Seroepidemiological study of Legionella infection in Denmark. A 28-month retrospective survey

An indirect immunofluorescence (IF) test for Legionella antibodies has been used since 1978 at Statens Seruminstitut, Copenhagen. An increasing annual number of blood specimens from all parts of the country has been tested by IF and the number of Legionella antigens in the test was increased from 4 over 10 to 13, resulting in an ever growing number of seropositive patients over the years. We investigated the occurrence of serologically diagnosed Legionella infections from November 1982 through February 1985, a period of 28 months during which the same 13 Legionella antigens were applied in the IF test. We used CDC's criteria for the serological diagnosis of a current Legionella infection: a greater than or equal to 4-fold rise in antibody titre to greater than or equal to 128 in the IF test. In a test of more than 5,000 blood specimens from 3,374 patients, 69 were found to have diagnostic titre rises. When analysed according to serological reactions with three groups of antigens, seroconversion to a L. pneumophila antigen was found to be more frequent in patients 30-59 years old than seroconversion to a non-L. pneumophila Legionella antigen, while in the age group 60-69 this relation was reversed. Thirteen of the 69 patients had acquired their infection abroad. Twelve of these were below the age of 60, and they had all seroconverted to a L. pneumophila antigen. Clinical data were in accordance with the assumption that Legionella may have been the aetiological agent of the disease in our patients selected by serological criteria.(ABSTRACT TRUNCATED AT 250 WORDS)