Until now it was believed that the virus only had a small impact on adults other than pregnant women. Most adults show little or no symptoms while others experience flu-like effects such as a fever, headache or joint pain.
The new research however paints a far more sinister picture of the effect that Zika could have on adults.
Experiments using mice engineered to mimic the human Zika infection show that the virus ‘wreaks havoc’ on immature brains cells vital to the development of learning and memory.
Over a longer period of time, the loss of these stem cells could then lead to shrinkage of the brain and finally, the same mental impairment that’s seen in people suffering from Alzheimer’s.
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Professor Sujan Shresta, a member of the team from the La Jolla Institute of Allergy and Immunology in California, USA, said: “Zika can clearly enter the brain of adults and can wreak havoc. But 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.”
The study is the first to focus on the impact of Zika infection on the adult brain.
The scientists used fluorescent biomarker “tags” to indicate when adult brain cells were invaded by the virus. Their results, published in journal Cell Stem Cell, show that the virus targets two specific regions of the adult brain critical to learning and memory.
Professor Joseph Gleeson, from Rockefeller University, said: “Our results are pretty dramatic - in the parts of the brain that lit up, it was like a Christmas tree.
“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.
“Based on our findings, getting infected with Zika as an adult may not be as innocuous as people think.”
While healthy individuals may be able to resist the virus, those with weakened immune systems could be at serious risk, said the researchers.
Prof Gleeson added: “In more subtle cases, the virus could theoretically impact long-term memory or risk of depression, but tools do not exist to test the long-term effects of Zika on adult stem cell populations.”
The mouse studies showed that Zika infection prevented neural progenitor cells replenishing neurons in learning and memory circuits.
The scientists still do not know to what extent the mouse model results apply to humans, or how permanent the brain damage is. But they say the research raises the disturbing possibility of long term mental impairment in Zika-infected adults.
“The virus seems to be travelling quite a bit as people move around the world,” says Prof Gleeson. “Given this study, I think the public health enterprise should consider monitoring for Zika infections in all groups, not just pregnant women.”
The epicentre of the current Zika epidemic is Brazil, where the Rio Olympic Games are in full swing.
In February the World Health Organisation declared the outbreak a “public health emergency of international concern” as evidence grew of Zika’s association with birth defects.
The virus is chiefly spread by the Aedes aegypti mosquito, which is common throughout the tropical and subtropical Americas.