A bit of further digging supports my suspicion that this story is bunk:
Cats and the Black Death
Relevant section states:
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The story that the stupid medieval people, at the instigation of their even more stupid clergy, killed thousands of cats and so died in even greater numbers during the 1340s epidemic as a result is popular and widespread. A quick Google on relevant key words will turn up a plethora of articles of the “strange true facts about history” clickbait variety that repeat the story, such as “
Cats and the Black Plague” or “
That One Time The Pope Banned Cats And It Caused The Black Plague” or “
Cat History: The Black Plague” or dozens of others. These articles are marked by a total lack of any reference to source material substantiating claims about a general medieval massacre of cats, a lack of any references to historical analysis of the Black Death or, if they have any references to the latter, a lack of any such references that actually mention any massacre of cats. Why? Because it did not happen.
The whole story is one of those pseudo historical urban myths that keeps getting repeated despite the fact it’s complete nonsense. And it gets repeated because it feels right to many people – it makes our ancestors look stupid and so make us feel smart, it blames the medieval Church for something that popular culture says is the kind of thing the medieval Church would do and it’s a nice story with an ironic ending. So no-one actually bothers to check on one rather important element: whether it is actually true.
The few versions of this story that bother to give anything like some substantiation claim that cats were declared servants of evil by Pope Gregory IX in 1232 or even that he declared that they should all be killed. That sounds highly specific and substantial, though some might notice that 1232 is over a century before the first appearance of the Black Death in 1347 and wonder why it took this long for any supposed cat massacre to cause the plague. Other versions of the story say the antipathy towards cats began with Gregory IX’s papal bull and then grew until the lack of cats in Europe made the plague particularly catastrophic.
But did Gregory IX declare all cats evil or order their destruction? Actually, no. The “1232” reference seems to be to Gregory’s papal bull Vox in Rama, issued in that year, which addressed an alleged outbreak of devil worship in Germany. This bull gives a description of the ceremonies of this group of “Luciferians”, which includes many standard tropes found in lurid medieval ideas about heretical practices. This involved visions of a giant toad, initiates kissing an emaciated pale man and finally a statue of a black cat coming to life and speaking with the initiates. Nowhere does the bull associate this diabolical cat with cats generally, condemn all cats or call for their slaughter. Yet the claim that this bull somehow did cause massacres of cats continues to be made, usually with no reference to any supporting evidence at all.
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