There are different types of Yersinia, which are bacteria that can grow with or without oxygen. This family of bacteria cause plague and are responsible for approximately 1 to 3 percent of diarrhea illnesses. Only 3 out of the 11 known species are infectious in humans.
Yersinia are Gram-negative bacteria that can grow with or without oxygen (a quality called facultative anaerobic). Although there are 11 named species in the genus Yersinia, only three are considered important human pathogens:
These bacteria were formerly classified in the Pasteurellaceae family, but based on their similarities to Escherichia coli (E. coli), the group has been reclassified as members of the Enterobacteriaceae family.
Yersinia pestis are the bacteria that cause plague. They are found in rodents and their fleas, and occur in many areas of the world, including the United States. Other animals can carry them as well. Rock squirrels and their fleas are the most frequent sources of human infection in the Southwestern United States. As for the Pacific states, the California ground squirrel and its fleas are the most common source.
Transmission from infected animals generally occurs in one of three ways:
- Bites from infected rodent fleas (about 85 percent of cases)
- Direct contact with infected tissue or bodily fluids
- Inhaling infected droplets.
(Click Plague Transmission for more information.)