Tularemia

Tularemia (also known as "rabbit fever") is a serious infectious disease caused by the bacterium Francisella tularensis. The disease is endemic in North America, and parts of Europe and Asia. The primary vectors are ticks and deer flies, but the disease can also be spread through other arthropods. Animals such as rabbits, prairie dogs, hares and muskrats serve as reservoir hosts. The disease is named after Tulare County, California.

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History

Francisella tularensis was discovered in 1911 during an outburst of rabbit fever, when the disease killed a large number of ground squirrels in the area of Tulare Lake in California. Scientists determined that tularemia could be dangerous to humans: a human being may catch the infection after contacting an infected animal. The ailment soon became frequent with hunters, cooks and agricultural workers.[1]

Epidemiology

The disease is endemic in North America, and parts of Europe and Asia. The primary vectors are ticks and deer flies, but the disease can also be spread through other arthropods. Rodents, rabbits, hares often serve as reservoir hosts,^[1] but waterborne infection accounts for 5 to 10% of all tularemia in the US.^[2]

In the United States, although records show that tularemia was never particularly common, incidence rates continued to drop over the course of the 20th century so that between 1990 and 2000, the rate was less than 1 per 1,000,000, meaning the disease is extremely rare in the US today.^[3]

Clinical manifestations and microbiological diagnosis

Depending on the site of infection, tularemia has six characteristic clinical syndromes: ulceroglandular (the most common type representing 75% of all forms), glandular, oropharyngeal, pneumonic, oculoglandular, and typhoidal.^[4]

The disease has a very rapid onset, with headache, fatigue, dizziness, muscle pains, loss of appetite and nausea. Face and eyes redden and become inflamed. Inflammation spreads to the lymph nodes, which enlarge and may suppurate (mimicking bubonic plague). Lymph node involvement is accompanied by a high fever. Death may result [2] but is less than 1% if therapy is initiated promptly.

The microbiologist must be informed when tularemia is suspected because F. tularensis requires special media for cultivation such as buffered charcoal and yeast extract (BCYE). It cannot be isolated in the routine culture media because the need for sulphydril groups donors (such as cystein). Serological tests (detection of antibodies in the serum of the patients) are available and widely used. Cross reactivity with Brucella can confuse interpretation of the results, and for this reason diagnosis should not rely only on serology. Molecular methods such as PCR are available in reference laboratories.

Mechanism of infection

Francisella tularensis is one of the most infective bacteria known; fewer than ten organisms can cause disease leading to severe illness. The bacteria penetrate into the body through damaged skin and mucous membranes, or through inhalation. Humans are most often infected by tick bite or through handling an infected animal. Ingesting infected water, soil, or food can also cause infection. Tularemia can also be acquired by inhalation; hunters are at a higher risk for this disease because of the potential of inhaling the bacteria during the skinning process. It has been contracted from inhaling particles from an infected rabbit ground up in a lawnmower (see below). Tularemia is not spread directly from person to person.

Francisella tularensis is an intracellular bacterium, meaning that it is able to live as a parasite within host cells. It primarily infects macrophages, a type of white blood cell. It is thus able to evade the immune system. The course of disease involves spread of the organism to multiple organ systems, including the lungs, liver, spleen, and lymphatic system. The course of disease is similar regardless of the route of exposure. Mortality in untreated (pre-antibiotic-era) patients has been as high as 50% in the pneumoniac and typhoidal forms of the disease, which however account for less than 10% of cases.^[5] Overall mortality was 7% for untreated cases, and the disease responds well to antibiotics with a fatality rate of about 2%. The exact cause of death is unclear, but it is thought to be a combination of multiple organ system failures.

Treatment and prevention

The drug of choice is Streptomycin.^[6] Tularemia may also be treated with gentamicin for ten days, tetracycline-class drugs such as doxycycline for 2-3 weeks,^[7] chloramphenicol or fluoroquinolones. An attenuated, live vaccine is available, but its use is restricted to high risk groups. Its use as post-exposure prophylaxis is not recommended.

Tularemia as a biological weapon

The Centers for Disease Control and Prevention regard F. tularensis as a viable bioweapons agent. The disease was used as a weapon by the Russians during World War II.[3] Practical research into using Tularemia as a bioweapon took place at Camp Detrick in the 1950s. It was viewed as an attractive agent because:

• it is easy to aerosolize
• it is highly infective; fewer than 10 bacteria are required to infect
• it is non-persistent and easy to decontaminate (unlike anthrax)
• it is highly incapacitating to infected persons
• it has low-lethality, which is useful where enemy soldiers are in proximity to non-combatants, eg civilians

No vaccine is available to the general public.^[8] The best way to prevent tularemia infection is to wear rubber gloves when handling or skinning rodents or lagomorphs (as rabbits), avoid ingesting uncooked wild game and untreated water sources, and wearing long-sleeved clothes and using an insect repellant to prevent tick bites.

