Surrounded by Viruses

Researchers calculate that there are more than 300,000 mammalian viruses still to be discovered.

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The estimation of at least

The estimation of at least 320,000 viruses that can affect mammals is simply based on the identification of 58 unique viruses in flying foxes multiplied by the number of known mammalian species, which was taken as 5,486. Simon Anthony and the other authors of the study acknowledge that this estimate assumes that all mammalian species are targeted by a similar number of viruses and that there is 100% host specificity. However, most viruses appear to be zoonotic and capable of infecting multiple species.

Using an N=1 with the Indian Flying Fox (Pteropus giganteus) as a typical representative of mammals in general is also somewhat tricky as these are extremely mobile and social creatures that reside in high density communities. In coming up with an estimation of viral diversity for a species, the best-studied mammals are actually humans, which admittedly are also atypical. Since 1901, at least 219 viruses have been identified that are capable of infecting humans, and this list has been growing by 3 to 4 new viruses each year (http://rstb.royalsocietypublishing.org/content/367/1604/2864.full).
The ViralZone webpage (http://viralzone.expasy.org/all_by_species/678.html) documents 124 pathogenic viruses that actually cause disease in humans. Of these, 74 (60%) are also found in other animal hosts including insects.

Using the average of the bat and human data, and a 40% host specificity value, the number of viruses than affect mammals can be calculated as equal to approximately 304,000 (=(219+58)/2 x 5,486 x 40%). This is very close to the estimate of at least 320,000 mammalian viruses offered by Simon Anthony and his colleagues at Columbia University.

The International Committee on Taxonomy of Viruses has catalogued a total of 2618 known viruses or viroids that either affect eukaryotes or prokaryotes in their latest list (http://talk.ictvonline.org/files/ictv_documents/m/msl/4440.aspx). The most recent list of known viruses from the Wikipedia: WikiProject Viruses features 3443 distinct viruses (http://en.wikipedia.org/wiki/List_of_viruses). Present knowledge about even the existence of all of the viruses on our planet is evidently extremely limited. Nevertheless, it is remarkable that that only a very tiny subset of the world’s viruses actually cause disease in humans and other mammals. This is perhaps because as a phylum mammals have been around for a much shorter time (i.e. 200 million years) than other vertebrates (over 540 million years), and mammals only really flourished after the demise of the dinosaurs 65 million years ago. Even today, mammals account for less than 0.1% of the biomass on our planet.

The ability of a virus to successfully infect a host requires co-evolution with the host species. Particularly virulent viruses that kill their hosts are not as effective in their propagation. More than half of the known viruses that infect humans cause disease, but the actual number of viruses that can stealthily replicate in humans without obvious evidence of morbidity may number in the multiples more. Future genomic sequencing of human fluid, stool and biopsy specimens will indubitably lead to the identification of much higher numbers of previously unknown human viruses.