Tularemia in the United States, 2011–2022

Tularemia is a rare disease caused by the bacterium Francisella tularensis, which passes to people from animals and the environment. Because it could be misused as a bioweapon, the organism is designated a tier-1 select agent. There is no vaccine. People can be infected through the bite of an arthropod such as a deer fly or tick, by handling infected animals improperly, by breathing contaminated aerosols or by drinking contaminated water.

Symptoms usually include fever plus local signs that depend on how the bacteria entered — skin ulcers, regional lymphadenopathy or pneumonia. Antibiotics treat it. The case fatality rate is typically under 2% but can reach 24%, depending on the strain and how the disease presents. Tularemia has been reported from every state except Hawaii and is nationally notifiable: state health departments report cases to CDC through the National Notifiable Diseases Surveillance System. This MMWR report reviews the cases reported from 2011 through 2022.

Counting cases

A confirmed case is a compatible illness with F. tularensis grown in culture, or a fourfold change in antibody level between samples taken early and later in the illness. A probable case is a compatible illness with a single raised antibody level, or bacteria detected in a clinical specimen by fluorescent assay; from 2017, detection by polymerase chain reaction (PCR) also counted.

What was reported

• 2,462 cases: 984 (40%) confirmed and 1,475 (60%) probable.
• A mean of 205 a year, from 149 in 2012 to 314 in 2015.
• Average yearly incidence of 0.064 per 100,000 people.
• Cases in residents of 743 counties in 47 states. Four central states reported half of all cases: Arkansas (18%), Kansas (11%), Missouri (11%) and Oklahoma (10%).
• Illness began from May through September for 78% of patients.

Reported tularemia cases by county of residence, 2011–2022. CDC figure.

Who got sick

Patients' median age was 48 (range 1–95), and 63% were male. Most were White (84%), followed by American Indian or Alaska Native (9%), Black or African American (2%) and Asian or Pacific Islander (1%) people; 5% were Hispanic or Latino.

Incidence was highest among:

• American Indian or Alaska Native people, at 0.260 per 100,000 — about five times the rate among White people (0.057);
• children aged 5–9, at 0.083;
• adult males, especially those aged 65–84, at 0.133–0.161.

The authors noted that several factors might raise the risk among American Indian and Alaska Native people, including the concentration of reservations in central states and cultural, social or work activities that bring more contact with infected wildlife or arthropods.

A rising rate

The average yearly incidence of 0.064 was 56% higher than in 2001–2010 (0.041), and the increase came mainly from more reports of probable cases. Probable cases made up 60% of the total, compared with 35% in the previous decade — a 71% increase. The incidence of probable cases (0.038) exceeded that of confirmed cases (0.026) over the period, had done so every year since 2015, and diverged from the confirmed trend starting in 2017. Probable incidence was highest among children aged 5–9 (0.052), American Indian or Alaska Native people (0.185), and residents of Arkansas (1.073), South Dakota (0.642), Kansas (0.432) and Oklahoma (0.367).

Tularemia incidence by case status and year, 2011–2022. CDC figure.

Probable cases were already increasing before PCR was added to the definition in 2017, so the new criteria do not fully explain the trend. More probable cases might mean more infections, better detection, or both. Another change: some commercial laboratories moved from agglutination tests to more sensitive but less specific enzyme-linked immunosorbent assays, whose results cannot readily be compared between early and later samples. Cases that would once have been confirmed by a fourfold change are now classed as probable.

Limitations

• CDC receives clinical and laboratory details for only a minority of cases, provided voluntarily by states, which limits understanding of what drives these patterns.
• Surveillance practices differ between states and have changed over time, so the data may not capture every infection.
• The COVID-19 pandemic may have affected health departments' ability to classify possible cases.

What the authors recommended

Tularemia remains a risk across the country, especially in some central states, and American Indian and Alaska Native people continue to face much higher risk than other groups. Because people can be exposed by breathing, swallowing, touching animals or being bitten, prevention education has to be tailored to those routes. Clinicians — particularly those serving tribal populations — need to know the disease's varied presentations, the testing options, and the importance of prompt treatment with an aminoglycoside, fluoroquinolone or tetracycline. They should consider tularemia in a compatible illness, such as fever with lymphadenopathy, after possible exposure, and alert the laboratory when they suspect it so that it can choose the right tests and take safety precautions.

Sources

• Rich SN, Hinckley AF, Earley A, et al. "Tularemia — United States, 2011–2022." MMWR 2024;73(51–52).