Outbreak of Pontiac fever due to Legionella anisa

An outbreak of Pontiac fever occurred among 34 of 56 people attending conferences at a hotel in Santa Clara County, California, in 1988. Two groups had an acute febrile upper respiratory illness, with a mean attack rate of 82% and a mean incubation period of 56 hours. Symptoms resolved spontaneously within 5 days. Legionella anisa, which had not previously been associated with outbreaks of Pontiac fever or legionnaires' disease, was isolated from a decorative fountain in the hotel lobby. In addition, 5 of 8 pairs of serum samples from cases showed a more than fourfold rise in antibody titre to the L anisa recovered from the fountain. 42% of hotel employees had titres greater than or equal to 256 against L anisa, whereas none of 48 serum samples from matched controls had titres greater than or equal to 128. The findings raise concern about water treatment protocols for extent of disease that might be caused by exposure to aerosols containing L anisa and other Legionella species.