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 ≥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 ≤1,2 to 3, 4 to 7, 8 to 12, and ≥13 years old,the percentages of positive sera were 0, 10.1, 30.3, 4.9 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 ≥13 years old, there was a significantly higher (P<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 indicate that infection was related directly to duration of exposure. A definitive demonstration of equinine infection will depend on isolation of the agent and repetition of this serological study with antigens obtained from organisms isolated from horses.