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Blood, chocolate, and Thayer-Martin agars can be used for the primary isolation of organisms belonging to the genus Francisella. Francisella organisms are facultative, intracellular pathogens that require cysteine, cystine, or another sulfhydryl and a source of iron for enhanced growth. They thus require a complex medium for isolation and growth.

General Characteristics

Organisms belonging to the genus Francisella are faintly staining, tiny, gram-negative coccobacilli that are oxidase and urease negative, catalase-positive, nonmotile, non–spore forming, strict aerobes. The taxonomy of this genus continues to be in flux. Current members of the genus share greater than 97% identity based on 16SrRNA sequence analysis. The most current proposed taxonomy is summarized in Table 38-1. For the most part, different subspecies are associated with different geographic regions.

TABLE 38-1

Most Recent Taxonomy of the Genus Francisella and Key Characteristics

Organism Primary Region Disease in Humans Requires Cystine/Cysteine
F. tularensis subsp. tularensis North America (United States and Canada) Most severe: Tularemia (all forms, see Table 38-2) +
F. tularensis subsp. holartica Europe, former Soviet Union, Japan, North America Least severe: Tularemia (all forms) +
F. tularensis subsp. mediasiatica Kazakhstan, Uzbekistan Severe: Tularemia +
F. noatunensis (formerly F. philomiragia subsp. noatunensis) North and South America Emerging pathogen of fish; no human infections identified _
F. novicida North America Mild illness; virulent in immunocompromised patients _
F. philomiragia (formerly Yersinia philomiragia) North America Mild illness; virulent only in immunocompromised individuals and near-drowning victims _


Epidemiology and Pathogenesis

Francisellaceae are widely distributed throughout the environment. F. tularensis is the agent of human and animal tularemia. F. novicida and F. philomiragia are present in the environment and are opportunistic human pathogens. Worldwide in distribution, F. tularensis is carried by many species of wild rodents, rabbits, beavers, and muskrats in North America. Humans become infected by handling the carcasses or skin of infected animals; by inhaling infective aerosols or ingesting contaminated water; through insect vectors (primarily deerflies and ticks in the United States); and by being bitten by carnivores that have themselves eaten infected animals. Some evidence indicates that francisellae can persist in waterways, possibly in association with amebae.

Most cases in the United States are sporadic, occurring during the summer months, and most cases are seen in the states of South Dakota, Arkansas, Missouri, and Oklahoma.

The capsule of F. tularensis appears to be a necessary component for expression of full virulence, allowing the organism to avoid immediate destruction by polymorphonuclear neutrophils. In addition to being extremely invasive, F. tularensis is an intracellular parasite that can survive in the cells of the reticuloendothelial system, where it resides after a bacteremic phase. Granulomatous lesions may develop in various organs. Humans are infected by fewer than 50 organisms by either aerosol or cutaneous routes. F. tularensis subsp. tularensis is the most virulent for humans, with an infectious dose of less than 10 colony forming units. F. philomiragia has been isolated from several patients, many of whom were immunocompromised or victims of near-drowning incidents. The organism is present in animals and ground water.

Spectrum of Disease

The disease associated with F. tularensis, known as tularemia, is recognized worldwide. In the United States the clinical manifestations have been referred to as rabbit fever, deer fly fever, and market men’s disease. The clinical manifestation depends on the mode of transmission, the virulence of the infecting organism, the immune status of the host, and the length of time from infection to diagnosis and treatment. The typical clinical presentation after inoculation of F. tularensis through abrasions in the skin or by arthropod bites includes the development of a lesion at the site and progresses to an ulcer; lymph nodes adjacent to the site of inoculation become enlarged and often necrotic. Once the organism enters the bloodstream, patients become systemically ill with high temperature, chills, headache, and generalized aching. Clinical manifestations of infection with F. tularensis range from mild and self-limiting to fatal; they include glandular, ulceroglandular, oculoglandular, oropharyngeal, systemic, and pneumonic forms. These clinical presentations are briefly summarized in Table 38-2.

TABLE 38-2

Clinical Manifestations of Francisella tularensis Infection

Types of Infection Clinical Manifestations and Description
Ulceroglandular Common; ulcer and lymphadenopathy; rarely fatal
Glandular Common; lymphadenopathy; rarely fatal
Oculoglandular Conjunctivitis, lymphadenopathy
Oropharyngeal Ulceration in the oropharynx
Systemic (typhoidal) tularemia Acute illness with septicemia; 30% to 60% mortality rate; no ulcer or lymphadenopathy
Pneumonic tularemia Acquired by inhalation of infectious aerosols or by dissemination from the bloodstream; pneumonia; most serious form of tularemia

Laboratory Diagnosis

F. tularensis is a Biosafety Level 2 pathogen, a designation that requires technologists to wear gloves and to work in a biologic safety cabinet (BSC) when handling clinical material that potentially harbors this agent. The organism is designated Biosafety Level 3 when the laboratorian is working with cultures; therefore, a mask is recommended for the handling of all clinical specimens and is very important for preventing aerosol acquisition of F. tularensis. Because tularemia is one of the most common laboratory-acquired infections, most microbiologists do not attempt to work with infectious material from suspected patients. It is recommended that specimens be sent to reference laboratories or state or other public health laboratories that are equipped to handle Francisella spp.

Specimen Collection, Transport, and Processing

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