Francisella



Francisella



Objectives



1. List the media of choice for optimal recovery and cultivation of Francisella tularensis.


2. Describe the optimal incubation conditions for F. tularensis.


3. Describe the normal habitat and means of transmission of Francisella spp.


4. Describe the symptoms of tularemia and differentiate the various clinical presentations, including ulceroglandular, glandular, oculoglandular, oropharyngeal, systemic (typhoidal), and pneumonic tularemia.



Genera and Species to be Considered




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.




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.




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


The most common specimens submitted to the laboratory are scrapings from infected ulcers, lymph node biopsies, and sputum. Whole blood is an acceptable specimen for all types of tularemia; however, false-negative results may occur during early stages of disease. Serum is generally collected from all patients early in disease and during convalescence. The blood should be separated from the serum as soon as possible, preferably within 24 hours, and may be stored at 2° to 8°C for up to 10 days. If long-term storage is required, the serum may be frozen. To minimize the loss of viable organisms, samples should be transported to the laboratory within 24 hours. If specimens are to be held longer than 24 hours, specimens should be refrigerated in Amie’s transport medium. F. tularensis should remain viable for up to 7 days stored at ambient temperature in Amie’s medium. Swab specimens should be placed in Amie’s transport media containing charcoal. Specimens for molecular testing should be placed in guanidine isothiocyanate buffer for up to 1 month.


Specimen collection for the identification of F. tularensis is highly dependent on the type of clinical manifestation. A detailed description of the recommended type of specimen associated with the patient’s clinical presentation is presented in Table 38-3. In light of recent events and concerns about bioterrorism, laboratories must keep in mind that isolation of F. tularensis from blood cultures might be considered a potential bioterrorist attack; F. tularensis is considered one of the Select Biological Agents of Human Disease (see Chapter 80).


Tags:
Aug 25, 2016 | Posted by in MICROBIOLOGY | Comments Off on Francisella

Full access? Get Clinical Tree
Get Clinical Tree app for offline access