Legionnaires' disease mimicking swine influenza (H1N1) pneumonia during the "herald wave" of the pandemic.
BACKGROUND
New York area hospitals were hit hard by the swine influenza (H1N1) pandemic in spring and summer 2009. During a pandemic, the initial cases may be difficult to recognize, but subsequent clinical diagnoses were relatively straightforward, given the high volume of cases and their typical clinical presentation. Swine influenza pneumonia presents as an influenza-like illness (ILI) with dry cough, fever >102 degrees F and myalgias. A variety of other viral pneumonias, eg, cytomegalovirus, human parainfluenza virus 3 (HPIV 3), and adenovirus, as well as bacterial community-acquired pneumonias (CAPs) that may present with some of the clinical and laboratory features of H1N1 pneumonia. Most adults admitted to hospitals with ILIs during the pandemic had, in fact, definite or probable H1N1 pneumonia. The Infectious Disease Division at Winthrop-University Hospital developed a diagnostic weighted point score to identify probable H1N1 cases in hospitalized adults with rapid negative influenza diagnostic tests (RIDTs).


METHODS
We present a case of an elderly male who presented with an ILI and negative RIDTs during the H1N1 pandemic. He was admitted with a diagnosis of possible H1N1, and placed on influenza precautions and oseltamivir. Although the patient had features consistent with H1N1 pneumonia, Legionnaires' disease was included in the differential diagnosis because of his elevated serum ferritin levels. A Legionella urinary antigen test was positive for Legionella pneumophila (serogroups 01-06).


RESULTS
The peak seasonal incidence of sporadic Legionnaires' disease occurs in the summer and fall. Even in the midst of a pandemic, clinicians should be on the alert for other infectious diseases that may mimic H1N1 pneumonia. In our experience, the best way to differentiate H1N1 from ILIs or other bacterial CAPs is through the Winthrop-University Hospital Infectious Disease Division's diagnostic weighted point score system for H1N1 pneumonia or its rapid simplified version, ie, the diagnostic swine influenza triad. Legionnaires' disease is the atypical CAP pathogen most likely to mimic H1N1 pneumonia.


CONCLUSIONS
Based on this and other nine cases at our institution during the "herald wave" of pandemic, we conclude that Legionnaires' disease may mimic swine influenza (H1N1) pneumonia.
Cunha
Burke A
BA
Infectious Disease Division, Winthrop-University Hospital, Mineola, New York 11501, USA.
Klein
Natalie C
NC
Strollo
Stephanie
S
Syed
Uzma
U
Mickail
Nardeen
N
Laguerre
Marianne
M
D000890:Anti-Infective Agents / Q000627:therapeutic use; D001372:Aza Compounds / Q000627:therapeutic use; D001799:Blood Sedimentation; D002097:C-Reactive Protein; D052279:Creatine Kinase, MB Form / Q000097:blood; D003937:Diagnosis, Differential; D004196:Disease Outbreaks; D024841:Fluoroquinolones; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype; D007251:Influenza, Human / Q000175:diagnosis* / Q000473:pathology; D007877:Legionnaires' Disease / Q000175:diagnosis* / Q000188:drug therapy / Q000473:pathology; D008223:Lymphoma; D008297:Male; D008875:Middle Aged; D000077266:Moxifloxacin; D011804:Quinolines / Q000627:therapeutic use; D000637:Transaminases / Q000097:blood
D000890:Anti-Infective Agents; D001372:Aza Compounds; D024841:Fluoroquinolones; D011804:Quinolines; D002097:C-Reactive Protein; D000637:Transaminases; D052279:Creatine Kinase, MB Form; D000077266:Moxifloxacin
Infection. 2008 Feb;36(1):92-3
18231716
Infect Dis Clin North Am. 2010 Mar;24(1):203-28
20171553
Semin Respir Infect. 1998 Jun;13(2):116-27
9643389
Infect Control Hosp Epidemiol. 2010 May;31(5):562-3
20367323
J Hosp Med. 2009 Mar;4(3):E1-2
19301376
Postgrad Med. 1996 Jan;99(1):123-8, 131-2
8539198
Heart Lung. 2008 Jul-Aug;37(4):311-20
18620108
Chest. 1998 Sep;114(3):945-6
9743194
MMWR Morb Mortal Wkly Rep. 2009 May 8;58(17):467-70
19444150
Heart Lung. 2009 Nov-Dec;38(6):534-8
19944879
Semin Respir Infect. 1998 Jun;13(2):109-15
9643388
MMWR Morb Mortal Wkly Rep. 2009 May 1;58(16):431-3
19407737
Eur J Clin Microbiol Infect Dis. 2009 Jul;28(7):817-20
19198903
Clin Infect Dis. 2003 Sep 15;37(6):870
12955665
Clin Infect Dis. 2009 Apr 15;48(8):1003-32
19281331
Semin Respir Infect. 1987 Dec;2(4):228-34
3328892
Scand J Infect Dis. 2009;41(10):782-4
19681019
Chest. 2004 May;125(5):1913-9
15136407
Infect Dis Clin North Am. 2004 Mar;18(1):141-55
15081510
Infect Dis Clin North Am. 2010 Mar;24(1):73-105
20171547
BMJ. 2009 Apr 30;338:b1791
19406879
Postgrad Med. 1979 Sep;66(3):95-102
471855
Clin Infect Dis. 2009 Nov 1;49(9):1454-6
19824851
Scand J Infect Dis. 2009;41(11-12):900-2
19922079
Clin Infect Dis. 2009 Oct 1;49(7):1090-3
19725784