Tularemia

█ BRIAN HOYLE

Tularemia is a plague-like disease caused by the bacterium Francisella tularensis . U.S. weapons stores of tularemia bacteria were reported destroyed in 1973. Until the demise of the Soviet Union, its biological weapons development program actively developed strains of the bacterium that were resistant to antibiotics and vaccines. As of March 2003, the whereabouts and disposition of some Soviet era tularemia stocks remains uncertain.

Tularemia is listed as potential bioterrorist weapon because it is easily obtained and potentially lethal.

World Health Organization (WHO) estimates hypothesize that if 50 kg of "weaponized" or highly virulent bacterium Francisella tularensis was dispersed in aerosol form over a large city, depending on weather and exposure patterns, there could be as many as 250,000 infections resulting in a projected 19,000 deaths.

Tularemia bacterium is transferred to humans from animals (i.e., a zoonosis) such as rodents, voles, mice, squirrels, and rabbits. Reflecting the natural origin of the disease, tularemia is also known as rabbit fever. Indeed, the rabbit is the most common source of the disease. Transfer of the bacterium via contaminated water and vegetation is possible as well.

The disease can easily spread from the environmental source to humans (although direct person-to-person contact has not been documented). This contagiousness and the high death rate among those who contract the disease made the bacterium an attractive bioweapon. Both the Japanese and Western armies experimented with Francisella tularensis during World War II. Experiments during and after that war established the devastating effect that aerial dispersion of the bacteria could exact on a population.

Tularemia naturally occurs over much of North America and Europe. In the United States, the disease is predominant in south-central and western states such as Missouri, Arkansas, Oklahoma, South Dakota, and Montana. The disease almost always occurs in rural regions. The animal reservoirs of the bacterium become infected typically by a bite from a blood-feeding tick, fly, or mosquito.

The causative bacterium, Francisella tularensis is a Gram-negative bacterium that, even though it does not form a spore, can survive for protracted periods of time in environments such as cold water, moist hay, soil, and decomposing carcasses.

The number of cases of tularemia in the world is not known, as accurate statistics have not been kept, and because illnesses attributable to the bacterium go unreported. In the United States, the number of cases used to be high. In the 1950s, thousands of people were infected each year. This number has dropped considerably, to less than 200 each year, and those who are infected now tend to be those who are exposed to the organism in its rural habitat (e.g., hunters, trappers, farmers, and butchers).

Humans can acquire the infection through breaks in the skin and mucous membranes, by ingesting contaminated water, or by inhaling the organism. An obligatory step in the establishment of an infection is the invasion of host cells. A prime target of invasion is the immune cell known as macrophages. Infections can initially become established in the lymph nodes, lungs, spleen, liver, and kidney. As these infections become more established, the microbe can spread to tissues throughout the body.

Symptoms of tularemia vary depending on the route of entry. Handling an infected animal or carcass can produce a slow-growing ulcer at the point of initial contact and swollen lymph nodes. When tularemia is inhaled, the symptoms include the sudden development of a headache with accompanying high fever, chills, body aches (particularly in the lower back) and fatigue. Ingestion of the organism produces a sore throat, abdominal pain, diarrhea, and vomiting. Other symptoms can include eye infection and the formation of skin ulcers. Some people also develop pneumonia-like chest pain. An especially severe pneumonia develops from the inhalation of one type of the organism, which is designated as Francisella tularensis biovar tularensis (type A). The pneumonia can progress to respiratory failure and death. The symptoms typically tend to appear three to five days after entry of the microbe into the body.

The infection responds to antibiotic treatment and recovery can be complete within a few weeks. Recovery produces a long-term immunity to re-infection. Some people experience a lingering impairment in the ability to perform physical tasks. If left untreated, tularemia can persist for weeks, even months, and can be fatal. The severe form of tularemia can kill up to 60% of those who are infected if treatment is not given.

A vaccine is available for tularemia. To date this vaccine has been administered only to those who are routinely exposed to the bacterium (e.g., researchers). The potential risks of the vaccine, which is a weakened form of the bacterium, have been viewed as being greater than the risk of acquiring the infection.

█ FURTHER READING:

BOOKS:

Chin, J. "Tularemia." In Control of Communicable Diseases Manual. Washington, DC: American Public Health Association, 2000.

Dennis, D. T. "Tularemia." In: Wallace, R. B. ed. Maxcy-Rosenau-Last Public Health and Preventive Medicine, 14th edition. Stamford: Appleton & Lange, 1998.

SEE ALSO

Bioterrorism, Protective Measures
Infectious Disease, Threats to Security