It’s not such a stretch to think that humans can catch the Ebola virus from monkeys and the flu virus from pigs. After all, they are all mammals with fundamentally similar physiologies. But now researchers have discovered that even a virus found in the lowly algae can make mammals its home. The invader doesn’t make people or mice sick, but it does seem to slow specific brain activities.

The virus, called ATCV-1, showed up in human brain tissue several years ago, but at the time researchers could not be sure whether it had entered the tissue before or after the people died. Then, it showed up again in a survey of microbes and viruses in the throats of people with psychiatric disease. Pediatric infectious disease expert Robert Yolken from Johns Hopkins University School of Medicine in Baltimore, Maryland, and his colleagues were trying to see if pathogens play a role in these conditions. At first, they didn't know what ATCV-1 was, but a database search revealed its identity as a virus that typically infects a species of green algae found in lakes and rivers.

The researchers wanted to find out if the virus was in healthy people as well as sick people. They checked for it in 92 healthy people participating in a study of cognitive function and found it in 43% of them. What’s more, those infected with the virus performed 10% worse than uninfected people on tests requiring visual processing. They were slower in drawing a line connecting a sequence of numbers randomly placed on a page, for example. And they seemed to have shorter attention spans, the researchers report online today in the Proceedings of the National Academy of Sciences. The effects were modest, but significant.

The slower brain function was not associated with any differences in sex, income or education level, race, place of birth, or cigarette smoking. But that doesn't necessarily mean the virus causes cognitive decline; it might just benefit from some other factor that impairs the brain in some people, such as other infectious agents, heavy metals, or pollutants, the researchers say.

To test for causality, the team injected uninfected and infected green algae into the mouths of mice. (They could tell that the mice became infected with the virus because they developed antibodies to it.) Infected and uninfected mice underwent a battery of tests. The two groups were about on par with how well they moved, but infected animals took 10% longer to find their way out of mazes and spent 20% less time exploring new objects—indications that they had poorer attention spans and were not as good at remembering their surroundings.

The researchers also studied gene activity in the animals' hippocampus, a part of the brain important for memory and understanding one's whereabouts. They found changes in the activity of almost 1300 genes in the infected animals. Some of those genes affect how the brain reacts to a key chemical messenger called dopamine, and others are important in immune function. Yolken has not yet found the virus in the brain but suspects it may affect the brain through its influence on the immune system, stimulating certain immune responses that might in turn affect gene expression in the brain.

The researchers and others have found this virus in samples around the world but have yet to test whether it’s present in people outside of Baltimore, where the study was done. And a few people carry antibodies to the virus, Yolken says. But he says they are still not 100% certain if and how the virus infects people.

The cognitive effects were small, notes Joram Feldon, a neuroscientist emeritus at the Swiss Federal Institute of Technology in Zurich who was not involved with the work. He praises the finding for being innovative, but says “if you ask me if I am worried about the existence of this virus, I am not.”

Allan Kalueff, the director of the ZENEREI Institute in Slidell, Louisiana, who was not involved with the work, suspects that other viruses may affect human sensory processing and behavior. He says he wonders whether these results indicate that there may be health risks to workers in the seafood industry or who work around water where they may be exposed to this algal virus. "But we clearly need more studies, including both animal and plant/algal studies."