Tularemia by age and sex/CDC
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. There are more than 12 tick vectors that can transmit tularemia. A. americanum and D. variabilis are most common in the Southeast and South-central 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 hunting season. Males are more likely to become infected, probably because they are outdoors/hunting more frequently. About 125 cases per year are reported to the CDC. As of September 30, 2015, 100 tularemia cases were reported just among residents of Colorado (n = 43), Nebraska (n = 21), South Dakota (n = 20), and Wyoming (n = 16). According to the CDC's Morbidity and Mortality Weekly Report published December 4, 2015 (64:1317-1318), "This is a marked increase (975%, 200%, 186%, and 70%, respectively) in the annual mean number of cases reported in each state during 2004 to 2014."
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.