tularemia

In the United States, although records show that tularemia was never particularly common, incidence rates have further dropped to below 0.1 per 100,000, meaning the disease is extremely rare in the US today.^[1]

Mechanism of infection

Francisella tularensis is one of the most infective bacteria known; fewer than ten organisms can cause disease leading to severe illness. 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 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. 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.^[2] 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 be a combination of multiple organ system failures.

Incubation period

A patient with tularemia will most often develop flu-like symptoms between 1-14 days after infection (most likely 3-5 days.) If the patient was infected through an insect or tick bite, an eschar may develop at the bite site.

Treatment

The drug of choice is Streptomycin. Tularemia can also be treated with gentamicin, tetracycline or fluoroquinolone antibiotics.

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-combattants, eg civilians

The Centers for Disease Control and Prevention regard F. tularensis as a viable bioweapons agent for use by terrorists.

No vaccine is available to the general public.^[3] The best way to prevent tularemia infection is to wear rubber gloves when handling or skinning rodents (especially rabbits), avoid ingesting uncooked wild game and untreated water sources, and wearing long-sleeved clothes and using an insect repellant to prevent tick bites.

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. The research may prove valuable in preventing bioterrorism.

In 2004, three researchers at Boston University Medical Center were accidentally infected with F. tularensis, after apparently failing to follow safety procedures.^[4]

In 2005, small amounts of F. tularensis were detected in the Mall area of Washington, DC the morning after an anti-war demonstration on Sept. 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.^[5]

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 - 60 percent. The rate-of-action was around three days, with a duration-of-action of 1 to 3 weeks (treated) and 2 - 3 months (untreated) with frequent relapses. UL was streptamycin resistant. The aerobiological stability of UL was a major concern, being sensitive to sun light, and losing virulence over time after release.

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.