs even after overt signs of infection are passed. The persistence of viral load in these bodily reservoirs underscores the importance of Zika transmission in household settings, where people are likely to come into contact with each other’s saliva and fluids.

The current emphasis on transmission has primarily focused on limiting exposure to mosquitos, but this new work suggests that additional emphasis on household transmission is needed.

Nature Medicine, 2016. DOI: 10.1038/NGEO2819 (About DOIs).