Laboratory Diagnosis, Treatment and Prevention of Francisella tularensis 5/5 (6)




Laboratory diagnosis of Francisella tularensis

Specimen

  • Scrapings from infected ulcers, lymph node biopsies, and sputum, whole blood.
  • Serum is generally collected from all patients early in disease and during convalescence.
  • 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.
  • Specimens for molecular testing should be placed in guanidine isothiocyanate buffer.

Laboratory diagnosis of Francisella tularensis

Direct detection method

  • Gram staining of clinical material is of little use with primary specimens unless the concentration of organisms is high, as in swabs from wounds or ulcers, tissues, and respiratory aspirates.
  • Gram negative coccobacilli are seen in Gram staining procedure.
  • A more sensitive and specific approach is direct staining of the clinical specimen with fluorescein labeled antibodies directed against the organism.
  • Fluorescent antibody stains and immunohistochemical stains are commercially available for direct detection of the organism in lesion smears and tissues and are typically available in reference laboratories.

Culture

  • The organism is strictly aerobic and is enhanced by enriched media containing sulfhydryl compounds (cysteine, cystine, thiosulfate or IsoVitaleX) for primary isolation.
  • Two commercial media for cultivation of the organism are available: glucose cystine agar and cystine-heart agar, both require the addition of 5% sheep or rabbit blood.
  • However, F. tularensis can grow on chocolate agar or buffered charcoal yeast extract (BCYE) agar, media supplemented with cysteine that are used in most laboratories.
  • They are slow-growing organisms and require 2 to 4 days for maximal colony formation.
  • They are weakly catalase positive and oxidase negative.
  • Identification is done by growth on chocolate agar but not blood agar (blood agar is not supplemented with cysteine) is also helpful.
  • The identification is confirmed by demonstrating the reactivity of the bacteria with specific antiserum (i.e., agglutination of the organism with antibodies against Francisella).

Antibody detection

  • Because of the risk of infection to laboratory personnel and other inherent difficulties with culture, diagnosis of tularemia is usually accomplished serologically by whole-cell agglutination (febrile agglutinins or newer enzyme-linked immunosorbent assay techniques).
  • Serum antibody detection is useful for all forms of tularemia.
  • After the initial specimen, a convalescent sample should be collected at 14 days and preferable up to 3 to 4 weeks after the appearance of symptoms.
  • Tularemia is diagnosed in most patients by the finding of a fourfold or greater increase in the titer of antibodies during the illness or a single titer of 1:160 or greater.
  • However, antibodies (including IgG, IgM, and IgA) can persist for many years, making it difficult to differentiate between past and current disease.

Molecular methods

  • Conventional and real-time polymerase chain reaction (PCR) assays have been developed to detect F. tularensis directly in clinical specimens.
  • Of significance, several patients with clinically suspected tularemia with negative serology and culture had detectable DNA by PCR.

Treatment of Francisella tularensis

  • The organism is susceptible to aminoglycosides, and streptomycin is the drug of choice.
  • Gentamicin is a possible alternative and now considered as drug of choice.
  • Doxycycline and ciprofloxacin can be used to treat mild infections.
  • Doxycycline and chloramphenicol also have been used, although these two agents have been associated with a higher rate of relapse after treatment.
  • F. tularensis strains produce β-lactamase, which renders penicillins and cephalosporins ineffective.
  • Fluoroquinolones appear promising for treatment of severe tularemia.

Prevention and control of Francisella tularensis

  • Preventing bites from ticks, flies, as well as mosquitos.
  • Refrain from drinking untreated water.
  • Wearing protective clothing and using insect repellents reduce the risk of exposure
  • If working with cultures of F. tularensis in the laboratory, wearing a gown, impermeable gloves, mask, and eye protection is must.
  • Prompt removal of the tick can prevent infection.
  • Persons who have a high risk of exposure (e.g., exposure to an infectious aerosol) should be treated with prophylactic antibiotics.

Laboratory Diagnosis, Treatment and Prevention of Francisella tularensis

(Visited 637 times, 1 visits today)

Please rate this note

0 1 2 3 4 5

The Author

Sagar Aryal

Sagar Aryal

I am Sagar Aryal, a passionate Microbiologist and the Scientific Blogger. I did my Master's Degree in Medical Microbiology and currently working as a Lecturer at Department of Microbiology, St. Xavier's College, Kathmandu, Nepal. I am particularly interested in research related to Medical Microbiology and Virology.

1 Comment

Add a Comment

Leave a Reply

Your email address will not be published. Required fields are marked *

Online Bacteriology Notes © 2018 Frontier Theme