Prevalence of antibodies to Legionella pneumophila in animal populations.

We examined more than 2,800 human and animal sera for antibodies to four serogroups of Legionella pneumophila by using the microagglutination test. Antibody titers of greater than or equal to 1:64 were considered positive. The occurrence of positive equine sera (31.4%) was significantly higher than the occurrence of positive sera in cattle (5.1%), swine (2.9%), sheep (1.9%), dogs (1.9%), goats (0.5%), wildlife (0%), and humans (0.4%). The highest titer measured in horses was 1:512. The occurrence of positive sera in horses was related directly to age. In horses less than or equal to 1, 2 to 3, 4 to 7, 8 to 12, and greater than or equal to 13 years old, the percentages of positive sera were 0, 10.1, 30.3, 44.9 and 58.1%, respectively. When we compared age-specific serogroup-specific rates in horses from Colorado and Pennsylvania, we found differences. With horses 8 to 12 and greater than or equal to 13 years old, there was a significantly higher (P less than 0.05) occurrence of sera that reacted to serogroups II and III in horses from Pennsylvania. Of 242 positive sera, 43.8% reacted to a single serogroup (serogroup III or I most commonly), and 56.2% reacted to multiple serogroups (serogroups II and III or serogroups I, II, and III most commonly). A high percentage of seropositive horses suggested that horses are commonly infected with L. pneumophila or related organisms, and the age-specific rates of occurrence indicated that infection was related directly to duration of exposure. A definitive demonstration of equine infection will depend on isolation of the agent and repetition of this serological study with antigens obtained from organisms isolated from horses.