Tularemia is caused by Francisella tularensis. Francisella is a small, gram-negative coccobacillus (GNCB). Tick bites account for half of the cases, but infection can also occur by the bite of a deer fly or mosquito or from handling infected rabbits. More than 12 tick vectors can transmit tularemia. A. americanum and D. variabilis are most common in the Southeast and Southcentral states. D. andersoni is the most common vector in the West. Drinking contaminated water and aerosols can also be routes of infection, but there is no person-to-person transmission.
Most cases are reported between June and September when the vectors are active and people are outdoors. There is a secondary peak of infections in December during the hunting season. Males are more likely to become infected, possibly due to exposure through hunting and outdoor work. About 125 cases per year are reported to the CDC, most commonly in the south central U.S., Pacific Northwest, and Martha's Vineyard in Massachusetts.
F. tularensis is a CDC category A select biological agent, which must be handled in a Class II or higher biological safety cabinet (BSC). Laboratory-acquired cases have been reported. This is a highly virulent organism, and it is important to have a high level of suspicion in these cases so that proper safety measures can be followed.
