Background
First described in Japan in 1837, tularemia is an infectious disease caused by the gram-negative pleomorphic bacterium, Francisella tularensis. The disease name relates to the description in 1911 of a plaguelike illness in ground squirrels in Tulare County, California, and the subsequent work performed by Dr Edward Francis. In 1928, Francis described his personal experience with more than 800 cases.
F tularensis is found worldwide in more than 100 species of wild animals, birds, and insects. This occurs in both terrestrial (rabbits, hares, ticks, and flies) and aquatic animals (muskrats and beavers). Four major strains, which differ in both virulence and geographic range, exist. The "tularensis" strain, found primarily in North America, is the most virulent.
The organism produces an acute febrile illness in humans. The route of transmission and factors relating to the host and the organism influence the presentation.
Pathophysiology
Categories of tularemia
Some authorities classify tularemia into 2 groups, which include the far more common ulceroglandular form (in which local or regional symptoms and signs predominate) and the more lethal typhoidal form (in which systemic symptoms dominate the clinical picture). More commonly, however, tularemia is divided into 6 forms:
- Ulceroglandular
- Glandular
- Oculoglandular
- Oropharyngeal
- Pneumonic
- Typhoidal
Each form reflects the mode of transmission. The organism gains access to the host by means of inoculation into skin or mucous membrane, inhalation, or ingestion. Although person-to-person transmission does not occur with F tularensis, the organism is extremely infectious, with as few as 10-50 inhaled organisms producing disease. It is therefore an organism that can infect laboratory technicians working with the organism, making it a candidate for use as a biological weapon.
Ulceroglandular form (70-80% of cases): The organism enters through a scratch, abrasion, or tick or insect bite and spreads via the proximal lymphatic system. Within the ulceroglandular form, more differentiation exists. A subcutaneous inoculum of as few as 10 organisms can cause disease.
Glandular form (rare): No ulcer is present, and the organism is presumed to have gained access to the lymphatic system and/or bloodstream through clinically unapparent abrasions.
Oculoglandular form (1% of cases): The organism enters through the conjunctiva from either a splash of infected blood or rubbing the eyes after contact with infectious materials (eg, blood from a rabbit carcass).
Oropharyngeal form (rare): This form occurs after ingestion of eating undercooked rabbit meat containing the organism.
Pneumonic form (uncommon): This form occurs when the organism is inhaled. This form is observed in laboratory workers and occasionally occurs naturally. Pneumonia also occurs in 10-15% of patients with ulceroglandular tularemia and in one half of those patients with typhoidal tularemia.
Typhoidal (or septicemic) form (10-15% of cases): This form is more severe than the others and often includes pneumonia. Ingestion may be the mode of transmission; however, in most cases, the portal of entry remains unknown.
Incubation
After an incubation period of 3-4 days (range, 1-14 d), a papule develops, accompanied by a high fever. The papule evolves into an ulcer associated with regional lymphadenopathy. Some patients infected by a second, less virulent strain (type B) have less dramatic presentations.
Carriers
Although numerous animals and insects can carry F tularensis, rabbits and ticks (especially Dermacentor and Amblyomma species) most commonly are implicated in human cases. The deer fly is another classic, although less common, vector.
Epidemiology
Frequency
United States
A few hundred cases of tularemia are reported annually in the United States. As with most such diseases, most cases are likely unreported or misdiagnosed. Although sporadic cases occur in all states, those with highest prevalence are Arkansas, Illinois, Missouri, Texas, Oklahoma, Utah, Virginia, and Tennessee. Some occupations confer risk for tularemia; they include laboratory workers, landscapers, farmers, veterinarians, hunters, trappers, cooks, and meat handlers.
The frequency of tularemia has decreased markedly over the last 50 years, and a shift from winter disease (usually from rabbits) to summer disease (more likely from ticks) has occurred. Although this decrease led to the Centers for Disease Control and Prevention (CDC) removing tularemia from its list of reportable diseases in 1994, it was reinstated in 2000 due to concerns about tularemia being used as a biological weapon.
International
Tularemia is found worldwide, but the incidence is unknown.
Mortality/Morbidity
Untreated, tularemia has a mortality rate of 5-15%; this rate is even higher with the typhoidal form. Appropriate antibiotics lower this rate to about 1%.
Sex
Biologically, no gender bias exists; however, young–to–middle-aged men may be more likely to engage in activities (eg, associated with tick bites, rabbit, and wild game exposure) that predispose them to tularemia. Recently, tularemia has been associated with the bite of a pet hamster.[1]
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