Yersinia

• TRANSMISSION ROUTE: Ingestion, inhalation, skin

Definition: Three species of Yersinia affect humans, two causing intestinal infections and one causing the plague, an acute, contagious disease

Natural Habitat and Features

Yersinia bacteria are typically rod-shaped, though at times they can approach a spherical shape, and are usually 0.5 micrometers (μm) in diameter and one to three μm in length. Yersinia are gram-negative. They are motile below 86° Fahrenheit (30° Celsius) except for some Y. ruckeri strains and Y. pestis, which is never motile. The optimal temperature for these bacteria is 82° to 86° F (28° to 30° C), although Y. enterocolitica is often found in cold climates.

Yersinia occur in a variety of habitats, including soil, water, and foods such as dairy products, and are present in birds, animalsespecially rodentsand humans. Most species are occasional human pathogens. Y. ruckerii causes red mouth in fish, and three speciespestis, pseudotuberculosis, and enterocoliticacause infections in humans and other mammals.

Draft sequencing of the various Yersinia species indicates that genes have been horizontally transferred and that virulence determinants have been gained and lost over time. There is a high degree of genetic redundancy among the Yersinia species. Later scientific developments have led to the ability to distinguish among the various species with a high degree of accuracy. A close evolutionary relationship exists among the enterocolitica clade (descendant) strains. Four speciesbercorieri, mollaretii, aldovae, and ruckerievolved from Y. enterocolitica. Y. pestis evolved from Y. pseudotuberculosis as early as 20,000 years ago with significant branching; this resulted in a clear split around 6,500 years ago.

Pathogenicity and Clinical Significance

Three species of Yersinia pose health threats to humans. The three species can be distinguished from each other by laboratory tests and by their symptoms. Most notable is Y. pestis, the causative agent of plague. There have been three plague pandemics dating from the plague of Justinian in the sixth century. The second, often referred to as the Black Death, first appeared in southern Europe in 1347 and then spread over much of the continent during the next four years. It is estimated that this pandemic killed between 30 and 60 percent of the population of Europe. Plague remained endemic to European society, with periodic epidemics until the early eighteenth century. The third pandemic began in western China in the 1860s, spread to Hong Kong by 1894, and to India, Java, Egypt, and San Francisco by 1900, killing more than one hundred million persons worldwide. Plague remains endemic to some parts of the world, such as Mongolia and the Four Corners region of the United States. The first plague pandemic probably originated in Africa, with later ones originating from Asia. Alexander Yersin identified Y. pestis as the agent of plague during the third plague pandemic. Although most scholars credit Y. pestis as the causative agent of the first and second plague pandemics, the diagnosis is disputed by some.

Three varieties of plague existbubonic, septicemic, and pneumonic. Bubonic is the most common form. It is transmitted by infected fleas, which bite humans after having bitten infected rats or other animals. Left untreated, bubonic plague has a mortality rate between 40 and 70 percent.

Septicemic plague arises as a secondary infection from a primary bubonic infection and is generally fatal. Pneumonic plague often occurs as a secondary infection but may also be a primary infection spread from human to human through nasal discharge. Untreated, it is fatal.

The incubation period for bubonic usually ranges from two to six days. It is followed by the sudden onset of chills, fever, and headache, followed by body aches and possibly diarrhea. Painful swollen lymph glands, often in the groin, are a telltale sign of bubonic plague. Septicemic plague arises from a bubonic infection, producing nausea and diarrhea. Buboes are uncommon in septicemic plague. The patient soon becomes moribund, has multiorgan failure, and dies. Pneumonic plague is often secondary to bubonic or septicemic plague, but is also spread from human to human. It exhibits sudden-onset fever with chest pain and purulent sputum; death often follows. Often, no buboes are present.

Y. entercolitica is a common form of Yersinia pathogen. It produces acute bacterial gastroenteritis that especially affects the young. A food-borne pathogen, it colonizes the small intestine and may exhibit symptoms similar to other intestinal ailments.

Y. pseudotuberculosis is also a food-borne pathogen, although it is less common than Y. entercolitica. It often mimics appendicitis and generally does not cause diarrhea. Together, Y. entercolitica and Y. pseudotuberculosis cause about 117,000 cases of yersiniosis every year.

Drug Susceptibility

Infections from Y. entercolitica and Y. pseudotuberculosis are often managed without antimicrobials. Antibiotic regimes, however, may be prescribed for some persons. Fluoroquinolones, third-generation cephalosporins, and aminoglycosides are prescribed at times for Y. pseudotuberculosis, and ciprofloxacin is prescribed for Y. entercolitica.

Early antibiotic treatment is essential for treating plague, as untreated cases often have a high mortality risk. Outside the United States, streptomycin is often prescribed. Gentamicin is comparable or superior to streptomycin and is often used to treat plague. Doxcycline is used for persons who cannot take aminoglycosides. It is also recommended for mass casualties, such as those from an act of bioterrorism. Drug-resistant cases have been reported, however.

Vaccines have been developed against the plague, although no country requires them. Killed whole-cell plague vaccines may cause severe side effects, require a six-month course of vaccination, and are ineffective against pneumonic plague. Largely because of the threat of the use of plague as a biological weapon by terrorists, scientists are working to develop second and third-generation vaccines that avoid the problems of the first-generation vaccines.

Bibliography

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Carniel, Elisabeth. “Evolution of Pathogenic Yersinia: Some Lights in the Dark.” In The Genus “Yersinia”, edited by Mikael Skurnick and José Antonio Bengoechea. Kluwer Academic, 2003.

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“Plague.” World Health Organization (WHO), 7 July 2022, www.who.int/news-room/fact-sheets/detail/plague. Accessed 11 Nov. 2024.

Watkins, Louise Francois, and Cindy Friedman. “Yersiniosis.” CDC, 1 May 2023, wwwnc.cdc.gov/travel/yellowbook/2024/infections-diseases/yersiniosis. Accessed 11 Nov. 2024.