Francisella tularensis: Symptoms, Causes, and Treatment

Tularemia is a rare infectious disease caused by the bacterium Francisella tularensis. It is sometimes referred to as “rabbit fever” or “deer fly fever” due to common ways people contract it.

Understanding Tularemia

Tularemia is caused by the bacterium Francisella tularensis, a highly infectious coccobacillus. This bacterium can cause illness in humans with exposure to as few as 10 to 50 organisms. The disease is primarily found in the Northern Hemisphere, including North America, Europe, and parts of Asia.

It is classified as a zoonotic disease, meaning it naturally spreads between animals and humans. Over 100 species of animals can host Francisella tularensis, with rabbits, hares, and rodents being common carriers. The bacterium can survive for several weeks in cold, moist environments such as animal carcasses, soil, and water.

How Tularemia is Contracted

Tularemia can be contracted through various routes. One common way is through the bites of infected insects, including ticks like dog, wood, and lone star ticks, as well as deer flies and mosquitoes. These bites often lead to skin ulcers and swollen lymph nodes.

Direct contact with infected animals or their tissues is another significant transmission route. This can occur during activities like hunting or skinning rabbits, muskrats, prairie dogs, and other rodents. The bacteria can enter the body through breaks in the skin or if individuals touch their face after contact with infected animal fluids or tissues. Domestic cats are also susceptible and have been known to transmit the bacteria to humans through bites or scratches.

Inhaling contaminated aerosols or dust can also lead to infection. This can happen during farming or landscaping activities, particularly when machinery like mowers runs over infected animal carcasses, aerosolizing the bacteria. This airborne transmission can result in pneumonic tularemia, which is a serious form of the disease.

Ingestion of the bacteria, though less common in the United States, is another possible route. This typically involves eating undercooked meat from an infected animal or drinking water contaminated with the bacterium. Water can become contaminated through contact with infected animals, and outbreaks have been linked to consuming untreated water from natural sources.

Recognizing Symptoms and Diagnosis

Symptoms vary depending on how the bacteria enter the body, with an incubation period typically ranging from 3 to 5 days, though it can extend up to 14 days. The most common form, ulceroglandular tularemia, results from insect bites or direct skin contact and presents with a skin ulcer at the infection site accompanied by swollen lymph nodes, often in the armpit or groin. Glandular tularemia is similar but without the presence of a skin ulcer.

Oculoglandular tularemia occurs when the bacteria enter through the eye, leading to eye irritation, inflammation, and swollen lymph nodes in front of the ear. If contaminated food or water is ingested, oropharyngeal tularemia can develop, characterized by a sore throat, mouth ulcers, tonsillitis, and swollen lymph nodes in the neck.

Pneumonic tularemia, often the most severe form, results from inhaling the bacteria and manifests with symptoms such as cough, chest pain, and difficulty breathing. This form can also develop if other types of tularemia are left untreated and the bacteria spread to the lungs through the bloodstream. Typhoidal tularemia is a more general form, presenting with fever, fatigue, and weight loss, without specific localizing symptoms.

Diagnosing tularemia often involves laboratory tests, particularly serology, which detects antibodies to Francisella tularensis in blood samples. Antibodies may not be detectable until 2 to 3 weeks after symptom onset, so repeat testing might be needed. Culture from clinical specimens like ulcer scrapings, lymph node aspirates, or blood can also confirm the diagnosis, though the bacterium grows slowly. Polymerase chain reaction (PCR) assays can also be used for rapid confirmation.

Treatment and Prevention

Tularemia is a treatable disease, with antibiotics being the primary course of action. Streptomycin and gentamicin are preferred antibiotics for severe infections. For milder cases, oral doxycycline or ciprofloxacin may be used. Prompt diagnosis and treatment are important to prevent complications. Antibiotic treatment typically ranges from 10 to 21 days.

Preventing tularemia involves measures for individuals who spend time outdoors or handle animals. Using insect repellents containing active ingredients like DEET or picaridin can help avoid tick and deer fly bites. Wearing long pants and long-sleeved shirts when in wooded or grassy areas provides a physical barrier against ticks and other biting insects.

When handling wild animals, particularly sick or deceased ones, wearing gloves is advisable to prevent direct skin contact with the bacteria. Thoroughly cooking game meat before consumption helps eliminate any potential bacteria. Avoiding drinking untreated water from natural sources like streams, lakes, or wells is also a safeguard. Taking precautions during landscaping activities, such as avoiding mowing over dead animals, can reduce the risk of inhaling contaminated aerosols. While a vaccine for tularemia exists, it is not widely available for the general public and is primarily used for laboratory personnel at higher risk of exposure.

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