Zika virus could have Alzheimer's-like effect in adults

Zika can “wreak havoc” on the brains of adults and cause major, lasting damage, according to a new study. The research could overthrow the assumption that the virus is only of major worry to pregnant women.

Until now, the mosquito-borne infection has been primarily linked to microcephaly, a serious defect where babies are born with small heads and brain damage. That has mean that pregnant women were warned to avoid coming into contact with the infection – but others have shown no obvious symptoms.

But a major new study on mice indicates that the impact of the Zika infection in other adults could be far more serious and sinister than had previously been thought.

Experiments on adult mice engineered to mimic human Zika infection show that the virus seems to attack immature cells in the adult brain. Those same cells are vital to learning and memory – and so losing them could have disastrous effects, comparable to those experienced by people with Alzheimer’s.

Over time, the gradual attack on those cells could lead to shrinkage of the brain and major impairment of cognitive processes, the scientist behind the study said.

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 look at the way that Zika attacks the adult brain. It was carried out by using fluorescent biomarker “tags” that could indicate where in the brain was invaded by the virus.

The scientists saw those attacks taking place in parts of the brain that are central to learning and memory, they write in the study published in the journal Cell Stem Cell.

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."

Healthy people might be able to resist those attacks. But people who already have weakened immune systems could be at risk.

"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,” said Professor Gleeson.

The scientists still aren’t sure the extent to which the behaviour in mice could apply to humans, or how permanent any damage sustained as a result of the virus might be. But they say that further work must be done to find out if Zika could cause long term mental impairment in adults.

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