Yersin and the plague bacillus

Plague reached Canton in March 1894, where by Alexandre Yersin's account it killed more than 60,000 people in a few weeks, and broke out in Hong Kong at the start of May. The French colonial ministry, afraid it would spread to Indochina, sent Yersin to study it. Born in Switzerland in 1863, he had worked with Émile Roux at the Institut Pasteur on diphtheria toxin before becoming a ship's doctor in Asia.

The Japanese bacteriologist Shibasaburo Kitasato, who had trained in Robert Koch's laboratory in Berlin, arrived three days before Yersin with six assistants and was given autopsies at once. Yersin came alone on 15 June, carrying a microscope, a sterilizer and culture supplies, and was kept from autopsies for several days. With British permission he built a straw hut in the grounds of the main hospital as his laboratory. On the day Yersin arrived, the colony's medical officer, James Lowson, wired The Lancet that Kitasato had found the cause.

Yersin found the pulp of the buboes packed with a short, stubby bacillus with rounded ends that took aniline dyes but did not stain by Gram's method. It grew on agar, and when injected into mice, rats and guinea pigs it killed them, leaving bacilli in their glands, spleen and blood. Noting many dead rats in the infected districts, he wrote that rats were probably the main carriers. He sent a sealed bubo specimen to Paris, where Albert Calmette and Amédée Borrel confirmed his findings, and published his report in the Annales de l'Institut Pasteur that September.

Kitasato also reported a bacillus, first found in blood, but he described it as gram-positive and in blood as a diplococcus, which led some to suspect contamination with the pneumococcus. Yersin's descriptions proved accurate, and since about 1970 the organism has been named Yersinia pestis.

In 1898 Paul-Louis Simond identified the rat flea as the carrier that passes the bacteria from rats to people. Yersin founded the Pasteur Institute at Nha Trang in Vietnam in 1895, treated plague patients in Canton and Amoy with an antiserum in 1896, and died at Nha Trang in 1943.