Until now it has been widely accepted that the Zika virus disproportionately causes brain abnormalities in developing foetuses, but new research suggests that adults may also be vulnerable to infection.

Scientists at Rockefeller University and La Jolla Institute for Allergy and Immunology found in tests with mice that adults infected with the virus were unable to form new neurons in certain regions of the brain.

Integration of new neurons into learning and memory circuits is crucial for neuroplasticity, which allows the brain to change over time, and the inability to incorporate new neurons is associated with cognitive decline and neuropathological conditions, such as depression and Alzheimer’s disease.

“Zika can clearly enter the brain of adults and can wreak havoc,” said professor at the La Jolla Institute of Allergy and Immunology, Sujan Shresta.

“It’s a complex disease; it’s catastrophic for early brain development, yet the majority of adults who are infected with Zika rarely show detectable symptoms. Its effect on the adult brain may be more subtle, and now we know what to look for.”

It was initially thought that Zika only affected developing foetuses because the virus targets neural progenitor cells, which are essentially the stem cells of the brain.

When we are developing in the womb our brains are comprised entirely of neural progenitor cells; adults’ brains retain niches of these neural progenitor cells that are vital for learning and memory.

The scientists suspected that if Zika could infect foetal neural progenitor cells, it wouldn’t be a far stretch for them to also be able to infect these cells in adults.

A laboratory experiement was set up that mimicked Zika infection in humans. The test, which involved monitoring fluorescent biomarkers in adult mice, revealed that adult neural progenitor cells could indeed be hijacked by the virus.

“Our results are pretty dramatic. In the parts of the brain that lit up, it was like a Christmas tree,” said adjunct professor at Rockefeller, head of the Laboratory of Pediatric Brain Disease, and Howard Hughes Medical Institute investigator, Joseph Gleeson.

“It was very clear that the virus wasn’t affecting the whole brain evenly, like people are seeing in the foetus. 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.”

Because of their findings, the researchers believe Zika , which has become widespread in Central and South America over the past eight months, is not simply a transient infection in adults, and all groups should guard against infection, especially since tools don’t currently exist to test the long-term effects of Zika on adult stem cell populations.

“The virus seems to be travelling 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.”