Severe Q fever community-acquired pneumonia (CAP) mimicking Legionnaires' disease: Clinical significance of cold agglutinins, anti-smooth muscle antibodies and thrombocytosis.

Atypical community-acquired pneumonia (CAP) may be caused by zoonotic or nonpulmonary pathogens. However, atypical pathogens are systemic infectious disease accompanied by pneumonia in contrast with typical bacterial pathogens with infection limited to the lungs and absent extrapulmonary findings. Clinically and radiologically, the atypical CAP pathogens that most closely resemble each other are psittacosis, Q fever, and Legionnaires' disease. Psittacosis can usually be readily suspected or eliminated on the basis of a recent psittacine bird contact history. The 2 atypical pneumonias that most closely resemble each other clinically are Q fever and Legionnaires' disease. The epidemiology of Q fever is related to livestock, and sporadic cases are related to contact to parturient cats. In nonendemic areas, Q fever CAP mimics Legionnaires' disease most closely. Both Q fever and Legionella CAP have several clinical and laboratory features in common. However, there are subtle but important differences that allow the astute clinician to differentiate between these 2 disorders on the basis of clinical and nonspecific laboratory findings before definitive diagnostic tests results are reported. We report a case of severe Q fever CAP mimicking Legionnaires' disease in a young adult normal host. Her initial zoonotic contact history was negative, and her clinical presentation suggested Legionnaires' disease as the most likely diagnosis. Against the diagnosis of Legionnaires' disease was the patient's age and occurrence of the disease in spring time. In contrast, Legionnaires' disease is usually an infection of older individuals and occurs in late summer/fall. Although the patient did not have splenomegaly, a common finding in Q fever CAP, she did have mild hepatomegaly. Hepatomegaly is a uncommon in Q fever CAP but is not a feature of Legionnaires' disease. In the absence of a positive zoonotic contact history, the cardinal findings pointing to the diagnosis of Q fever in this case were "multiple round opacities" on chest computed tomography scan and thrombocytosis during her hospitalization. Against the diagnosis of Legionnaires' disease was the absence of hypophosphatemia and highly elevated ferritin levels. In patients with atypical pneumonias in whom the clinical presentation and nonspecific laboratory findings suggest Legionnaires' disease, but in addition have findings not associated with Legionnaires' (eg, hepatomegaly, thrombocytosis), Q fever serology should be ordered. We conclude that Q fever may closely mimic Legionnaires' disease. Severe atypical CAP with "multiple round opacities" on chest x-ray/computed tomography chest scan with elevated anti-smooth muscle antibodies or thrombocytosis should suggest the diagnosis of Q fever and prompt specific testing for Q fever. Rarely, Q fever CAP may be associated with elevated cold agglutinin titers.