15.24D: Tularemia
- Recall the mode of transmission and symptoms associated with tularemia
Tularemia (also known as Pahvant Valley plague, rabbit fever, deer fly fever, and Ohara’s fever) is a serious infectious disease caused by the bacterium Francisella tularensis . A Gram-negative, nonmotile coccobacillus, the bacterium has several subspecies with varying degrees of virulence.
The most important of these is F. tularensis tularensis (Type A), which is found in lagomorphs (rabbits and similar animals) in North America, and it is highly virulent in humans and domestic rabbits. F. tularensis palaearctica (Type B) occurs mainly in aquatic rodents (beavers, muskrats) in North America and in hares and small rodents in northern Eurasia. It is less virulent for humans and rabbits.
The primary vectors are ticks and deer flies, but the disease can also be spread through other arthropods. The disease is named after Tulare County, California and most commonly occurs in North America and parts of Europe and Asia. Although outbreaks can occur in the United States, they are rare.
Depending on the site of infection, tularemia has six characteristic clinical symptoms: ulceroglandular, glandular, oropharyngeal, pneumonic, oculoglandular, and typhoidal. The incubation period for tularemia is one to 14 days; most human infections become apparent after three to five days.
In most susceptible mammals, the clinical signs include fever, lethargy, anorexia, signs of septicemia, and possibly, death. Fever is moderate or very high. Tularemia bacilli can be isolated from blood cultures at this stage. The face and eyes redden and become inflamed. Inflammation spreads to the lymph nodes, which enlarge and may suppurate (mimicking bubonic plague), accompanied by a high fever. Death occurs in less than 1% if therapy is initiated promptly.
Francisella tularensis is an intracellular bacterium, meaning it is able to live as a parasite within host cells. It primarily infects macrophages and is able to evade the immune system. The course of disease involves the spread of the organism to multiple organ systems, including the lungs, liver, spleen, and lymphatic system; and differs according to the route of exposure.
Tularemia is primarily treated with streptomycin but can also be treated with gentamicin for ten days and tetracycline-class drugs such as doxycycline for two to three weeks, chloramphenicol, or fluoroquinolones. An attenuated, live vaccine is available, but its use is only for high-risk groups.
Key Points
- Tularemia is an infection common in wild rodents that is passed to humans through contact with infected animal tissues or by ticks, biting flies, and mosquitoes.
- Symptoms vary depending upon the site of infection. The main clinical signs include fever, lethargy, anorexia, muscle pains and signs of septicemia.
- Although tularemia can be life-threatening, most infections can be treated successfully with antibiotics.
Key Terms
- Francisella tularensis : Francisella tularensis is a pathogenic species of gram-negative bacteria and the causative agent of tularemia or rabbit fever.
- macrophage : A white blood cell that phagocytizes necrotic cell debris and foreign material, including viruses, bacteria, and tattoo ink. It presents foreign antigens on MHC II to lymphocytes. Part of the innate immune system.
- tularemia : An infectious disease caused by the bacterium Francisella tularensis.