I attended a religious university (whose affiliation and history with the Catholic Church is mixed at best) for a brief period as a history major and studied the bubonic plague outbreak of 1346-1353 one semester. I came across the idea that one reason the Black Death was particularly virulent was because Pope Gregory IX issued a Papal Bull declaring that cats were often instruments of Satan and encouraged Christians to kill cats throughout Europe. This led to a rapid decline in the cat population which were often used to curb rat infestations.

My question is this: How likely is it that Pope Gregory IX's Papal Bull on cats contributed to the devastation of the bubonic plague in 14th century Europe? Is this just an anti-Catholic legend?