Credited to Hong Kong. Kitasato reported a bacillus on June 14, 1894, days before Yersin, and was credited for some years. His description was imprecise and expert opinion later gave the credit to Yersin. Both worked in Hong Kong, so the place of discovery is not in question.
How it began
Early in 1894 bubonic plague broke out in Canton and Hong Kong, the start of the third plague pandemic. Two bacteriologists worked independently in Hong Kong to find the cause. A Japanese team led by Kitasato Shibasaburo and Aoyama Tanemichi arrived on June 12, and on June 14 Kitasato reported that he had found the likely plague bacteria. Alexandre Yersin of the Pasteur Institute arrived three days after Kitasato. Refused hospital laboratory space, he set up a rudimentary laboratory in a hut near the hospital and within about a week found the bacillus in pus taken from the buboes of plague victims.
Before it existed
For many centuries diseases such as bubonic plague, smallpox and cholera were thought to come from miasmas, disease-carrying vapors rising from corpses, rotting matter or the breath of the sick. Others blamed plagues such as the Black Death on earthquakes, comets or divine punishment. In Hong Kong in 1894 the acting hospital superintendent James Lowson at first did not accept germ theory, and changed his position after Kitasato reported his findings.
How it works
Yersin inoculated mice with pus taken from a bubo on a corpse. When a mouse died, its spleen contained very small, stocky, round-tipped bacilli that were hard to stain. In the following week he grew pure cultures of the bacillus on medium and showed for the first time that the same bacillus was present in rats as well as in human cases, which pointed to how the disease might travel. Kitasato had found bacilli in the bubo pus, blood and organs of a victim, cultured them in broth and inoculated mice and other animals.
What changed first
Both men announced isolation and culture of the bacillus during June 1894. Lowson reported Kitasato's finding to The Lancet, and Yersin published in the Annales de l'Institut Pasteur under the title La peste bubonique a Hong Kong. Back at the Pasteur Institute in 1895, Yersin began work with Amedee Borrel and Albert Calmette on an anti-plague serum, returned to Hong Kong in 1896 hoping to test it, and continued trials in Bombay in 1897.
What lasted
Kitasato was credited with the discovery for some years, but his description lacked precision and Yersin's account is now considered the more accurate. Yersin's suspicion about rats led Paul-Louis Simond to show in Karachi in 1898 that fleas carried the bacillus between rats. Around 1900 Waldemar Haffkine developed a killed plague vaccine, and later workers produced further killed and live vaccines. The organism was first named Bacterium pestis and was renamed several times before becoming Yersinia pestis in 1970.
