By Roberta Attanasio
Chlorella viruses, or chloroviruses, infect green algae, single-celled organisms present throughout the world in freshwater ecosystems such as lakes and ponds. Now, it seems that chlorovirues also infect humans, causing changes in cognitive functions — the processes by which information is perceived, registered, stored, retrieved, and used. In other words, chloroviruses influence the ability to acquire and use knowledge. These novel findings — published on line in the scientific journal Proceedings of the National Academy of Sciences (October 27, 2014) — confirm that viruses may be able to jump from one kingdom (plants) to another (animals), something that has been shown only in very few instances.
For the study (Chlorovirus ATCV-1 is part of the human oropharyngeal virome and is associated with changes in cognitive functions in humans and mice), the researchers used throat swabs collected from 92 adults without a psychiatric disorder or serious physical illness. The researchers found that 40 of these adults carried the genetic material of one of the chloroviruses — Acanthocystis turfacea chlorella virus 1, or ATCV-1 for short. They also found that the presence of ATCV-1 was associated with a modest but measurable decrease in cognitive functions. Such a decrease was not associated with age, sex, race, socioeconomic status, cigarette smoking, travel history, or place of birth.
However, this first set of findings did not show “causality” — the ability of ATCV-1 to cause the observed decline in cognitive functions. Thus, the researchers infected mice with the virus. After several months, as expected, the infected mice exhibited signs of cognitive impairment. Moreover, the researchers found that, in both humans and mice, exposure to ATCV-1 was associated with decreases in performance on tasks calling for visual spatial abilities, which are important for everyday life — for example, when using a map or merging into high-speed traffic.
Robert Yolken, one of the study’s authors, said in a press release: “This is a striking example showing that the ‘innocuous’ microorganisms we carry can affect behavior and cognition. 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.”
The researchers speculate that the immune system might be involved in the cognitive decline associated with the presence of ATCV-1. In mice infected with the virus, they found changes in several pathways involved in antigen processing and immune cell functioning. These changes were located in the hippocampus, the region of the brain involved in emotions, learning and memory formation. About 35% of the mice mounted an immune response following infection. The researchers propose that a type of chemical messengers called proinflammatory cytokines — which are produced during the activation of the immune system — may affect brain functions, leading to behavioral abnormalities.
Another author of the study, Mikhail Pletnikov, said in an additional press release: “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.”
Allan Kalueff, the director of the ZENEREI Institute in Slidell, Louisiana, who was not involved in the study, told that he suspects other viruses may affect human sensory processing and behavior. He also wondered about health risks to people working in the seafood industry or around waters where they may be exposed to ATCV-1. However, he also said that additional studies are necessary, including animal and plant/algal studies. of Science News