Tularemia 

Tularemia is an acute, febrile, granulomatous, infectious zoonosis caused by the aerobic gram-negative pleomorphic bacillus Francisella tularensis. Although tularemia has likely existed since ancient times,^[1] the disease was first described in Japan in 1837. In 1911, a plaguelike disease in ground squirrels was described in Tulare County, California (tulare is an Aztec word for the tule reed, a marsh plant commonly found in that area), and was later found to be caused by the bacterium now known as F tularensis. Edward Francis studied the causative organism further, named the disease, and, in 1928, described his experience.^[2]

Worldwide, more than 100 species of animals, birds, amphibians, and arthropods host F tularensis. The bacillus may also be found in mud and water. F tularensis produces acute infectious illness in humans. The mode of transmission and factors related to the host and organism influence the clinical presentation (ulceroglandular, glandular, oculoglandular, oropharyngeal, pneumonic, typhoidal).

Humans become infected with F tularensis after introduction of the bacillus by inhalation, intradermal injection, or oral ingestion. The clinical form of tularemia reflects the mode of transmission. Some authors classify the disease as typhoidal (predominance of systemic symptoms), pneumonic (pulmonary findings), or ulceroglandular (regional symptoms).

To cause tularemia in humans via intradermal injection or inhalation, 10-50 bacilli are required. To transmit the disease orally, 100 million organisms are required.

Ulceroglandular tularemia (80% of reported cases) usually occurs after F tularensis enters through the skin. Reportedly, bacilli can penetrate intact skin, but an abrasion (which may be clinically inapparent), a tick bite, or an insect bite is more likely to allow entry. Inoculation of the oral mucosa or conjunctiva may follow (1) contact with hands or fingers contaminated with tissue fluids or (2) contact with infectious aerosols.

Following an incubation period of 3-5 days (range, 1-14 d), a papule develops. In 2-4 more days, the papule may ulcerate, usually accompanied by fever and regional lymphadenopathy. Humans are probably bacteremic during this phase. The bacilli become entrapped in the reticuloendothelial system, where they may survive for a prolonged period. Caseating granulomata, with or without multinucleated giant cells, may develop.

Many animals and arthropods may carry F tularensis; however, ticks (especially Dermatocentor and Amblyomma species) and rabbits are the most common vectors implicated in cases of human tularemia. Domestic cats are increasingly recognized as associated with human tularemia. The deer fly is also a classic vector, although a less commonly reported one.

Inhalation of F tularensis may lead to pulmonic tularemia, while oral ingestion may cause oropharyngeal tularemia. Conjunctival inoculation may follow contact with contaminated tissue fluids. The portal of entry is unknown in most cases of typhoidal tularemia.

Frequency

United States

A few hundred cases of tularemia are reported annually. Many cases are probably undiagnosed, misdiagnosed, or unreported. Tularemia has been reported in all states except Hawaii. Most reported cases occur in Arkansas, Tennessee, Texas, Oklahoma, Kansas, Utah, and Missouri.

In the past, tularemia infections reportedly occurred more frequently during the cold-weather months (eg, rabbit-associated disease); however, recently, tularemia has been reported more frequently during warm-weather months (eg, tick-associated disease).

International

Tularemia occurs throughout the Northern Hemisphere, except for in the United Kingdom. Cases have been reported in the United States, the former Soviet Union, Japan, Canada, Mexico, and Europe.^{[3, 4, 5]} Tularemia has not been reported in Africa and South America.

Mortality/Morbidity

Overall, untreated tularemia carries a mortality rate of approximately 8%. The mortality rate of untreated typhoidal tularemia is 2-3 times higher. With early diagnosis and appropriate treatment, the mortality rate is less than 1%.

Race

All races are equally susceptible to tularemia.

Sex

Both sexes are equally susceptible to tularemia; however, societal activities common to young men may predispose them to F tularensis exposure.

Age

People of all ages are susceptible to the tularemia; however, young-to-middle-aged people are more likely to participate in activities that predispose them to exposure.