Learning biology in school, I became interested in the fact that there aren't any diseases by prions which are globally infectious (as far as I know), unlike diseases by viruses (ex. COVID-19, SARS etc.) or by bacteria (ex. plague, ...). Why is it?

cf. Here are some my hypothesis

  1. Prions are not efficient pathogens because they don't have DNAs.
  2. Such prions are not yet discovered or does not exist.
  3. There exists lots of people who are immune to such diseases.

Are there any answers to the questions? Or is there any answers?

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    $\begingroup$ Add #4: Prions are bound up in cells, and not readily transmitted through the environments. You might contrast respiratory viral diseases like COVID or influenza, that are transmitted through the air, with ones like AIDS that require exchange of body fluids. $\endgroup$ – jamesqf May 11 at 3:30
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    $\begingroup$ Kuru, a neurodegenerative spongiform encephalopathy, is considered by many to have reached epidemic proportions in Papua New Guinea in the 1950s, caused by cannibalism of dead relatives, mainly by women & children. Carleton Gajdusek received a Nobel prize for his work on Kuru. (For a disturbing BBC documentary on Carleton Gajdusek, see Storyville The Genius And The Boys) $\endgroup$ – user338907 May 11 at 9:10
  • $\begingroup$ I think it requires cannibalism. $\endgroup$ – David May 11 at 18:11
  • $\begingroup$ I thought CJD could be spread by surgical instruments (and that 'conventional' sterilization methods is not enough to 'kill' the prion)? (See Iatrogenic Creutzfeldt-Jakob disease via surgical instruments, for example) $\endgroup$ – user338907 May 11 at 20:48
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    $\begingroup$ @Jamesqf hepatitis and AIDS are pandemics affecting millions of people, so why would that vector not a threat? quote: Worldwide, 500 million people are estimated to be infected with hepatitis B or C. ... isn't that a pandemic, because it's not transmitted readily through the environment? $\endgroup$ – DeltaEnfieldWaid May 12 at 8:55

I think that you could argue that some prion diseases do form epidemics or outbreaks. In some cases it does appear that people have referred to BSE/vCJD as pandemics. There is even some journalism on the topic, and see also here.

Infection by prion from the environment does occur at some rate, and it is true that it is these prions are the more transmissible. See Table 1 in that linked paper for more information; it appears that the discovery of prion excretion through e.g. feces is relatively recent.

For less transmissible variants it is true that you usually have to directly ingest a fair amount of prion from (i.e. eat) an infected individual to be affected, which is a harder means of transmission because it implies cannibalism (which is not that uncommon biologically, but still).

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    $\begingroup$ Might be noted that the pandemic-like outbreak of BSE supposedly was the result of (unintentional) cannibalism, as cow feed producers included waste from meat processing in their products. $\endgroup$ – jamesqf May 11 at 3:26
  • $\begingroup$ @jamesqf a good point. So which transmission mechanism is most efficient can be expected to vary a lot across environments; obviously a feedlot / meatpacking supply chain has a significantly different ecology from a "natural" population. $\endgroup$ – Maximilian Press May 11 at 18:03
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    $\begingroup$ In order to get large outbreaks via cannibalism/consumption one typically needs not just cannibalism but group cannibalism, i.e. one individual gets eaten by several others royalsocietypublishing.org/doi/full/10.1098/… $\endgroup$ – Ben Bolker May 14 at 0:57

Viruses mutate relatively fast, i.e. Aids mutates 100,000 times faster than Eurkariotic DNA. Actually, viruses mutate on average once every time they reproduce into a new copy / per genome replication.

The DNA segment that encodes healthy PRPC protein which can be hyjacked and converted by a prion has 3kb base pairs, and there are only 30 mutant version of PRPC known as prions detailed since 1970, compared to thousands of different versions of Covid-19 since 2019. The mutant prions dont use RNA, they actually latch on to the healthy versions inside the human body and changes them to mutants.

We can say that prions evolve 10,000 to 100,000 times slower than other bodily fluid viruses like hepatitis and AIDS. Those mutations serve to keep the viruses fit, resistant to new environments, new species, new biochemistries.

  • $\begingroup$ Prions don't have a nucleotide component, it is entirely protein, so prions and viruses (or any nucleotide containing organisms) aren't comparable directly in terms of evolution. It's also that they are incredibly slow - takes years (decades?) to see the effects in any individual. $\endgroup$ – bob1 May 12 at 8:47
  • $\begingroup$ Precisely, the question was comparing prions to quote: like diseases by viruses (ex. COVID-19, SARS etc.) or by bacteria (ex. plague, ...). So I detailed the difference in replication process and mutation rate... $\endgroup$ – DeltaEnfieldWaid May 12 at 8:52

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