Serological investigations of Legionella infections in Denmark.

During the period 1 July 1978 to 1 January 1982, sera from 728 patients suspected of suffering from Legionella infection were tested for Legionella antibodies by an indirect fluorescent antibody test. The antigens used were prepared from heat-killed strains of L.pneumophila serogroups 1-4. In 1981 we further incorporated antigens from serogroups 5 and 6 of L.pneumophila together with L.micdadei, L.bozemanii, L.dumoffii and L.gormanii. According to the criteria put forward by CDC, Atlanta, USA Legionella infection was confirmed in 78 patients with pneumonia. Sera from 35 showed at least a four-fold rise in titre to greater than or equal to 128, while 43 had single or static titres of greater than or equal to 256. There were 59 men and 19 women (3.1:1) with a median age of 51 and 49, respectively. More than half the patients showed maximum titres of antibodies to L.pneumophila serogroup 1. In an additional patient, who was not tested serologically, the diagnosis was made post mortem at CDC by demonstration of Legionella in lung tissue. There were 11 fatal cases among the 79 patients (13.9%). Thirty-two had contracted the disease on a journey abroad. All cases except the seven first cases among tourists from Garda, Italy, were sporadic. The number of cases diagnosed has increased heavily while the proportion of imported cases has decreased.