Francisella is a bacterium known for causing tularemia, a disease that can affect both humans and animals. Though less commonly known than some other pathogens, this organism is significant due to its ability to cause a range of clinical presentations, from mild to severe. Understanding Francisella and the disease it causes is important for public health, helping to identify risks and implement preventive measures.
The Francisella Bacterium
Francisella is a Gram-negative bacterium, appearing as small, rod-shaped or oval-shaped cells (coccobacilli). Francisella species are facultative intracellular pathogens, meaning they can survive and multiply inside host cells, particularly immune cells like macrophages. This intracellular lifestyle allows Francisella to evade the host’s immune system. While the genus includes several species, Francisella tularensis is the primary cause of human disease. F. tularensis is also notably resilient, capable of surviving for extended periods in various environmental conditions outside a host.
Tularemia: The Associated Disease
Tularemia, often called “rabbit fever” or “deer fly fever,” is caused by Francisella tularensis. Symptoms typically appear three to five days after exposure, though this incubation period can range from one to fourteen days. The specific manifestations of tularemia vary depending on how the bacteria enter the body.
Ulceroglandular tularemia is the most common form, often resulting from insect bites or direct contact with infected animals. This form presents with a skin ulcer at the site of infection, accompanied by painful swelling of nearby lymph nodes, commonly in the armpit or groin. Glandular tularemia is similar, involving swollen and painful lymph nodes, but without the characteristic skin ulcer.
Oculoglandular tularemia affects the eyes when bacteria enter, perhaps by touching the eyes after handling contaminated material. Symptoms include eye pain, irritation, redness, light sensitivity, and swelling of lymph nodes around the ear. An ulcer may also form inside the eyelid.
Oropharyngeal tularemia results from consuming contaminated food or water. Individuals with this form may experience a sore throat, mouth ulcers, swollen tonsils, and swollen lymph nodes in the neck, sometimes with digestive symptoms like vomiting and diarrhea.
Pneumonic tularemia develops from inhaling the bacteria or from spread to the lungs from other infection sites. It causes symptoms similar to pneumonia, including cough, chest pain, and difficulty breathing.
Typhoidal tularemia is characterized by general symptoms such as high fever, chills, headache, and muscle aches, without the specific localized signs seen in other forms. This form can be more difficult to diagnose due to its non-specific presentation.
Transmission and Risk Factors
Francisella tularensis can spread to humans through several routes. One common method of transmission is through the bites of infected arthropods, such as ticks and deer flies. In the United States, ticks like the dog tick, wood tick, and lone star tick are known vectors.
Direct contact with infected animals, particularly rabbits, hares, and rodents, is another significant route of transmission. This can occur when handling sick or dead animals, or through activities like hunting and trapping. The bacteria can enter the body through breaks in the skin or mucous membranes.
Inhaling contaminated aerosols or dust containing the bacteria also leads to infection. This might happen during activities that disturb soil or animal carcasses, such as farming or landscaping, especially when machinery runs over infected animals.
Although less common, consuming contaminated food or water can transmit tularemia, leading to oropharyngeal or gastrointestinal forms of the disease. For example, drinking untreated water can pose a risk.
Diagnosis and Treatment
Diagnosing tularemia often begins with a healthcare provider evaluating a patient’s symptoms and exposure history, which can provide important clues. Because tularemia is rare and its symptoms can mimic other illnesses, laboratory tests are usually needed to confirm the diagnosis. These tests include blood work to detect antibodies against Francisella tularensis, indicating the body’s immune response.
Culturing the bacteria from clinical specimens, such as tissue samples from ulcers, lymph node aspirates, or respiratory secretions, can also confirm the diagnosis. However, F. tularensis is a slow-growing bacterium, and laboratories must be specifically notified if tularemia is suspected due to the need for appropriate safety precautions and media. Polymerase chain reaction (PCR) tests can also identify the bacteria’s genetic material in specimens, offering a rapid diagnostic tool.
Treatment for tularemia typically involves antibiotics, and early therapy is important for a favorable outcome. Aminoglycoside antibiotics, such as streptomycin and gentamicin, are commonly used and often considered the preferred treatment options. Other effective antibiotics include doxycycline, ciprofloxacin, and levofloxacin. The choice of antibiotic and duration of treatment may depend on the severity and form of the disease.
Preventive Strategies
Preventing tularemia involves reducing exposure to the bacteria and its common sources. One important strategy is to avoid direct contact with sick or dead animals, especially rabbits, hares, and rodents. Hunters and trappers should wear gloves when handling animal carcasses and ensure proper disposal.
Using insect repellent containing DEET or picaridin helps deter ticks and biting flies, which are common vectors for the disease. When in areas where these insects are prevalent, wearing long pants and sleeves provides a physical barrier against bites. Regularly checking for and promptly removing ticks after outdoor activities also reduces the risk of transmission.
Ensuring proper handling and cooking of wild game meat is another preventive measure. Water that may be contaminated, particularly from natural sources in endemic areas, should be disinfected before use. Practicing good personal hygiene, such as washing hands thoroughly after outdoor activities or contact with animals, further minimizes the chance of infection.