Big game hunting in virology: pandoraviruses and their cousins

Earlier this month, the discovery of two new “giant” viruses, dubbed pandoraviruses, was published in Science (Philippe, et al., 2013). This discovery by husband-and-wife team, Jean-Michel Claverie and Chantal Abergel, and their collaborators in Marseille, France, has received a lot of press coverage, including Science's own report (Pennisi, 2013) and a New York Times piece by Carl Zimmer (2013). If you google “pandoravirus” you can find other coverage, including this summary by Ker Than for Inside Science News Service. The pandoravirus work reinforces the earlier breakthrough discovery of the first “giant” virus, mimivirus, a decade earlier, as well as subsequent discoveries of other “giant” viruses and related developments. Zimmer (2011) wrote about some of these earlier findings in the Epilogue of his superb (and short) expository book on virology for laypersons, A Planet of Viruses.

Pandoravirus on the cover of Science

The original serendipitous discovery of the mimivirus, as well as the more directed effort that discovered the pandoraviruses, focus on amoeba-infecting viruses. Algae and possibly other forms of sea life can also be infected by mimiviruses (Zimmer, 2011). As a layperson myself, I wondered whether more complex organisms, including humans, need to worry about “giant” viruses. It initially occurred to me that agents for most of the major infectious diseases of humans, and plants & animals of economic significance, had probably been identified. If any of these agents had been “giant” viruses, wouldn't we have known about them earlier?

Apparently not! Zimmer (2011) notes that the mimivirus has been found in the lungs of hospital patients with pneumonia. “It's not clear yet if mimiviruses actually cause pneumonia...or if they just colonize people who are already sick” (p. 91). More interestingly, Zimmer (2013) cites a preprint which reports finding another “giant” virus in the blood of (apparently) asymptomatic blood donors (Popgeorgiev, et al., in press). Some blood donors even have antibodies to these viruses.  Such viruses had never been seen before in human blood because conventional virus testing uses ultrafiltration, which removes “giant” viruses before you have a chance to find them. Popgeorgiev, et al. (in press) point to an earlier paper (Lagier, 2012) reporting yet another “giant” virus found in a human stool sample of (again) an asymptomatic person, suggesting that such viruses may be present in the gut flora. (Both of these papers are from the Marseille-area researchers involved in the original mimivirus discovery.)

The tentative evidence suggests, therefore, that “giant” viruses may be ubiquitous, despite their recent discovery. These viruses may be residents of the human viriome, and presumably the viriomes of other plants and animals. Naturally, a great deal of research needs to be done, but the hypothesis, once posed, seems eminently plausible. The “giant” viruses' influence on the health and sickness of their hosts remains to be known. While the pandoraviruses have deservedly attracted the headlines, the papers from Marseille on “giant” viruses in the human viriome foreshadow how such research could potentially benefit us in everyday life.

References

E. Pennisi, 2013: Even-bigger viruses shake tree of life. Science, 341: 226-227.

J.-C. Lagier, et al., 2012: Microbial culturomics: paradigm shift in the human gut microbiome study. Clinical Microbiology and Infection, 18: 1185-1193.

N. Philippe, et al., 2013: Pandoraviruses: Amoeba viruses with genomes up to 2.5 Mb reaching that of parasitic eukaryotes. Science, 341: 281-286.

N. Popgeorgiev, et al., in press: Giant blood Marseillevirus recovered from asymptomatic blood donors. J. Infectious Diseases, in press.

C. Zimmer, 2011: A Planet of Viruses. University of Chicago Press.

C. Zimmer, 2013: Changing view on viruses: not so small after all. New York Times, Science Desk, July 18, 2013.