@CliftonR My understanding is, as of the time Hansen discovered the bacterium, about 80%–90% of Europeans had familial immunity against leprosy and were not suspectible, but since epidemiology was a very new study at the time — I mean, a certain pump's...

[riley@toot.cat]

@CliftonR My understanding is, as of the time Hansen discovered the bacterium, about 80%–90% of Europeans had familial immunity against leprosy and were not suspectible, but since epidemiology was a very new study at the time — I mean, a certain pump's handle hadn't been removed on Broad street of London even twenty years earlier —, plus leprosy has notoriously long incubation period, this used to be not understood, and the familiar clusters used to be misinterpreted as infectional clusters. 80% general immunity contributes to herd immunity, but, depending on the infection rate, not necessarily enough to make the pathogen non-viable, so leprosy used to be a problem even though most people were immune.

Also, AFAIU, the immunity rate might have been significantly lower in Europe a thousand years ago.

I'll see if I can find some online sources for this. My original source was a book that I read many years ago and can't easily access right now.

[cliftonr@wandering.shop]

@riley

FWIW I did check the Wikipedia article on leprosy and found no mention of any such connection.

I did find another article suggesting something quite different for the lowered rate of leprosy in Europe:

That because leprosy increased the susceptibility to catching the bubonic plague from flea bites, the bacterium causing it nearly died out in Europe during the repeated waves of the "Black Death" from the Middle ages to early modern times.

[riley@toot.cat]

@CliftonR That's an interesting theory, but soon after the post-Plague repopulation, the Hundred-Year War and then the Renaissance hit, cross-European travel became commonplace again, and by the time of the High Age of Sail, Europeans still had enough of a leprosy reservoir to take it all the way to the New World, where it hadn't been a thing before, and now haunts innocent armadillos in Florida. Yet, leprosy prevalence in Europe never again reached the former peaks. For a context of what epidemiologyc patterns look like after a disease's temporary defeat due to killing too many people and temporarily burning out its stomping grounds, consider the waves of plague epidemic recurring on a generational schedule after the first wave. Contrariwise, smallpox is not returning because its wild reservoir has been fully cleansed of it. Leprosy's wild reservoir didn't go away at around the time of the Black Death.

If there's a 'leprosy wounds helped The Plague spread' connection, perhaps it was that people dying from The Plague were preferentially those who lacked the pre-existing genetic immunity for leprosy? But then again, the Black Death was big in Europe, but it was not an Europe-only thing, and it's strange that it wouldn't have the same effects in Asia.

[riley@toot.cat]

@CliftonR , @oblomov , @WhyNotZoidberg : This is an interesting section from the International Textbook of Leprosy Dot-Org:

From the 14th century onwards, leprosy was in decline, particularly in Europe. However, more recently, the infection has increased in some parts of the world such as Brazil and southeast Asia. There have been many possible reasons suggested for this decline. Four of the most cited are considered here:

• The segregation of people affected. However, not everyone was segregated, some people were not diagnosed, and some evaded diagnosis.
• The Black Death affecting weaker people, such as those with leprosy. However, leprosy was in decline before the Black Death [68]. Furthermore, people with leprosy would have been at no more risk than people with other health challenges.
• The climate in 1300–1500 AD Europe, when temperatures were lower, shortening the survival time of M. leprae [69], [70]. However, leprosy has been found in human remains in the Old World ranging from the Mediterranean to the cooler North and Baltic seas. This evidence suggests that a decline in temperature is unlikely to have led to the decline of leprosy.
• Tuberculosis (TB), due to the nature of cross immunity between the two infections [71], [72], [73]. Both TB and leprosy are caused by mycobacteria and, therefore, increasing exposure to TB caused leprosy to die out, making the population immune to leprosy. This hypothesis has not been thoroughly tested in the archaeological record, but it is interesting to note that the pattern of bioarchaeological evidence for TB mirrors that for leprosy [74]. This hypothesis, therefore, cannot be supported with hard scientific evidence yet.

Donoghue et al. have also suggested that people with LL are more susceptible to TB [75] and that co-infection may be a cause for leprosy’s decline [76]. Thus, both the cross-immunity and co-infection hypotheses of leprosy’s decline could be important.

Source: https://www.internationaltextbookofleprosy.org/chapter/bioarchaeology-leprosy-learning-skeletons.

[cliftonr@wandering.shop]

@riley

Oh, that's an interesting source and cite! Thanks for digging that up - I'll read through it.

It does make it sound like no scholars are completely certain what was going on with that decline, one way or another.

[riley@toot.cat]

@CliftonR @oblomov @WhyNotZoidberg

Perhaps the famished Europeans ate so many squirrels during the hungriest centuries that they eliminated a major zoonotic transmission pathway, and that is, or used to be, more common, at least in the European environmental conditions, than the human-to-human transmission?

Among the questions that still linger is how leprosy spread across Europe in centuries past. Researchers can’t be certain, but they did make an intriguing discovery while reconstructing the genome of an individual from Great Chesterford, England, whose remains date between 415 and 545 A.D. The individual represents one of the oldest known leprosy cases in the United Kingdom, and the M. leprae strain extracted from the skeleton is the same one that has been found in modern-day red squirrels.

Source: https://www.smithsonianmag.com/smart-news/did-leprosy-originate-europe-180969061/.

[riley@toot.cat]

@CliftonR Yep. It's kind of a two-edged sword. On one hand, the mysteries that haven't been solved yet are often the most interesting ones, on the other hand, I'm not in a position to work towards solving this one.