A 54-year-old man comes to the emergency department due to worsening fever, cough, and shortness of breath. The patient initially presented a week ago with a fever, headache, myalgia, nasal discharge, and a sore throat. He tested positive for influenza A and was discharged home with symptomatic treatment. The patient's symptoms gradually improved, but over the past 2 days he has had high fevers, cough productive of yellowish sputum, sharp right-sided chest pain, and shortness of breath. The patient has no other medical problems and does not use tobacco, alcohol, or illicit drugs. Temperature is 38.9 C (102 F), blood pressure is 118/66 mm Hg, and pulse is 110/min. Physical examination reveals right-sided lung crackles. Chest x-ray reveals infiltrates in the right lung. Which of the following pathogens is most likely to be isolated from this patient's sputum?
Influenza virus attacks the tracheobronchial epithelium and results in decreased cell size and a loss of cilia, which promotes bacterial colonization. The influenza surface protein neuraminidase also cleaves sialic acid off host glycoproteins, leading to an increased amount of free sugar in the respiratory tract, which fosters bacterial growth. These changes put patients recovering from influenza at increased risk for secondary bacterial infections in the respiratory tree.
Secondary bacterial pneumonia should be suspected when a patient with influenza has improved symptoms and then develops worsening fever and lower respiratory tract manifestations such as productive cough, pleuritic chest pain, and shortness of breath. Physical examination and chest x-ray findings typically demonstrate pulmonary consolidation. Most cases are caused by Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae. Although the elderly are affected most commonly, post-influenza pneumonia due to S aureus frequently develops in young, previously healthy patients.
(Choice A) Cytomegalovirus may cause pneumonia in patients who are immunocompromised (eg, AIDS, post-transplantation). It is not associated with post-influenza infections and usually causes bilateral interstitial infiltrates on chest x-ray.
(Choice B) Klebsiella pneumoniae most commonly causes nosocomial pneumonia or aspiration pneumonia in patients who abuse alcohol or use intravenous drugs. It is classically associated with thick, blood-tinged "currant jelly" sputum due to significant inflammation and necrosis. This pathogen does not typically cause post-influenza pneumonia.
(Choice C) Older patients and those who are immunocompromised are at increased risk for Listeria monocytogenes sepsis and meningitis; pneumonia would be atypical.
(Choice D) Mycobacterium avium-intracellulare causes disseminated disease in AIDS, M kansasii may cause pulmonary tuberculosis-like symptoms, M leprae is responsible for cutaneous leprosy, and M scrofulaceum causes cervical lymphadenitis in children.
Educational objective:
Influenza infection alters the respiratory epithelium and can increase the risk of secondary bacterial pneumonia. The leading pathogens are Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae. The elderly are affected most commonly, but S aureus can cause secondary pneumonia in young, previously healthy patients.