Legionellosis differential diagnosis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Legionellosis must be differentiated from other causes of fever, dyspnea, cough, and sputum production, such as bacterial pneumonia, viral pneumonia, and other causes of atypical pneumonia.

Differential Diagnosis

• Legionnaires' disease
• Mycoplasma pneumonia
• Pneumococcal pneumonia
• Pneumocystis carinii pneumonia
• Pulmonary embolism
• Streptococcal pneumonia
• Viral pneumonia

Differentiating legionellosis from other causes of atypical pneumonia

Legionellosis must be differentiated from other diseases that cause atypical pneumonia such as Q fever and mycoplasma pneumonia

Disease	Prominent clinical features	Lab findings	Chest X-ray
Q fever	Q fever is characterized by abrupt onset of fever, myalgia, headache, and other constitutional symptoms. Cough is the most prominent respiratory symptom and it is usually dry. Cough is associated with dyspnea and pleuritic chest pain.	Antibody detection using indirect immunofluorescence (IIF) is the preferred method for diagnosis. PCR can be used if IIF is negative, or very early once disease is suspected. C. burnetii does not grow on ordinary blood cultures, but can be cultivated on special media such as embryonated eggs or cell culture. A two-to-three fold increase in AST and ALT is seen in most patients.	Q fever pneumonia - - Case courtesy of Royal Melbourne Hospital Respiratory, Radiopaedia.org, rID 21993
Mycoplasma pneumonia	Mycoplasma pneumonia can be asymptomatic. Headache, nausea, and malaise usually precede the onset of symptoms. Cough is intractable and nonproductive.	Postitve Coombs test Leukocytosis Thrombocytosis	Mycoplasma pneumonia - Case courtesy of Dr Alborz Jahangiri, Radiopaedia.org, rID 45781
Legionellosis	Legionellosis is characterized by cough that is slightly productive. Constitutional symptoms such as chills, myalgia, and arthralgia. Gastrointestinal symptoms such as diarrhea, nausea, and vomiting.	Labs are nonspecific for diagnosing legionellosis Renal and hepatic dysfunction Thrombocytopenia and leukocytosis Hyponatremia	Legionella pneumonia - Case courtesy of Dr Henry Knipe, Radiopaedia.org, rID 31816
Chlamydia pneumonia	There are no specific clinical features of chlamydia pneumonia. Symptoms appear gradually. Chlamydia infection is usually associated with upper respiratory tract symptoms (pharyngitis, sinusitis, etc). It might be associated with extrapulmonary maifestations such as meningitis and Guillain-Barre syndrome.	Chlamydia pneumonia is usually associated with normal WBC count. Diagnosed with the presence of antichlamydial antibody (through complement fixation or direct immunofluoroscence) or direct antigen detection.	Chlamydia-pneumonia - Case courtesy of Dr Andrew Dixon, Radiopaedia.org, rID 14567