Viruses are occasionally known to jump, infecting one type of organism before jumping over to another. Such is the fear with the avian flu, for instance. But usually those jumps occur between organisms with minor or intermediate taxonomic differences — say, from mammal to mammal, or at least from vertebrate to vertebrate. Rarely does a virus jump between whole taxonomic kingdoms.

So scientists were surprised when they discovered an algae virus capable of making the jump to infect humans and other mammals, reports Phys.org. The chlorovirus — Acanthocystis turfacea chlorella virus 1, or ATCV-1 — made just such a leap. In addition, it was found to cause measurable cognitive impairments in the people it infects. 

"Chloroviruses are worldwide," said senior author James Van Etten, who helped discover the existence of these viruses in green algae more than 30 years ago. "They're very common among inland bodies of fresh water such as lakes and ponds. But I don't know of many examples of viruses jumping from one kingdom to another. If this turns out to be true, this is quite rare and a total surprise."

The researchers stumbled upon traces of DNA resembling the virus while taking throat swabs from healthy subjects during a study on cognitive functioning. Subjects that tested positive for ATCV-1 performed measurably worse on tests of visual processing and spatial orientation. 

To test the correlation further, researchers infected mice with the chlorovirus and tested their ability to navigate mazes. Sure enough, mice also carrying ATCV-1 exhibited similar cognitive deficits. Further research on the mice found that the chlorovirus altered the expression of genes in the rodent hippocampus, an area of the brain associated with memory and spatial navigation.

"This is a striking example showing that the 'innocuous' microorganisms we carry can affect behavior and cognition," said lead investigator Robert Yolken. "Many physiological differences between person A and person B are encoded in the set of genes each inherits from parents, yet some of these differences are fueled by the various microorganisms we harbor and the way they interact with our genes."

Even more alarming, chloroviruses like this one are easy to encounter. They are common in freshwater lakes and ponds around the world. 

Unlike better-known and more widely feared viruses like HIV, influenza or Ebola, chloroviruses are monsters. They can feature 40 times as many genes as more conventional viruses. This is one of the reasons — in addition to the rarity of viral jumping — that they are often overlooked. 

"Viruses are almost always thought to be very small. Researchers filter out other components when they're identifying viruses, and chloroviruses are so big that they would've been caught in those filters," explained Van Etten.

It's not known if ATCV-1 can replicate in humans and other mammals. It's possible we are not suitable hosts, but since we can be infected by the virus, it opens the door to a whole other kingdom of new health research.

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