Rat collection station, Philadelphia, date unknown but shortly after 1900.
Whatever happened to the Bubonic Plague?
Like so much that is fundamental to history (Whatever happened to the Dust Bowl? Why did the Japanese bomb Pearl Harbor?), this question is sidestepped by most grade-school teachers.
The simple truth is that RATS ended Bubonic Plague pandemics in Europe -- an odd but true story that, no doubt has been suppressed by public health officials uncomfortable with such inconvenient truths.
First, a little history. Plague has probably swept through the Old World time and time again. Plague-like pestilences are mentioned in the Bible (the so-called “scourge of the Philistines”), and at least five great outbreaks are noted by historians.
For modern purposes, however, the "big plague" was the so-called "Black Death" that swept through Europe beginning in the 14th Century and carried forward, with fits and starts, until the middle of the 17th Century.
The first wave of this plague killed off one-third of the population of Europe within two years of its arrival in the port of Messina, Sicily in 1346.
The vector, or transmission agent, for this wave of Bubonic Plague was the black rat Ratus ratus, which was host to the black rat flea, Xenopsylla cheopis, which in turn was host to the bacterium Yersinia pestis that actually causes the Plague.
Large-scale incidents of plague in Europe ended with the arrival of a very aggressive new immigrant -- Ratus norvegicus, aka the Brown or "Norwegian" rat.
In fact this rat is not Norwegian. It probably originated in Asia, and got to Europe through the Middle East, first arriving in England on a load of timber from Norway (hence the name given to it by the British)
The Brown Rat and the Black Rat look somewhat similar, but they have very different temperaments.
A Brown Rat is not only larger that its Black Rat cousin, it is also far more aggressive. When the Brown Rat arrived in Europe and began to multiply, it quickly pushed the smaller and more mouse-like Black Rat out of buildings, alleys, storage sheds and sewers. In fact, over time, it pushed the Black Rat almost totally out of existence in the temperate world.
Though fleas and lice are opportunists, they tend to gravitate towards, and specialize in, certain hosts. Different species of bird lice, for example, specialize in different species of birds. In fact, many species of bird lice can only be found on very specific bird species. The extinction of a bird species may, in turn, result in the extinction of one or more species-specific types of bird lice.
Many types of flea also gravitate towards, and special in, certain kinds of hosts. Though a species of flea may theoretically be able to draw a blood meal from a wide variety of mammalian hosts, most thrive on a specific list of hosts and generally fail to thrive if these particular hosts are not around.
So it is with Xenopsylla cheopis, the oriental rat flea, which is the flea most likely to be implicated in transmission of the Bubonic Plague.
The oriental rat flea thrives on a few species of rodents, and the Black Rat is far and away the most common of its rodent-host carriers.
With the rapid spread of the Brown Rat in Europe, the Black Rat was bullied and beaten into extirpation across most of the civilized world.
Today the Black Rat is commonly found only in the tropics. Even there it is most likely to be found high up (running along roofs and feeding at the tops of date palms) in order to avoid running into the neighborhood bully, the Brown Rat.
Bottom line: the Bubonic Plague was brought to Europe by fleas riding on Black Rats, while Brown Rats largely drove that species of rat out of Europe (and much of the rest of the world), thus eliminating the oriental rat flea and the Yersinia pestis bacteria that brought with it the Bubonic Plague.