Tularemia

Tularemia is a potentially serious infection caused by the bacterium Francisella tularensis. Infection in humans most often occurs from handling animal carcasses or bites from an infected insect. Fever, chills, and headache are some of the symptoms of infection. Antibiotics are effective against the bacteria, but untreated infections can be fatal. Airborne bacteria are a potential bioterrorist weapon.

The appearance of a typical skin lesion from tularemia, here from handling infected animal carcasses. Source:CDC

Other Names

• Rabbit fever
• Deerfly fever
• Francis disease

Symptoms

The signs and symptoms people develop depend on how they are exposed to tularemia. If the bacteria are inhaled, symptoms can include abrupt onset of fever, chills, headache, muscle aches, joint pain, dry cough, and progressive weakness. People with pneumonia can develop chest pain, difficulty breathing, bloody sputum, and respiratory failure. Tularemia can be fatal if the person is not treated with appropriate antibiotics. Symptoms of tularemia usually appear 3 to 5 days after exposure to the bacteria, but they may not appear for up to 14 days. Symptoms of tularemia include the following:

• sudden fever
• chills
• headaches
• diarrhea
• muscle aches
• joint pain
• dry cough
• progressive weakness
• pneumonia

The fever may be accompanied by chest pain, bloody sputum, and trouble breathing (in severe cases breathing may stop). Besides pneumonia in the lungs, other sites of infection include the bone, spleen, and the sac around the heart.

Some symptoms of tularemia are dependent on the route of exposure to the bacteria. For example, skin ulcers can form at a break in the skin where the bacteria enter the body. Other examples include mouth ulcers, swollen and painful lymph nodes, swollen and painful eyes, and a sore throat.

Causes

Tularemia is caused by infection with bacteria of the genus Francisella. The species Francisella tularensis causes the most severe forms of tulaermia. ^[1]These bacteria can be acquired in several ways:

Wild rabbits are a common source of infection. They transmit tularemia when their carcasses are handled. Source:Wikimedia Commons

Muskrats are another common source of infection for humans. Source: Wikimedia Commons

• Muskrats
• Bite from an infected tick, deerfly, or other insect
• Handling infected animal carcasses (e.g., those of wild rabbits)
• Eating or drinking contaminated food or water
• Breathing in the bacteria, F. tularensis

Most cases of tularemia are acquired from handling infected animal tissues or tick bites. Some outbreaks have been linked to mowing grass, which suggests that aerosolized bacteria can also transmit disease. Tularemia is not known to be spread from person to person.

Diagnosis

Tularemia is a septicemia, which is an infection of the blood. Organisms can be cultured from sputum, from gastric-lavage fluid, and less often, from blood. Tularemia grows on a special media that must be requested by a doctor. The laboratory must be informed that F. tularensis may be present in the specimens, not only because special medium must be used, but also because of the danger of laboratory-acquired infections. Since it is so contagious, laboratory work with the bacteria is extremely dangerous. Tularemia is usually diagnosed using serology, which is obtained from a special, specific blood test. A major problem in the diagnosis of tularemia is that it is so rare. Physicians usually must ask questions about contact with rabbits, insect bites, and travel to or residence in areas of endemic disease. ^[2]

Treatment

Antibiotics are used to treat tularemia. Usually streptomycin is used. This medicine is given as an injection into the muscle. Another antibiotic called tetracycline is given by mouth at the same time as streptomycin. Treatment is given for at least four days after the fever has cleared. The disease can be fatal if it is not treated with the right antibiotics. A vaccine is under review by the Food and Drug Administration, but it is not currently available in the United States.

Prevention

Tularemia occurs naturally in many parts of the United States. The Centers for Disease Control and Prevention (CDC) makes many recommendations for preventing infection with F. tularensis:

• Use of insect repellent containing DEET, or treat clothing with repellent containing permethrin. This prevents insect bites.
• Frequent hand washing with warm water and soap, especially after handling animal carcasses
• Cook food thoroughly with water from a safe source.
• Note any change in the behavior of pets (especially rodents, rabbits, and hares) or livestock, and consult a veterinarian if they develop unusual symptoms.

Chances of Developing Tularemia

Anyone can get tularemia if they spend time outdoors in areas where infected animals, deerflies or ticks, can be found. Rabbit hunters, trappers, and laboratory workers exposed to the bacteria are at higher risk.

