Many well-known human viruses, including poliovirus, rabies virus, West Nile virus, can infect cells of the nervous system, leading to alterations in the function of that organ. Could a virus that infects algae also cause human neurological alterations?
Chloroviruses are large DNA-containing viruses that infect unicellular algae called zoochlorellae. Unexpectedly, chlorovirus DNA sequences were found in the oropharynx of 40 of 92 individuals (43.5%) who had no known physical or psychiatric illness. The clinical specimens had been obtained as part of a study of cognitive function, and it was possible to determine that presence of chlorovirus DNA was associated with a slight but statistically significant decreased performance in tests for visual motor speed, delayed memory, and attention.
When mice were fed chlorovirus-infected algae, they showed decreased performance in tests of cognitive function, such as recognition memory and sensory-motor gating. Some of these animals developed antibodies against the virus, suggesting that viral replication took place. Furthermore, feeding of chlorovirus to mice was associated with changes in gene expression in the hippocampus, the part of the brain essential for learning, memory, and behavior.
It is not known if the chlorovirus replicates in humans or in mice; only viral nucleic acids were detected. No mention is made of attempts to isolate infectious chloroviruses from humans or mice. The amount of chlorovirus in the oropharynx is not known. However the results of sequence analysis, in which low numbers of sequences were found in each person suggest very low numbers of genomes. Of course, it is possible that virus replication took place some time ago, and its effects linger after replication has subsided.
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