Yersinia pestis bacteria -- the organism responsible for plague -- can grow with or without oxygen. Animals that are known to carry the bacteria include rats, prairie dogs, and fleas. During an outbreak, the bacteria can survive for months in cool, moist conditions, such as a rodent hole. Approximately 10 to 20 people in the United States develop an infection each year from flea or rodent bites.
Yersinia pestis are the bacteria that cause plague. This is a Gram-negative bacteria that can grow with or without oxygen (a quality called facultative anaerobic).
Yersinia pestis was formerly classified in the Pasteurellaceae family, but based on its similarities to Escherichia coli (E. coli), the Yersinia group has been reclassified as members of the Enterobacteriaceae family.
Although there are 11 named species in the genus Yersinia, only three are considered important human pathogens:
- Yersinia pestis
- Yersinia pseudotuberculosis
- Yersinia enterocolitica.
Yersinia pseudotuberculosis is the closest genetic relative to Yersinia pestis, but it can be distinguished from Yersinia pestis by the symptoms it causes and by laboratory test results. Neither of these bacteria frequently infect humans, in contrast to Yersinia enterocolitica, which accounts for 1 to 3 percent of diarrhea cases caused by bacteria.
Yersinia pestis is found most commonly in rats, but occasionally in other animals, such as:
- Prairie dogs
- Wood rats
Other, less frequent sources include wild rabbits and wild carnivores that pick up their infections from wild rodent outbreaks.
Deer mice and voles (field mice) are thought to maintain Yersinia pestis in animal populations, but are less important as sources of human infection.