CDC Reports a Decade-Long Rise in Human Cases of Rabbit Fever

The US Centers for Disease Control and Prevention (CDC) reports a significant increase in cases of tularemia, also known as ‘rabbit fever,’ across the United States over the past decade.

Tularemia, caused by the bacterium Francisella tularensis, can spread to humans through various means, including bites from infected ticks and deer flies or direct contact with infected rabbits, hares, and rodents, which are highly vulnerable to the disease.

Uncommon Transmission Routes

More unusual transmission routes also exist: mowing over nests of infected animals can aerosolize the bacteria, potentially infecting unsuspecting gardeners.

This was first documented at a Massachusetts vineyard in 2000, where a tularemia outbreak persisted for six months, leading to 15 confirmed cases and one reported death.

At least one of several cases reported in Colorado during 2014 and 2015 was also tied to a lawn-mowing incident.

The CDC closely monitors Francisella tularensis due to its classification as a Tier 1 Select Agent by the US government, highlighting its potential use in bioterrorism. Additionally, the disease can be fatal if left untreated.

“Tularemia’s case fatality rate is typically below 2 percent but can be higher depending on the clinical presentation and infecting strain,” the report’s authors explain.

Tularemia remains relatively rare: from 2011 to 2022, 2,462 cases were recorded across 47 states. In comparison, the CDC estimates approximately 1.35 million cases of Salmonella poisoning occur annually in the US.

While the tularemia rate translates to roughly one case per 200,000 people, it marks a 56 percent increase from 2001-2010.

Improved Case Recording Contributes to Rising Numbers

This rise may partly reflect improvements in case recording. Starting in 2017, the CDC included cases where F. tularensis was detected via polymerase chain reaction (PCR) in the “probable case” category, which previously required both symptoms and specific molecular markers suggesting the bacteria’s presence.

For a tularemia case to be classified as “confirmed,” either the bacteria must be isolated from the patient’s body, or a significant change in antibody levels must be detected through serum testing.

Between 2011 and 2022, 984 cases were confirmed, while 1,475 were classified as probable, accounting for 60 percent of all cases. This marks a significant increase from 2001-2010, when only 35 percent of cases fell into the probable category.

“Increased reporting of probable cases might reflect an actual rise in human infections, improved detection of tularemia, or both,” the CDC team explains. Variations in commercially available laboratory tests during this period may also have influenced the data.

Higher Incidence Rate Among Native American Populations

The incidence rate among individuals categorized by the CDC as “American Indian or Alaska Native” was approximately five times higher than that observed among White individuals.

“Several factors may contribute to the increased risk of tularemia in this population, including the concentration of Native American reservations in central states and sociocultural or occupational activities that heighten exposure to infected wildlife or arthropods,” the report’s authors note.

Other groups most affected by the disease include children aged five to nine, men aged 65 to 84, and individuals living in the central United States.

Diagnosing tularemia is challenging due to its highly variable symptoms, which depend on the transmission route. However, greater awareness of these transmission methods can reduce exposure risks and help doctors promptly recognize and treat the disease with antibiotics.

Read the original article on: Science Alert

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