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A virus that usually attacks algae, has been found in the throats of humans and researchers believe that it may produce a subtle change in an infected people's cognitive function.

The virus called ATCV-1 is typically found in algae and seems to be responsible for poorer spatial awareness and a shorter attention span in affected humans.

The discovery sheds new light on a class of viruses that has been previously been deemed non-infectious to humans, highlighting the ability of certain microorganisms to trigger delicate physiologic changes without causing full-blown disease.

A virus called ATCV-1 that usually attacks algae (pictured), has been found in the throats of humans and researchers believe that it may produce a subtle change in an infected people's cognitive function

'This is a striking example showing that the "innocuous" microorganisms we carry can affect behaviour and cognition,' said lead author of the PNAS-published study, Dr Robert Yolken, a virologist and paediatric infectious disease specialist at John Hopkins University.

He and colleagues from the University of Nebraska stumbled across the algae virus by accident while they were studying microbes living in the throats of healthy humans for a non-related study.

By analysing the DNA of swabs, they discovered DNA matching that of ATCV-1 (Acanthocystis turfacea Chlorella virus 1), known to infect green algae, which includes more than 7,000 water-dwelling organisms resembling plants that are commonly found in ponds, lakes and the sea.

A total of 40 people from the 92 participating in the study tested positive for the algae virus.

Mice infected with the virus exhibited deficits similar to those observed in humans, including recognition memory and spatial awareness

The researchers found that people infected with ATCV-1 performed 'measurbly lower' in a set of tasks measuring the speed and accuracy of visual processing, than healthy people. 

For example, people who harboured the virus scored, on average, nearly nine points lower on a test that measured how quickly they could draw a line between sequentially numbered circles on a piece of paper. Viral carriers also scored an average of seven points lower on tests measuring attention.

To extend the study, the scientists infected a group of mice and analysed their performance during a battery of tests designed to measure their brain power.

Animals infected with the virus exhibited deficits similar to those observed in humans, including lesser recognition memory and spatial awareness.

For example, they found it more difficult to navigate a maze and spent 30 per cent less time examining a new object than healthy mice - a finding that suggests a shorter attention span.

While the scientists are cautious in linking mouse behaviour with that of humans, they say that the similarities seen in the tests are 'striking'.

Dr Mikhail Pletnikov, director of the Behavioural Neurobiology and Neuroimmunology Laboratory at the university, said: 'The similarity of our findings in mice and humans underscores the common mechanisms that many microbes use to affect cognitive function in both animals and people.'

'This commonality is precisely what allows us to study the pathologies that these microorganisms fuel and do so in a controlled systematic way.'

Analysis of brain samples from virus-infected mice revealed changes in the expression of multiple genes found in the hippocampus, which is the part of the brain that sorts and catalogues short-term and long-term memories and guides spatial orientation.

The new findings follow several recent studies showing that microbes can play an important role in the origin of cognitive and mental health disorders. For example, some have previously shown that infection with the cat-borne parasite Toxoplasma gondii (pictured) can alter behaviour in genetically predisposed people

Some of these alterations involved genes that regulate brain response to dopamine - a neurotransmitter that affects a wide range of cognitive functions - as well as genes involved in immune cell regulation.

Because multiple genes are involved, the scientists think that numerous mechanisms may explain the effects seen in the study.

The new findings follow several recent studies showing that microbes can play an important role in the origin of neurologic, cognitive and mental health disorders.

For example, Dr Yolken and others have previously shown that infection with the cat-borne parasite Toxoplasma gondii can alter behaviour in genetically predisposed people.