Most patients with legionellosis have been diagnosed retrospectively by detection of a fourfold rise in anti-Legionella antibody with an indirect fluorescent antibody (IFA) test. Serum specimens should be tested no closer than 2 weeks apart. Confirmation of disease is accomplished by a fourfold rise in titer to more than 128. A single serum with a titer of more than 256 and a characteristic clinical picture may be presumptive for legionellosis; however, because as many as 12% of healthy persons yield titers as high as 1:256, this practice is strongly discouraged.9 Unfortunately, individuals with legionnaires' disease may not exhibit an increase in serologic titers until as long as 10 weeks after the primary illness or they may never display significant antibody titer increases. Commercially prepared antigen-impregnated slides for IFA testing are available from numerous suppliers.
antimicrobial susceptibility testing and therapy
In vitro susceptibility studies are not predictive of clinica) response and should not be performed for individual isolates of legionellae. Because newer agents such as fluoroquinolones and the newer macrolides (e.g., claitf thromycin and azithromycin) are more active againsti, pneumophila, erythromycin has been replaced. Alterna, rive regimens include doxycycline and the combination of erythromycin and rifampin. Clinical response usually follows within 48 hours after the introduction of effective therapy. Penicillins, cephalosporins of all generations, and aminoglycosides are not effective and should not be used.
Although under development, a vaccine against Legi<% nella infections is not currendy available. The effectivd? ness of other approaches to the prevention of legionella, infections, such as the elimination of its presence from cooling towers and potable water, is uncertain,8-11
A6-month-old infant was diagnosed clinically witfr pneumonia. She was treated with intramuscular ceftriaxone followed by an oral cephalosporin for 3 days The next day she was found to be unresponsive and rushed to the hospital. She was afebrile but tachypnea (increased breathing) and tachycardic (increased heart rate); she had an increased white blood cell (WBC): count predominated with lymphocytes. A broncho-alveolar lavage was collected and was positive for Legionella by direct fluorescent antibody. A culture grew Legionella pneumophila serogroup 6 after 8 days. Despit^ appropriate therapy with erythromycin and rifampin» the infant's pulmonary disease was fatal. No underlying disease was found in the baby.
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