Animal research suggests Zika could affect the adult brain
"Zika virus may cause long-term memory damage, similar to Alzheimer's disease," The Daily Telegraph reports. At the moment such a claim is pure speculation as it is based on research into mice.
Currently, the effects of the Zika virus are thought to be short-term in adults, only presenting a threat to unborn babies. The short-term symptoms in adults are usually similar to the flu, such as fever and joint pain.
This latest research involved mice bred to have an immune deficiency to the Zika virus. Researchers found that after injecting the virus into their blood, it went on to have an effect on areas of the brain where new brain cells are created. If a similar effect occurred in humans there could be a potential impact on memory and thinking skills.
The last major Zika outbreak occurred in French Polynesia in 2013-2014. During this time the World Health Organization recorded an increase in the number of cases of Guillain-Barre syndrome (GBS); usually a rare neurological condition that can cause muscle weakness due to nerve damage. But the picture was complicated as the area was also in the grip of a dengue outbreak, which has also been associated with GBS.
As this was exploratory research, we don't yet know the implications of these findings for adults. The picture may become clearer once more data is analysed from the ongoing outbreak in the Americas.
If travelling to Zika-affected areas, then following standard advice about avoiding mosquito bites would be wise.
Where did the story come from?
The study was carried out by researchers from the Howard Hughes Medical Institute, the University of California and the La Jolla Institute for Genomic Medicine, all in the US. It was funded by the National Institutes of Health, the Simons Foundation of Autism Research Initiative and the Howard Hughes Medical Institute. There appear to be no conflicts of interest.
The study was published in the peer-reviewed medical journal Cell Stem Cell on an open-access basis, which means you can read it for free online or download it as a PDF.
The UK media generally reported it accurately and, refreshingly, made clear from the start that this was animal research. Though as is so often the case, many headlines were needlessly alarmist, such as the Mail Online's choice of language about the virus possibly "devastating" human brains.
What kind of research was this?
This animal study in mice aimed to look at the effects of infection with Zika virus on the brains of adult mice.
Recent global attention has been drawn to the Zika virus outbreak and its link with cases of microcephaly, when the brain does not develop properly in babies. It has also been linked to Guillain-Barre syndrome, when the body's immune system attacks part of the peripheral nervous system.
Until now, the virus was thought only to affect pregnant women's babies, not the rest of the adult population. Short term symptoms experienced by some adults include a fever, rash, joint pain, headaches and vomiting.
Although Zika is considered a short-term infection in adult humans, the long-term effects on the adult brain have yet to be studied.
Animal studies are often used in the early stages of research to see how biological processes may happen in humans. However, we are not identical to animals and the implications for humans may need to be tested in other ways, especially to see if humans can develop immunity to the virus quickly.
What did the research involve?
This was complex laboratory research using mice to observe the effect of Zika virus on adult brain cells.
Mice were bred with immune deficiencies. At between five and six weeks old, researchers infected the mice with an Asian strain of the Zika virus.
Mice were injected in a way that introduced the virus into the bloodstream rather than directly into in the brain, to mimic the way the virus enters the bloodstream in humans.
To examine the potential for viral infection in the brain, the researchers screened sections of the brain from both infected mice and mock-treated mice.
The impact on brain cell division and loss was assessed using cell cycle markers. These are essentially fluorescent tags that allow the researchers to track how the virus spread through brain cells.
What were the basic results?
The researchers found that Zika virus was concentrated in the two sections of the brain where there is active cell division in adult mice. These were the subventricular zone (SVZ) of the anterior forebrain and the subgranular zone (SGZ) of the hippocampus. These are both areas of the brain where new brain cells are produced (neurogenesis).
The researchers found that when the Zika virus entered the bloodstream of the mice, there was pronounced evidence of Zika infection in these two brain areas, which led to cell death and reduced cell division.
The changes were found in the three mice infected with the Zika virus and not in the three mice who weren't infected.
The results suggested an increase in the death of neural cells in these two areas. The areas of the brain that were not associated with cell division were not affected by the virus.
How did the researchers interpret the results?
The researchers concluded that "the virus was able to infect SVZ and SGZ niche cells to a much greater degree than non-neurogenic regions."
They also "recognize that healthy humans may be able to mount an effective antiviral response and prevent entry into the CNS, but it remains a possibility that some immunocompromised humans and even some apparently healthy humans may be susceptible in ways modeled by the TKO mice [mice where certain immune cells had been 'knocked out']".
Conclusion
This experimental study in mice investigated the effect of Zika virus on adult brain cells, hoping to increase knowledge of the long-term outcomes of Zika virus on the adult brain. Zika was thought to be a short-term virus for adult humans without many long-term effects.
The researchers' experiments in mice found that the two small areas in the adult mouse brain containing cells active in cell division can be susceptible to pronounced Zika infection leading to cell death and reduced cell division.
While healthy humans may be able to mount an effective immune response to the virus, it is possible that immunocompromised humans may be susceptible in ways demonstrated by the mice.
However, as the authors point out, the study only used a single virus strain, a single mouse strain and was at a single point in time. There is more information needed before the implications for humans are understood.
Future studies are needed in infected humans to describe the effects of the Zika virus on the adult brain.
Public Health England provide an up-to-date overview of the current state of the Zika virus outbreaks in the Americas, as well as specific advice for certain groups, such as pregnant women or women planning to become pregnant.
