|smallpox (variola major)|
The largest known DNA viruses infect Acanthamoeba and belong to two markedly different families. The Megaviridae exhibit pseudo-icosahedral virions up to 0.7 μm in diameter and adenine–thymine (AT)-rich genomes of up to 1.25 Mb encoding a thousand proteins. Like their Mimivirus prototype discovered 10 y ago, they entirely replicate within cytoplasmic virion factories. In contrast, the recently discovered Pandoraviruses exhibit larger amphora-shaped virions 1 μm in length and guanine–cytosine-rich genomes up to 2.8 Mb long encoding up to 2,500 proteins. Their replication involves the host nucleus. Whereas the Megaviridae share some general features with the previously described icosahedral large DNA viruses, the Pandoraviruses appear unrelated to them. Here we report the discovery of a third type of giant virus combining an even larger pandoravirus-like particle 1.5 μm in length with a surprisingly smaller 600 kb AT-rich genome, a gene content more similar to Iridoviruses and Marseillevirus, and a fully cytoplasmic replication reminiscent of the Megaviridae. This suggests that pandoravirus-like particles may be associated with a variety of virus families more diverse than previously envisioned. This giant virus, named Pithovirus sibericum, was isolated from a >30,000-y-old radiocarbon-dated sample when we initiated a survey of the virome of Siberian permafrost. The revival of such an ancestral amoeba-infecting virus used as a safe indicator of the possible presence of pathogenic DNA viruses, suggests that the thawing of permafrost either from global warming or industrial exploitation of circumpolar regions might not be exempt from future threats to human or animal health.
In multiple reports they have stated that, "The discovery of Pithovirus shows how incomplete our understanding of microscopic biodiversity is when it comes to exploring new environments. And the potentially bad news: the reemergence of other viruses is no longer the domain of science fiction. These include viruses that were long considered eradicated such as smallpox, whose replication process is similar to that of pithovirus. This is an indication that viruses pathogenic for human or animals might also be preserved in old permafrost layers, including some that have caused planet wide epidemics in the past, said Jean Michel Claverie, one of the study's co-authors." Prof. Claverie warns that exposing deep layers could expose new viral threats. He has been quoted as saying that 'ancient strains of the smallpox virus, which was declared eradicated 30 years ago, could pose a risk. "If it is true that these viruses survive in the same way those amoeba viruses survive, then smallpox is not eradicated from the planet--only the surface." "By going deeper we may reactive the possibility that smallpox could become a disease of humans in modern times." See: http://www.bbc.com/news/science-environment-26387276
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