A case report of Legionella and Mycoplasma pneumonia: Co-incidence or co-infection?

Rationale Concurrent or sequential coinfections of Legionella pneumophila and Mycoplasma pneumoniae have been reported in the past though infrequently. Distinguishing a true co-infection from cross reactivity is often challenging as the diagnosis is mostly dependent on serological testing. Patient concerns A 77-year-old male presented with worsening dyspnea, cough with yellow sputum, diarrhea and fever of 2-days duration. Patient had history of chronic obstructive pulmonary disease (COPD) on home oxygen, bronchiectasis, rheumatoid arthritis (on methotrexate and leflunomide), treated pulmonary tuberculosis and 30-pack-year smoking. Chest X-ray showed bilateral interstitial changes with left lower lobe infiltrate. On day 5, his urine antigen for L pneumophila serogroup 1 was reported positive. The following day his serum M pneumoniae IgM antibody titers were reported elevated at 6647 U/mL. Patient was started on antibiotics and placed on non-invasive positive pressure ventilation. Diagnosis The patient was diagnosed with possible Legionella and Mycoplasma co-infection. Outcomes Sputum Mycoplasma polymerase chain reaction (PCR) and serum cold agglutinins were obtained on day 6 and later reported negative. He was treated with azithromycin for 10 days with clinical improvement. Lessons Serological testing alone is an indirect measure with poor sensitivity and specificity and has its own limitations. Urine antigen detection confirms L pneumophila serogroup 1 infection in a patient with suggestive symptoms. However, diagnosis of M pneumonia should be based on combination of tests including serology and PCR to confirm true co-infection.