Two cases of Legionnaires' disease due to Legionella cardiaca.

Background: Legionella cardiaca is a Gram-negative bacterium initially isolated from the heart valve of a patient with endocarditis. Since its species description in 2012, there have been no other report of infections. Case Summary: Here, we describe two cases of L. cardiaca pneumonia. The patients were a 74-year-old man from New Orleans, Louisiana, USA, with underlying multiple myeloma and a 67-year-old man from Oklahoma City, Oklahoma, USA, with chronic lymphocytic leukemia. They developed cough and fever while undergoing antineoplastic therapy in August 2022 and July 2024, respectively. Laboratory examinations showed leukocytosis in the man with multiple myeloma and pancytopenia in the other with leukemia. In both patients, chest computed tomography showed ground-glass and consolidative opacities in the lung fields to suggest pneumonia. The cultures of the bronchoalveolar lavage fluids grew slender Gram-negative bacilli on buffered charcoal yeast extract (BCYE) agar with growth dependence on L-cysteine. The bacilli were identified by sequencing analyses of the 16S rRNA gene and Mip gene as L. cardiaca. The patients were treated with a combination of cefepime and other antibiotics and recovered. Both cases were sporadic infections, with likely exposure in the patients' hometowns of New Orleans and Oklahoma City, respectively. Conclusion: These results suggest that L. cardiaca is an opportunistic pathogen and may cause pneumonia, or Legionnaires' disease, in immunocompromised patients. Culture with BCYE agar and gene sequencing analysis may be required to recover and identify the bacterium for an etiologic diagnosis.