WASHINGTON • The more researchers learn about the Zika virus, the worse it seems.
A growing body of research has established that the virus can cause severe birth defects - most notably microcephaly, a condition characterized by an abnormally small head and often incomplete brain development. The virus also has been linked to cases of Guillain-Barre syndrome in adults, a rare autoimmune disorder that can result in paralysis and even death.
Now, in a study on mice, researchers have found evidence that suggests adult brain cells critical to learning and memory also might be susceptible to the Zika virus.
"This was kind of a surprise," Professor Joseph Gleeson of Rockefeller University, one of the co-authors of the study published on Thursday in the journal Cell Stem Cell, said in an interview. "We think of Zika health concerns being limited mostly to pregnant women."
In a developing foetus, the brain is made primarily of "neural progenitor" cells, a type of stem cell. Researchers believe these cells are especially susceptible to infection by the Zika virus, which can hinder their development and disrupt brain growth. Most adult neurons are believed to be resistant to Zika, which could explain why adults seem less at risk from the virus' most devastating effects.
But some neural progenitor cells remain in adults, where they replenish the brain's neurons over the course of a lifetime. These pockets of stem cells are vital for learning and memory.
This was kind of a surprise. We think of Zika health concerns being limited mostly to pregnant women.
DR JOSEPH GLEESON, a professor at Rockefeller University, on finding that adult brain cells could be affected by Zika.
Prof Gleeson and his colleagues suspected that if Zika can infect foetal neural progenitor cells, the virus might have the same ability to infect adult neural progenitor cells. That is precisely what they found.
"We asked whether (these cells) were vulnerable to Zika in the same way the foetal brain is," he said. "The answer is definitely yes."
Prof Gleeson is the first to admit that the findings represent only an initial step in discovering whether Zika can endanger adult human brain cells. For starters, the study was conducted only on mice, and only at a single point in time.
More research will be necessary to see whether the results of the mouse model translate to humans, and whether the damage to adult brain cells can cause long-term neurological damage or affect behaviour.
But the initial findings suggest that the Zika virus, which has spread to the United States and more than 60 other countries over the past year, may not be as innocuous as it seems for adults, most of whom do not realise they have been infected.
Researchers found that infected mice had more cell death in their brains and reduced generation of new neurons, which is key to learning and memory.
The possible consequences of damaged neural progenitor cells in humans would include cognitive problems and a higher likelihood for conditions such as depression and Alzheimer's disease.
"Zika can clearly enter the brain of adults and wreak havoc," Associate Professor Sujan Shresta, from the La Jolla Institute of Allergy and Immunology who is another study co-author, said in a statement. "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."