Tularemia

Tularemia is a rare bacterial zoonotic infection caused by Francisella tularensis, which is a small, intracellular, Gram-negative coccobacillus . F. tularensis can be found throughout North America in a variety of animal hosts; including lagomorphs (rabbits and hares), aquatic rodents (muskrats, beavers, and water voles), other rodents (water and wood rats and mice), squirrels, and cats. F. tularensis can survive for weeks in contaminated water, soil, and vegetation. Airborne infections result from inhalation of dust contaminated with rodent feces that is generated during agricultural or landscaping activities. 

Most human infections result from bites by deer flies and ticks or direct contact with infected animals. Ingestion of contaminated water or the inhalation of aerosolized bacteria can also cause disease. Between 2011 and 2022, a total of 2,462 tularemia cases were reported in the United States. A mean of 205 cases was reported per year. Arkansas, Kansas, Missouri, and Oklahoma accounted for 50% of all reported cases. 

F. tularensis is highly infectious; as few as 10 bacteria can cause human disease through cutaneous or respiratory routes. The pathogenicity of F. tularensis is related to its capacity to evade and suppress the innate immune system while it replicates intracellularly. High infectivity and the potential for airborne transmission pose serious hazards for medical laboratory technologists. The clinical laboratory should be notified whenever tularemia is suspected so that additional safety precautions can be taken.

Tularemia can cause a wide range of symptoms in humans, depending upon the route of inoculation. After an incubation period of 2 to 6 days, the illness typically begins with the sudden onset of fever, chills, headache, and myalgias. Headache may be the dominant symptom. Pulse–temperature dissociation, which is a pulse rate that is lower than expected for a given elevation in temperature, is often present. Other manifestations of tularemia reflect the portal of entry: skin ulcers, tender lymphadenopathy, conjunctivitis, exudative pharyngitis, and pneumonia. Pneumonic tularemia can result from direct inhalation of organisms or bacteremic spread from an extrapulmonary site of infection. Common radiographic findings include unilobar or multifocal opacities, pleural effusions, and hilar adenopathy. The differential diagnosis of pneumonic tularemia includes other causes of atypical pneumonia, such as psittacosis and Q fever. 

The diagnosis of tularemia is most often confirmed serologically. Patients should be tested for both IgM and IgG antibodies against Francisella tularensis. A positive IgM result and negative IgG result suggest current or recent infection. Repeat testing in 1 to 2 weeks to detect seroconversion of IgG is helpful to confirm the diagnosis. A positive IgG antibody result, by itself, suggests recent or past infection. False negative results can occur if specimens are collected too soon following symptom onset. False positive results may occur in patients previously or currently infected with Brucella species. 

F. tularensis can also be isolated from blood, respiratory samples, and tissue-biopsy specimens, but it grows slowly on routine laboratory cultures. Recovery can be expedited with the use of cysteine-enriched media. Polymerase chain reaction can provide more rapid diagnosis, but is not widely available. 

Tularemia is a nationally notifiable disease in the United States. Cases should be reported to the National Notifiable Diseases Surveillance System. Francisella tularensis has been designated a tier-1 agent because of its potential for misuse as a bioweapon. No vaccine is available. 

References

Rich SN, Hinckley AF, Earley A, Petersen JM, Mead PS, Kugeler KJ. Tularemia — United States, 2011–2022. MMWR Morb Mortal Wkly Rep 2025;73:1152–1156. DOI: http://dx.doi.org/10.15585/mmwr.mm735152a1

Nelson CA, Sjöstedt A. Tularemia: a storied history, an ongoing threat. Clin Infect Dis 2024;78(Suppl 1):S1–3.

Nelson CA, Winberg J, Bostic TD, Davis KM, Fleck-Derderian S. Systematic review: clinical features, antimicrobial treatment, and outcomes of human tularemia, 1993–2023. Clin Infect Dis 2024;78(Suppl 1):S15–28.

CDC. Tularemia: clinical testing and diagnosis for tularemia. Atlanta, GA: US Department of Health and Human Services, CDC; 2024. https://www.cdc.gov/tularemia/hcp/diagnosis-testing/index.html