How Tularemia is Spread

People can get tularemia many different ways:

• being bitten by an infected tick, deerfly or other insect
• handling infected animal carcasses
• eating or drinking contaminated food or water
• breathing in F. tularensis

Tularemia is not known to be spread from person to person. People who have tularemia do not need to be isolated. People who have been exposed to the tularemia bacteria should be treated as soon as possible. The disease can be fatal if it is not treated with the right antibiotics.

History

In 1922 the American microbiologist Edward Francis first recognized tularemia in humans and F. tularenis as the cause of the disease. Several of the early investigators, including Edward Francis, became ill, and took several months to recuperate, after trying to isolate the bacterium.

The genus was named in honor of Edward Francis., who, in 1922, first recognized F. tularensis (then named Bacterium tularensis) as the causative agent of tularemia.^[3][2]

Epidemiology

Infections in humans are extremely rare in the United States. Between the years 1990 and 2000, less than 1 in 1,000,000 to cases were reported annually.^[4] The first recognized outbreak of tularemia occurred in California in 1911. The disease first killed numerous squirrels, and soon cooks, farmers, and other agricultural workers became infected.

F. tularensis is widely distributed in the Northern hemisphere and is capable of infecting hundreds of different animals. The most important vector species involved in transmission to humans are voles, mice, squirrels, muskrats, and beavers. Infectious transfer to humans and other mammals can occur through aerosolization of infected material, the bite of an insect (ticks), and contact with infected animal products. F. tularensis is present throughout the continental United States. Most cases of tularemia occur in Midwestern, Western and Southern states.^[1]

Bioweapon

F. tularensis is considered a potential bioweapon (biological warfare agent) because of its extreme virulence, low infectious dose, ease of aerosol dissemination, and capacity to cause severe illness and death. Indeed, inhalation of as few as 10 bacteria is sufficient to cause disease in humans and most untreated infections can be fatal. Humans have very low natural immunity to the bacteria. If F. tularensis were used as a weapon, the bacteria would likely be made airborne to expose the most people. Airborne bacteria could cause severe respiratory illness, including life-threatening pneumonia and infections throughout the body.

The bacteria that cause tularemia occur widely in nature. They could be isolated and grown in large quantities in a laboratory. Fortunately, manufacturing an effective aerosol weapon would require considerable sophistication. Because of this threat, the CDC operates a national program for bioterrorism preparedness and response that incorporates a broad range of public health partnerships. The CDC has put forth many preventative measures:

• Antibiotic stockpiles
• Coordinating a nation-wide program where states share information about tularemia
• Creating new education tools and programs for health professionals, the public, and the media.

F. tularensis has a long history of use in warfare. Japanese germ warfare scientists studied F. tularensis and the Japanese may have used it Chinese civilians, Russian troops, and American prisoners of war between 1932 and 1945. Also, some think the occurrence of tularemia in thousands of Russian and German troops at the siege of Stalingrad may have been the result of deliberate use by the Soviets. However, a natural cause for this outbreak has not been eliminated, and military personnel may have acquired F. tularensis from mice and rats whose numbers multiplied owing to the widespread disruption of sanitation and hygiene during battle. During the Cold War both the Soviet Union and the United States prepared and stockpiled tons of infectious agents for potential use against the civilian populations of their enemies. In 1969, a World Health Organization expert committee estimated that aerosol dispersal of 50 kg of virulent F. tularensis bacteria over a metropolitan area with five million inhabitants would result in 250,000 casualties requiring extensive medical care and 19,000 deaths.

References

1. ↑ ^{1.0 1.1} McLendon MK, Apicella MA, Allen LA. Francisella tularensis: taxonomy, genetics, and Immunopathogenesis of a potential agent of biowarfare. Annu Rev Microbiol. 2006;60:167-85. Abstract | Full Text
2. ↑ ^{2.0 2.1} Hornick, R. Tularemia Revisited N Engl J Med 2001 345: 1637-1639 Full Text.
3. ↑ Francis E, Tularemia. I. The occurrence of tularemia in nature as a disease of man.1921.Public Health Rep 36:1731–53.
4. ↑ Centers for Disease Control and Prevention. Tularemia--United States, 1990-2000. MMWR Morb Mortal Wkly Rep. 2002 Mar 8;51(9):181-4.Full Text

External Links

Tularemia as a biological weapon

Video from CDC about potential use of tularemia in bioterrorism