LEGIONELLA - World Health Organization

Parenteral therapy is usually given at least until there is a clinical response, although outpatients
with mild disease do well with oral therapy. Most patients recover from fever within 3–5 days.
Total duration of treatment is normally 10–14 days (5–10 days for azithromycin), but a
21-day course has been recommended for immunosuppressed patients, as well as for those
with severe disease (e.g. extensive evidence of disease on chest radiographs). However, chest
X-rays are not effective for monitoring the success of the therapy (see Section 1.1.1).
Adverse effects
The principal adverse effects of a macrolide treatment include gastrointestinal symptoms,
such as diarrhoea and vomiting, and effects on the ears (when given in high doses). Relatively
frequently, the respiratory fluoroquinolones cause adverse effects such as gastrointestinal
symptoms and central nervous system disturbances. Alteration to the electrocardiogram
(ECG or EKG) (i.e. prolongation of the QT interval — the duration of the contraction of the
heart’s main chambers) precludes the use of these quinolones in patients with severe electrolyte
imbalances, irregular heartbeats or severe congestive heart failure. Erythromycin has also been
reported to cause ventricular fibrillation (disorganized twitching of the heart muscle) and QT
prolongation, and should be used with caution in patients with heart disease, especially when the
drug is rapidly administered into the bloodstream via a central venous catheter.
1.4 Types of organism causing disease
1.4.1 Taxonomy
Since Legionnaires’ disease was recognized, characterization of the strains isolated from patients
has led to the creation of a new bacterial genus, Legionella, belonging to the family Legionellaceae
(Brenner, Steigerwalt & McDade, 1979). Some investigators (Garrity, Brown & Vickers, 1980;
Brown, Garrity & Vickers, 1981) have proposed placing the legionellae in three separate
genera — Legionella, Fluoribacter and Tatlockia — on the basis of low DNA (deoxyribonucleic
acid) hybridization values between some Legionella species (Fox & Brown, 1993). However,
other studies have shown that the family Legionellaceae forms a single subgroup, sharing a common
ancestor within the gamma-s subdivision of the Proteobacteria. Data using 16S ribosomal
RNA (ribonucleic acid) analysis support a single family, showing that all legionellae studied
are more than 95% related (Fry et al., 1991).
Within the genus Legionella, the DNA relatedness between strains of a given species is unusually
high (>90%), whereas DNA relatedness between one species and another is less than 70%
(Brenner, 1986). Many definitions for bacterial genera and species have been suggested; however,
it is likely that the integrated use of phylogenetic and phenotypic characters is necessary for
the delineation of bacterial taxa at all levels (Murray et al, 1990). The nearest genetic relative
to Legionellaceae is Coxiella burnetti (Marti, Garcia & Bustillo, 1990), the cause of Q fever.
The Legionellacae and C. burnetti have similar intracellular lifestyles, and may have common
genes associated with the infection processes in their hosts.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS