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Mystery Rays from Outer Space
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| Krishna, milking a cow |
Vaccinia virus is a widespread virus whose natural host remains unknown. It turns out to be pretty good at jumping across species.
Vaccinia, of course, is the vaccine against smallpox. Even though smallpox is eliminated in the wild,1 vaccinia is still very widely used in research and even, to some extent, in the clinic, because the broad and deep experience with the virus gained from its importance in vaccination has carried over into other fields.
When Jenner developed his vaccine against smallpox, he used the cowpox virus. But — in spite of a widespread misconception — vaccinia is not cowpox. They’re quite distinct viruses, though they are related. At some point along the centuries of vaccine use cowpox was replaced by vaccinia. (It’s also worth pointing out that the disease Jenner called cowpox, may not have been cowpox as we know it today. 2 It may have been a distinct strain of virus, or it may have been a different virus altogether.)
Remember that for a couple of hundred years, there was no tissue culture to grow the virus in, and it was basically propagated by continually re-infecting animals and collecting virus from their scabs. At some point, presumably a cow that was being used as a vaccine incubator was infected with vaccinia instead of cowpox, and the vaccinia proved more effective, or perhaps safer or more convenient, as a vaccine, crowding out the vaccine cowpox.
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| “Cowpox inoculation sites” Douzhen dinglun (Definitive Treatise on Pox Diseases) by Zhu Chunxia, 1888 |
(By the way, it’s interesting to note that cowpox has an MHC class I immune evasion function,3 whereas vaccinia virus does not. Obviously this immune evasion doesn’t prevent cowpox from acting as a strong immunogen, because it was an effective vaccine for decades if not centuries, but perhaps it’s one reason vaccinia was a more popular vaccine.)
Where vaccinia virus came from — what animal it was infecting before it jumped into cattle — no one knows. Although vaccinia must have (or have had) a natural host at one point, the true host for the virus is now cultured cells in the lab incubator.
Does that mean vaccinia isn’t …