A recent report from the US Centers for Disease Control and Prevention (CDC) shows that there has been a significant increase in cases of tularemia, commonly known as “rabbit fever”, in the United States over the past ten years.
Tularemia is an infectious disease caused by the bacterium Francisella tularensis. Humans can become infected with the disease in a variety of ways, including bites from infected ticks and deer flies; and direct skin contact with infected animals such as rabbits, hares and rodents, which are highly susceptible to the disease, science alert Informed.
More dangerous transmission methods have been documented: mowing over the nests of infected animals can aerosolize the bacteria, which can inadvertently infect the person operating the mower.
This mode of transmission was first observed in a Massachusetts vineyard in 2000, where an outbreak of tularemia persisted for six months, resulting in 15 confirmed cases and one death. Similarly, at least one of the many cases reported in Colorado during 2014–2015 was linked to lawn mowing.
CDC Monitors this bacterium closely, not only because it has been classified as a Tier 1 selection agent by the US government due to its potential use in bioterrorism, but also because, when transmitted naturally, So it can be fatal without proper treatment.
“The case fatality rate for tularemia is usually less than 2%, although it may be higher depending on the clinical presentation and bacterial strain,” the authors of the CDC report say.
Tularemia is relatively rare: between 2011 and 2022, only 2,462 cases were reported in 47 states. The CDC estimates that about 1.35 million cases of salmonella poisoning occur annually. Despite its rarity – only one case per 200,000 people – the incidence rate of tularemia during this period was 56% higher than in 2001–2010.
This increase is partly due to better case detection. Starting in 2017, the CDC began including cases where Francisella tularensis was detected via polymerase chain reaction (PCR) in the “probable case” count. Previously, probable cases required symptoms and molecular markers suggestive of bacteria.
To confirm a tularemia diagnosis, a bacterial isolate must be obtained from the patient’s body, or a significant change in antibody levels must be seen in a blood test. Between 2011 and 2022, there were 984 confirmed cases and 1,475 probable cases, with probable cases accounting for 60% of the total. This is a significant change from 2001–2010 when only 35% of cases were classified as probable.
The CDC notes, “Increased reporting of probable cases may reflect an actual increase in human infection, improved detection methods, or both.” Changes in available laboratory testing during this time may also have affected the data.
The disease disproportionately affects certain groups. The incidence rate among Native Americans and Alaska Natives, as defined by the CDC, was approximately five times higher than among white individuals. The report’s authors suggest that factors such as the density of Native American reservations in central states and increased exposure to infected wildlife or arthropods from cultural or commercial activities may contribute to this disparity.
Other high-risk groups include children aged 5–9, men aged 65–84, and individuals living in Central American states.
Tularemia is challenging to diagnose because of its highly variable symptoms, which depend on the mode of transmission. However, increased awareness of its transmission routes may help prevent exposure and enable rapid diagnosis and treatment with antibiotics.
