‘Rabbit Fever’, A Rare Disease, On The Rise In US
A recent report from the US Centers for Disease Control and Prevention (CDC) reveals a significant increase in tularemia cases, commonly known as “rabbit fever,” within the United States over the past ten years.
Tularemia is an infectious disease caused by the bacterium Francisella tularensis. Humans can contract this disease through various means, including bites from infected ticks and deer flies; and direct skin contact with infected animals such as rabbits, hares, and rodents, which are highly vulnerable to the disease, the Science Alert reported.
More alarming transmission methods have been documented: mowing over nests of infected animals can aerosolize the bacteria, unknowingly infecting the person operating the mower.
This mode of transmission was first observed in 2000 at a Massachusetts vineyard, where a tularemia outbreak persisted for six months, resulting in 15 confirmed cases and one death. Similarly, at least one of several cases reported in Colorado during 2014-2015 was also linked to lawn mowing.
The CDC closely monitors this bacterium, not only because it is classified as a Tier 1 Select Agent by the U.S. government due to its potential use in bioterrorism but also because, when naturally transmitted, it can be deadly without proper treatment.
“The case fatality rate of tularemia is typically below 2%, though it can be higher depending on the clinical presentation and bacterial strain,” note the authors of a CDC report.
Tularemia is relatively rare: between 2011 and 2022, just 2,462 cases were reported across 47 states. The CDC estimates approximately 1.35 million cases of Salmonella poisoning annually. Despite its rarity- just one case per 200,000 people- the incidence rate for tularemia during this period was 56% higher than in 2001-2010.
This increase is partly attributed to improved case identification. 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 indicative of the bacteria.
To confirm a tularemia diagnosis, a bacterial isolate must be obtained from the patient’s body, or a significant change in antibody levels in blood tests must be observed. 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 shift from 2001-2010 when only 35% of cases were classified as probable.
The CDC notes, “Increased reporting of probable cases might reflect an actual rise in human infections, improved detection methods, or both.” Variations in available laboratory testing during this time may have also influenced the data.
The disease disproportionately affects certain groups. Incidence rates among Native Americans and Alaska Natives, as defined by the CDC, were about five times higher than those of White individuals. The report’s authors suggest that factors like the concentration of Native American reservations in central states and cultural or occupational activities increasing exposure to infected wildlife or arthropods might contribute to this disparity.
Other high-risk groups include children aged 5-9, men aged 65-84, and individuals residing in central U.S. states.
Diagnosing tularemia is challenging due to its highly variable symptoms, which depend on the mode of transmission. However, increased awareness of its transmission routes can help prevent exposure and enable faster diagnosis and treatment with antibiotics.
