Yersinia bacteria is responsible for the disease yersiniosis, a diarrheal infection contracted after eating contaminated food. This disease is caused by two species, Yersinia enterocolitica and Y. pseudotuberculosis, both related to Y. pestis, the bacterium responsible for the plague or “black death.” All are of the family Enterobacteriaceae to which Salmonella also belongs. Y. enterocolitica is a Gram-negative facultative bacterium, frequently isolated from pigs who carry the bacteria in their throats. It resembles the Gram-positive Listeria monocytogenese.
Y. pestis is a Gram-negative coccobacillus that displays bipolar staining in laboratory procedures. A variety of toxins are associated with Y. pestis, the most significant being the V and W toxins. Y. pestis is able to multiply in a various mammal and insect hosts, causing three forms of the disease.
The first or natural form is found in wild rodents, and called ‘sylvatic’ plague. This type of plague is transmitted by fleas to over 200 animal species. Some die of the disease, while others show few signs.
The second form occurs when the microorganism is spread to urban-type animals – mainly rats. This is called the ‘urban’ or ‘domestic’ plague. Urban rats frequently die of the disease, which causes their fleas to seek a new host. With humans living nearby, it is natural that the fleas seek them out, starting the third cycle or form of the disease.
This third cycle is called the ‘human’ plague and begins with the fleas – principally the Xenopsylla cheposis, the Oriental rat flea – biting the legs of a person. This causes the bacteria to travel to the closest lymph nodes, usually in the groin, where they multiply extensively. The swollen and incredibly painful lymph nodes are called buboes. It is this form of the disease that is referred to as ‘bubonic’ plague.
The buboes usually appear less than a week after the flea bite. Fever, chills, nausea, malaise, and pains usually precede the development of the buboes. Since the spread of the bacteria does not stop at the lymph nodes, bacteremia results. The presence of Y. pestis in the blood is referred to as ‘septicemia’ plague, which results in large purple skin lesions that spread over the entire body, resulting in the term “black death.”
Pneumonic plague results when the disease spreads to the lungs from untreated bubonic plague. This is the only form that can be spread from person to person. Transmission is via their infected breath rather than from the bite of a flea. Bacterial emboli become trapped in the lungs, causing this deadly form to develop. Those contracting pneumonic plague develop severe signs of the disease and die within two or three. The death rate from untreated bubonic plague is 50%-75%, whereas pneumonic plague claims almost 100% of its victims. Another form, ‘plague meningitis,’ results as the bacteria stream through the blood spreading infection everywhere, including the covering of the spinal cord.
Y. pestis is highly susceptible to streptomycin, tetracyclines, and chloramphenicol. Early treatment is extremely important, especially for pneumonic plague, because little can be done if treatment is not meted out the first day. Proper treatment of bubonic plague has reduced mortality to less than 5%. Both living and killed plague vaccines are available. Those travelling to southeast Asia, in particular, are urged to take the three doses of killed vaccine before leaving.
The first well documented pandemic of the plague occurred in 550 CE, resulting in an estimated 100 million deaths over a sixty-year period. The next major pandemic occurred in the 14th century, claiming 25% of the European population. The last major epidemic occurred in China toward the end of the 1800s. China’s population was reduced from 123 million to 65 million in less than 200 years. During the Renaissance, Paris had a death rate of 800 per day, and Jews were blamed for the outbreak. They were exterminated at a rate not seen again until the Holocaust. In Strasbourg alone, 16,000 were murdered. Bubonic plague in 1346 killed twenty million people, or about 60% of the population worldwide. To put this into perspective, AIDS has killed only about two million from a worldwide population of six billion – or roughly 3.3%.
A more common disease caused by Yersinia is from Y. enterocolitica. Most outbreaks from Y. enterocolitica can be traced to raw milk, chocolate milk, mussels, oysters, tofu, pork, and contaminated water. Infection is characterized by acute abdominal pain, profuse and sometimes bloody diarrhea, and headache. Vomiting can also occur, but not always. These symptoms have often been misdiagnosed, causing unnecessary appendectomies. Patients can often show signs of inflammed abdominal lymphatic structures (mesenteric adenitis). Recovery is usually uneventful and complete.
Y. enterocolitica can also reproduce quite well under refrigeration, as well as in undercooking. It has been found ten days after refrigeration where a few hundred cells multiplied into millions. Another interesting feature of this organism is that it is motile when grown at room temperature, but not when grown at normal body temperature of 37C (98.6°F).
