A review was conducted of the 106 wild-type F. tularensis isolates voluntarily deposited in the UNMC collection during 1998–2012. These isolates were obtained from infected humans, animals, or ticks predominantly residing in Nebraska; however, isolates from several other regions were also included. Wild-type F. tularensis isolates had been transferred to UNMC from other locations, according to requirements of the national Select Agent Program (2). None of these isolates had been solicited from veterinary or environmental reference laboratories. Species identity for locally detected wild-type F. tularensis isolates to which humans had been exposed was confirmed by the Nebraska Public Health Laboratory in Omaha. Viable culture material was manipulated by authorized persons following select agent–approved Biosafety Level 3 criteria. For genotyping, we used the PCR-based differential insertion sequence amplification (DISA) method with the CR10 C+L+S primer set and pulsed-field gel electrophoresis (PFGE) of PmeI-digested Francisella spp. DNA, as previously described (9).

Of the 106 wild-type F. tularensis isolates in the repository, 54 (51%) were from humans for whom the source of exposure was unknown or undocumented, 29 (27%) were from cats (21) or from humans (8) with tularemia linked to infected cats, 5 (5%) were from humans with tularemia linked to infected ticks, 1 (1%) was from a human with tularemia linked to an infected rabbit, 16 (15%) were from animals with tularemia, and 1 (1%) was from an unknown host. Of the 29 isolates derived from feline-associated cases, 28 were associated with domestic cats and 1 involved a feral cat (Figure). Eight cases of human tularemia occurred through a cat bite: 7 of these cases involved adults and 1 involved a 6-year-old child. In 2 cases, the person was bitten while taking a rabbit from a cat. None of the 8 humans died.

We also analyzed a subset of the 29 F. tularensis isolates obtained from cats or humans bitten by cats. All but 2 of the 29 isolates were available for genotyping. DISA and PFGE results showed that 1 type B and 26 subtype A.I strains were responsible for the 27 cases of tularemia (Figure). Of the 8 cat bite–associated human tularemia cases, 1 was caused by a type B and 7 by subtype A.I strains. None of the feline-associated tularemia cases were caused by a subtype A.II strain.

PFGE demonstrated that the feline-derived F. tularensis A.I strains could be further divided into 4 subpopulations. For strains in 1 of these subpopulations, DISA results showed chromosomal polymorphisms in which a 1.2-kbp or a 1.5-kbp amplicon was produced. This finding is consistent with 2 unique origins for the strains, even though they were obtained from the same geographic area.

Subset analysis of Nebraska cases showed that 48% (24/50) of the wild-type F. tularensis isolates were feline-associated, and most of those 24 isolates (71%, 17/24) were associated with tularemia cases that occurred in 1 city in eastern Nebraska. The highest number of tularemia cases in Nebraska was reported in 2012, a year noted for extreme heat and drought conditions with warm weather beginning in early January and lasting through December. No feline-associated tularemia cases in Nebraska were reported during 1998–2004; however, 18 were reported during 2005–2011.

Two different molecular methods demonstrated that F. tularensis subtype A.I was responsible for most of the tularemia cases in cats (96%, 26/27); 1 of the 27 cases was caused by a type B strain. This finding is consistent with subtype A.I strains being found predominantly east of the 100th meridian in the United States, whereas A.II strains appear to be restricted to the west of this meridian (4). More importantly, this study confirms that the highly virulent A.I strains are frequent causes of tularemia in cats and humans, a finding that is in agreement with those in a previous report (4), and that the type B clade can cause disease in these hosts.

Although cats are a well-known vector of F. tularensis, the strikingly high percentage of disease associated with cats in Nebraska was unexpected (48% in Nebraska vs. 27% in the UNMC repository). Of note, the highest number of feline-derived tularemia cases occurred in a city known to have multiple feral cat colonies and during a year with extended warm weather and extreme drought conditions. Warmer temperatures can accelerate the tick life cycle, augmenting the potential for transmission of vector-borne diseases (10,11), and droughts can contribute to impaired host immunity, thereby increasing the likelihood for the transmission of zoonotic pathogens at the limited sources of water (12). Therefore, although the natural reservoir of F. tularensis is unknown, we speculate that these environmental conditions exacerbated the spread of tularemia by increasing the abundance of ticks harboring and transmitting F. tularensis to outdoor animals. Nevertheless, the high relative contribution of cats to disease may be due to underrecognition of an existing problem, a new emerging threat caused by environmental factors, and/or reporting or other biases.

A 2012 systematic review reported that free-ranging domestic cats are probably responsible for the greatest percentage of bird and mammal deaths (13). This information and the findings from the current study support the supposition that although cats are considered incidental hosts for F. tularensis, their inherent predatory behavior increases the likelihood that they will acquire tularemia through the consumption of infected animals and exposure to contaminated ticks, animals, or water harboring this pathogen. A vaccine for tularemia is not available in the United States, and the overall prognosis for infected persons is poor if antibiotic drug treatment is not administered early in the infection process. Therefore, cat owners should protect themselves against potential bites and/or scratches and consider limiting outdoor access for domestic cats. In addition, because tularemia is often initially misdiagnosed (14), confirmatory testing by public health laboratories is important and will provide critical surveillance information regarding F. tularensis in the environment.

The high percentage of feline-associated tularemia cases reported in this study demonstrates that outdoor domestic cats are frequent vectors of F. tularensis, particularly in areas where the bacterium is endemic. The cases reported in this study were primarily from urban areas with nearby access to veterinarians and with cat owners/caretakers motivated to treat infected animals; therefore, our findings may not accurately estimate the prevalence of this pathogen within the feral and rural domestic cat populations. These findings do emphasize the importance of taking appropriate measures to reduce the transmission of feline-associated tularemia to humans and the need for a vaccine that protects against the highly virulent pathogen F. tularensis.