Epidemiology – Natural Reservoirs

• Humans acquire plague most often through the bites of infected rodent fleas. For naturally occurring cases of plague, the Oropsylla montana flea is the primary vector for plague. This type of flea is found mostly in rural rodent species, particularly the Rock Squirrel in New Mexico and Arizona. Urban plague from rats has not occurred in the U.S. in over 70 years. This is due to good public health surveillance and control and improved sanitation measures. If an urban plague event (natural or bioterrorism related) was to occur, the Xenopsylla cheopsis flea or "rat" flea would be the flea of most concern.
• The flea ingests a blood meal from a bacteremic animal. The bacteria multiply and block the gut of the flea. When the flea attempts to feed again, it regurgitates bacteria into human or animal mammalian host.
• The most common reservoirs for the bacteria are ground squirrels and wood rats.