Francisella tularensis is the causative agent of the infectious disease tularemia. The pathogen is a rod-shaped bacterium of the Pasteurellacae family.
What is Francisella tularensis?
The bacterium Francisella tularensis is a gram-negative pathogen. Unlike gram-positive bacteria, gram-negative bacteria have an outer cell membrane in addition to the thin peptidoglycan layer of murein. The pathogen Francisella tularensis is pleomorphic. Pleomorphic bacteria are multiform. They change their cell shape depending on environmental conditions. Their appearance also depends on the stage of development. Francisella tularensis belongs to the coccoid rod-shaped bacteria. Rod-shaped bacteria are actually elongated, whereas coccoid rod-shaped bacteria are somewhat roundish in shape. There are four different subtypes of the pathogen. However, the three clinically significant forms are identical in serology. Biochemically and genotypically, two groups of Francisella tularensis can be distinguished. The bacterium Francisella tularensis biovar tularensis of Jellison type A is highly virulent and causes severe courses of disease that are often fatal. The bacterium Francisella tularensis biovar holarctica of Jellison type B is less virulent. However, this bacterium can also cause severe disease.
Occurrence, distribution, and characteristics
Francisella tularensis is native to Scandinavia, Russia, China, Japan, the United States, and Canada. Francisella tularensis biovar tularensis type A is most common in North America. Francisella tularensis biovar palaearctica occurs worldwide. Pathogen reservoirs are hares, rats, squirrels, mice, and rabbits. However, the pathogen can also be found in soil and water. The small mammals become infected either through contact with contaminated water or soil material or through blood-sucking parasites such as flies, ticks or mosquitoes. The bacterium is transmitted to humans through mucosal or skin contact with contaminated animal material. The consumption of inadequately heated, infectious meat is also a possible source of infection. In particular, consumption of rabbit meat has been shown to be a possible route of infection. Inhalation of contaminated dust (for example, from hay, straw, or soil) can also lead to infection. The same is true for contact with infected mosquitoes, ticks, or flies. The infection cannot be transmitted from person to person. However, when handling pathogens or inhaling aerosols containing pathogens, people can become infected in the laboratory. More often, however, the rural population is affected by infection with Francisella tularensis. Here, infection usually occurs through the processing of game meat or other agricultural products. The pathogen Francisella tularensis is highly contagious. This means that even small amounts of the pathogen are sufficient to trigger an infection. The incubation period is three to five days. However, depending on the infectious dose, infectious route, and pathogen virulence, the incubation period can be up to three weeks.
Diseases and symptoms
Tularemia is a notifiable zoonosis. Although the disease is rather rare, it is often severe and life-threatening. A distinction can be made between an external (localized) and an internal (invasive) form. The external ulceroglandular form is the most common form of tularemia. It begins very suddenly with a sharp rise in fever. Ulcers form at the site of entry of the pathogen. The local lymph nodes are inflamed with pus. In oculoglandular tularemia, also known as parinaud conjunctivitis, the site of entry of the pathogen is at the conjunctiva of the eye. It is recognizable as a yellow nodule. In the eye, the pathogen causes a painful inflammation of the conjunctiva (conjunctivitis). The lymph nodes on the neck and in front of the ear are swollen. In glandular tularemia, no portal of entry is seen. There is also no formation of ulcers. Only the regional lymph nodes are swollen and painful. Glandulo-pharyngeal tularemia is found primarily in children. Here, ulcers form in the oral cavity and pharynx. The lymph nodes in the angle of the jaw are swollen. When the pathogens are inhaled or reach the internal organs through the bloodstream, the internal or invasive form of the disease develops. Typhoid tularemia develops primarily during slaughter or laboratory work.The lungs and respiratory tract are often affected. Patients have a high fever and suffer from headaches and sweating. A feared complication of typhoid tularemia is lung abscesses. In addition, the meninges (meningitis) can become inflamed. Inflammation of the mediastinal cavity (mediastinitis) or the pericardium (pericarditis) is also possible. Other complications include rhabdomyolysis and osteomyelitis. Typhoid tularemia is also called septic or generalized tularemia. It is very dangerous and is associated with high lethality. Intestinal tularemia probably develops from consumption of contaminated meat that has been inadequately heated. Typical symptoms include vomiting, nausea, pharyngitis, diarrhea, and abdominal pain. The second most common form of tularemia is pulmonary tularemia. It manifests as pneumonia. Patients have cough with sputum, shortness of breath, and pain in the thorax. Abdominal tularemia presents with a typhoid-like clinical picture. The liver and spleen are swollen. Patients suffer from diarrhea and abdominal pain. Tularemia is treated with antibiotics. Streptomycin in particular has been shown to be effective. There is resistance to penicillin and sulfonamides. Even with antibiotic treatment, five percent of all invasive forms are fatal. Without treatment, mortality exceeds 30 percent. The lethality is significantly higher for the American forms of tularemia than for the European strains of Francisella tularensis.
