Experimental infections of horses with Legionella pneumophila

Attempts to infect horses with Legionella pneumophila were undertaken to determine pathogenicity and to evaluate the possibility that horses serve as a reservoir for the organism. A previous study showed that the prevalence of antibodies to L pneumophila in the equine population exceeded 30% of over 600 sera examined. Horses were infected experimentally with the Philadelphia 1 or Bloomington 2 strain of L pneumophila IV or by aerosolization. Signs of clinical illness were restricted to a transient febrile response. A transient decrease in circulating lymphocytes occurred 2 days after inoculation. At necropsy, only moderate generalized lymphadenopathy was noted. Histologically, the lungs contained evidence of a low-grade inflammatory response characterized by focal proliferation of alveolar lining cells, with few neutrophils and eosinophils. Lymph nodes had evidence of reactive hyperplasia. The tissue response to Bloomington 2 strain was slightly more pronounced than that to Philadelphia 1. Attempts to reisolate L pneumophila from blood and nasal or pharyngeal swabs were unsuccessful. The organism was not isolated by culturing tissues obtained at necropsy, nor was it demonstrated by tissue-staining techniques. However, all horses exhibited a marked increase in agglutinating antibodies to L pneumophila serogroups (SG) 1 and 3 as early as 4 days after inoculation. The serologic response was confirmed by indirect immunofluorescence and was shown to consist predominantly of immunoglobulin M by 2-mercaptoethanol treatment. Agglutinating antibodies persisted at least 4 months after infection. On the basis of these studies, the pathogenicity of L pneumophila SG 1 and 3 for the horse appears to be low. There is no evidence to support a role for the horse in the maintenance of these organisms in nature. Horses may be exposed in the environment and maintain a relatively long-lived serologic response to L pneumophila. However, it is also possible that they become infected with other strains of L pneumophila or Legionella-like organisms more pathogenic for horses, or other non-Legionella bacteria, which elicit a cross-reacting serologic response to L pneumophila SG 1 to 4.