Notes from the Field: Increase in Human and Animal Tularemia Cases ¡ª Minnesota, 2024

Investigation and Outcomes

MDH obtained medical records and interviewed tularemia patients to collect information on their illness and exposures and obtained veterinary records and interviewed animal owners to learn about tularemia-positive animal illnesses, identify exposures, and assess zoonotic exposures in the household. Exposures of clinical laboratory staff members were also assessed. Veterinary personnel were assessed for occupational exposures to tularemia-positive animals. A high-risk exposure to an infected animal was defined as a bite, scratch, needlestick injury, or exposure to aerosolized infectious material from an infected animal. Any person with a high-risk household, veterinary, or clinical laboratory exposure was recommended antibiotic postexposure prophylaxis (PEP), generally with doxycycline or ciprofloxacin (2). Site investigations (i.e., tick drag sampling and site characterization using a standardized data collection form) were conducted for animal cases with illness onset in the past month or any human case in the Minneapolis-St. Paul metropolitan area (after obtaining consent) (3).

Human Cases and Human Exposures

In 2024, the MDH Public Health Laboratory identified five laboratory-confirmed human tularemia cases (all F. tularensis subsp. tularensis) (Table), among whom four patients had the pneumonic form of tularemia. During 2000–2023, among 32 tularemia cases identified in Minnesota, two (6.3%) were pneumonic tularemia. All five patients were hospitalized for a median 6 (IQR = 3) days. In three cases, tularemia was not identified as the cause of infection until after hospital discharge. Once tularemia was identified, patients were treated with oral ciprofloxacin or doxycycline, and all recovered without complication. Two clinical laboratorians manipulated cultures outside of a biosafety cabinet and were advised to self-monitor for tularemia signs or symptoms for 14 days; neither experienced illness.

Animal Cases

Twenty-seven tularemia-positive animals were reported by direct laboratory report or veterinarians via telephone or email in 2024. Twenty-one (78%) animal cases were in domestic cats, five (19%) in domestic dogs, and one (4%) in a wild rabbit. Typhoidal tularemia (nonlocalized infection) and oropharyngeal tularemia (fever, oral ulcers, and lymphadenopathy) were the most common clinical forms identified. Most pets (78%) survived, four (15%) died from their infection, and two (7%) were euthanized because of cost or prognosis. Three pet owners and one veterinary staff member were determined to have been exposed, and one pet owner was advised to receive PEP after a scratch from an infected cat. The pet owner took the course of PEP as advised. No cases of tularemia occurred among exposed pet owners.

Site investigations for three human and two animal cases were conducted at the likely exposure location and nearby public spaces. No ticks were found by drag sampling. Two patients with pneumonic disease who consented to a site investigation reported mowing over animal carcasses during their exposure period; a rabbit and mouse carcass were found at the likely exposure sites but were too decomposed for testing.

Preliminary Conclusions and Actions

Compared with the median number of tularemia cases reported in Minnesota during 2000–2023, human tularemia cases increased from one to five and animal cases increased from two to 27 in 2024. Increased veterinary awareness after tularemia-related communications in 2023 likely contributed to the increase in animal tularemia case reporting, in addition to a true increase in cases. On June 20, MDH issued a press release to notify the public of the increased risk for tularemia in Minnesota (4). Veterinarian alert emails sent through the Minnesota Board of Animal Health veterinarian email distribution list and physician alerts provided on regularly scheduled infectious disease clinician calls raised awareness of the increase in cases.

Health care providers should consider tularemia in febrile patients with a history of tick or deer fly bites, contact with ill animals, or mowing over a rabbit or rodent, even in areas of the United States, such as Minnesota, where tularemia is less commonly reported. When ordering testing for a patient in whom tularemia is suspected, providers should alert the laboratory to ensure that laboratorians take appropriate precautions such as working in a biosafety cabinet and wearing gloves, gowns, and eye protection. Veterinarians should consider tularemia in cats and dogs with compatible symptoms, including high fever, oral ulcers, and lymphadenopathy. Although dogs are less likely to experience acute clinical illness than are cats, dogs might be at risk for severe infection (5). Persons who have higher-risk exposure to an animal with tularemia through bites, aerosolization, or direct contact with an animal’s body fluids, should promptly receive PEP (2). Animal cases can be used to direct protective public health messaging to pet owners and others at higher risk for exposure.