Documented outbreaks

In summer 2000, an outbreak of tularemia in Martha's Vineyard resulted in one fatality, and brought the interest of the CDC as a potential investigative ground for aerosolized Francisella tularensis. Over the following summers, Martha's Vineyard was identified as the only place in the world where documented cases of tularemia resulted from lawn mowing.^[9] The research may prove valuable in preventing bioterrorism^{[citation needed]}.

An outbreak of tularemia occurred in Kosovo in 1999-2000 [4].

In 2004, three researchers at Boston University Medical Center were accidentally infected with F. tularensis, after apparently failing to follow safety procedures.^[10]

In 2005, small amounts of F. tularensis were detected in the Mall area of Washington, DC the morning after an anti-war demonstration on September 24, 2005. Biohazard sensors were triggered at six locations surrounding the Mall. To this date, no cases of tularemia infection have been reported as a result.^[11]

In 2007, a lab of Boston University's Center for Advanced Biomedical Research, where F. tularensis were being kept for research, was evacuated after smoke set off alarms. An investigation has later determined that an electrical problem was the culprit, and no bacterial contamination was found.

In July 2007, an outbreak was reported in the Spanish autonomous region of Castile and León and traced to the plague of voles infesting the region.

Biological Warfare

By the late 1950's the US biological warfare program was focused mostly on tularemia as a biological agent. The Schu S4 strain was standardized as Agent UL for use in the M143 bursting spherical bomblet. It was a lethal biological with an anticipated fatality rate of 40 to 60 percent.[THIS CONTRADICTS STATEMENTS ABOVE] The rate-of-action was around three days, with a duration-of-action of 1 to 3 weeks (treated) and 2 to 3 months (untreated) with frequent relapses. UL was streptomycin resistant. The aerobiological stability of UL was a major concern, being sensitive to sun light, and losing virulence over time after release.^{[citation needed]}

The United States later changed the military symbol for UL to TT (wet-type) and ZZ (dry-type) in an effort to retain security on the identity of military biologicals. When the 425 strain was standardized as agent JT (an incapacitant rather than lethal agent), the Schu S4 strain's symbol was changed again to SR.^{[citation needed]}

References

1. ^ Morner T (1992). "The ecology of tularemia". Rev Sci Tech 11: 1123–30.
2. ^ Jellison WL, Owen C, Bell JF, Kohls GM (1961). "Tularemia and animal populations". Wildl Dis 17: 1–22.
3. ^ Hayes E, Marshall S, Dennis D, et al. (2002). "Tularemia—United States, 1990–2000". MMWR 51 (09): 182–4.
4. ^ Plourde PJ, Embree J, Friesen F, Lindsay G, Williams T (1992). "Glandular tularemia with typhoidal features in a Manitoba child". Can Med Assoc J 146: 1953–5.
5. ^ http://www.cidrap.umn.edu/cidrap/content/bt/tularemia/biofacts/tularemiafactsheet.html#_Overview_1
6. ^ Enderlin G, Morales L, Jacobs RF, Cross JT (1994). "Streptomycin and alternative agents for the treatment of tularemia: review of the literature". Clin Infect Dis 19: 42–7.
7. ^ CDC FAQs re Tularemia
8. ^ http://www.niaid.nih.gov/factsheets/tularemia.htm
9. ^ Feldman KA, Enscore R, Lathrop S, et al. Outbreak of primary pneumonic tularemia on Martha's Vineyard. N Engl J Med 2001;345:1601--6.
10. ^ Smith S. "City tells BU to bolster safety of its medical labs", Boston Globe, 2005-03-29. Retrieved on 2007-05-09. 
11. ^ Dvorak P. "Health Officials Vigilant for Illness After Sensors Detect Bacteria on Mall: Agent Found as Protests Drew Thousands of Visitors", Washington Post, 2005-10-2, p. C13. Retrieved on 2007-05-08. "A week after six bioterrorism sensors detected the presence of a dangerous bacterium on the Mall, health officials said there are no reports that any of the thousands of people in the nation's capital Sept. 24 have tularemia, the illness that results from exposure to the bacteria." 

• Tularemia, NIAID Fact Sheet, April 2005. Retrieved on 2007-01-07.