Zika may damage brain cells in adults, not just in developing fetuses

The Zika virus is known to cause microcephaly and other brain abnormalities in developing fetuses, but new research in mice out Thursday showed that adult brain cells critical to learning and memory may be vulnerable to infection as well.

Although more research is needed to determine if this damage has long-term biological implications or the potential to affect behavior, the findings suggested the possibility that the Zika virus may be more harmful than previously believed, according to the study published in the U.S. journal Cell Stem Cell.

"This is the first study looking at the effect of Zika infection on the adult brain," said Joseph Gleeson, a professor at the Rockefeller University and one of the co-authors of the study.

"Based on our findings, getting infected with Zika as an adult may not be as innocuous as people think."

In developing fetuses, the brains are comprised entirely of neural progenitor cells -- the kind of stem cells that in healthy individuals grow and divide rapidly to become fully formed neurons.

Current evidence suggested that Zika can target neural progenitor cells in developing fetuses, leading to microcephaly and a wide variety of developmental disabilities.

The mature brain retains some niches of these neural progenitor cells, which the researchers suspected are also vulnerable to Zika infection.

These niches exist primarily in the subventricular zone of the anterior forebrain and the subgranular zone of the hippocampus, two regions vital for learning and memory in mice.

Gleeson and his colleagues created a mouse model and then injected a modern Zika strain into the mice's bloodstream to mimic Zika infection in humans.

The results showed adult neural progenitor cells could indeed be hijacked by the virus.

"It was very clear that the virus wasn't affecting the whole brain evenly, like people are seeing in the fetus," Gleeson said.

"In the adult, it's only these two populations that are very specific to the stem cells that are affected by virus. These cells are special, and somehow very susceptible to the infection."

The researchers recognized that healthy humans may be able to mount an effective immune response and prevent the virus from attacking.

However, they suggested that some people, such as those weakened immune systems, may be vulnerable to the virus in a way that has not yet been recognized.

"In more subtle cases, the virus could theoretically impact long-term memory or risk of depression," said Gleeson, "but tools do not exist to test the long-term effects of Zika on adult stem cell populations."

Although there were still many unanswered questions, these findings raised the possibility that Zika is not simply a transient infection in adult humans, and that exposure in the adult brain could have long-term effects.

"The virus seems to be traveling quite a bit as people move around the world," said Gleeson. "Given this study, I think the public health enterprise should consider monitoring for Zika infections in all groups, not just pregnant women." Endit