Serum antibodies to Legionella pneumophila in patients with cystic fibrosis.

We studied the prevalence of serum antibodies to Legionella pneumophila in patients with cystic fibrosis (CF) by indirect immunofluorescence. Thirty-two (29.4%) of 109 patients with CF had a titer of 1:256 or higher, the level presumptive of previous infection, compared with (1) three (11.5%) of 26 patients without CF but with chronic suppurative bronchitis of childhood, (2) three (1.7%) of 178 age-matched persons without pulmonary disease, (3) zero of 22 parents and normal siblings, and (4) zero of 113 adult patients with chronic pulmonary diseases. (Addition of patients with CF who had a titer of 1:128 or higher would raise the prevalence of serum antibodies to L pneumophila in patients with CF to 43.2%.) Fifteen of 32 patients with CF who had antibody titers of 1:256 or higher to L pneumophila (46.9%) had titers of 1:1,024 or higher. Using a standard scoring system, the clinical condition of patients with CF who exhibited antibodies to L pneumophila was worse than that of patients with CF who lacked antibodies to L pneumophila. We conclude that patients who suffer from CF exhibit a high prevalence of antibodies to L pneumophila by the conventional indirect immunofluorescent test. The clinical importance of this single epidemiologic observation awaits further clarification.