Differentials
Coronavirus disease 2019 (COVID-19)
SIGNS / SYMPTOMS
More common cause of severe morbidity and mortality in older adults and people with underlying chronic medical conditions.
No differentiating signs/symptoms.
INVESTIGATIONS
Real-time reverse transcription polymerase chain reaction (RT-PCR): positive for SARS-CoV-2 RNA. Rapid antigen tests may also be used.
Community-acquired pneumonia
SIGNS / SYMPTOMS
No differentiating signs/symptoms.
INVESTIGATIONS
Diagnostic studies should be considered based on local guidance as well as microbial patterns in a particular community.
Isolation of organisms such as Streptococcus pneumoniae and Staphylococcus aureus from sputum and blood culture, and through response to typical therapy.
Chest x-ray findings for typical pneumonia are consistent with consolidation.
Positive Asian lineage A(H7N9) virus-specific tests do not exclude the possibility of coinfections or bacterial super-infections. Bacterial coinfections have not been detected in most Asian lineage A(H7N9) cases; when they have occurred, bacterial species associated with hospital-associated infections and ventilator-associated pneumonia accounted for the majority of bacterial coinfections. Methicillin-resistant S aureus (MRSA) coinfection has been reported. Coinfection with bacteria associated with community-acquired pneumonia is more common in patients with seasonal influenza.
Atypical pneumonia
SIGNS / SYMPTOMS
No differentiating signs/symptoms.
INVESTIGATIONS
Confirmation of infection by atypical pathogens (including atypical pneumonia pathogens such as Mycoplasma pneumoniae, Legionella pneumophila, and Chlamydophila pneumoniae) by sputum culture, blood culture, or other specific tests.
A diagnosis of atypical pneumonia does not rule out Asian lineage A(H7N9) virus infection, but coinfection with Asian lineage A(H7N9) virus and atypical pneumonia pathogens has not been reported.
Endemic respiratory infections
SIGNS / SYMPTOMS
No differentiating signs/symptoms.
Respiratory infections due to pathogens endemic to the region where infection occurred should be considered (e.g., endemic mycotic infection, melioidosis in parts of Southeast Asia).
INVESTIGATIONS
Diagnostic tests confirming infection caused by another respiratory virus do not rule out Asian lineage A(H7N9) virus infection, but coinfection with Asian lineage A(H7N9) virus and endemic respiratory infections has not been reported.
Seasonal influenza A or B virus infection
SIGNS / SYMPTOMS
More common cause of severe morbidity in young children, older adults, and people with underlying chronic medical conditions (e.g., cardiopulmonary disease, immunosuppressed). More likely to be a self-limited condition with milder symptoms among previously healthy people.
No differentiating signs/symptoms, but severe lower respiratory tract disease can also occur among previously healthy children, young adults, pregnant women, and morbidly obese people.
Seasonal influenza and Asian lineage A(H7N9) virus infection can have rapid onset of fever, cough, and pneumonia.
INVESTIGATIONS
Confirmation by diagnostic testing of infection by another respiratory virus does not rule out Asian lineage A(H7N9) virus infection. Coinfections with Asian lineage A(H7N9) and seasonal A(H3N2) and seasonal A(H1N1)pdm09 viruses have been reported.[75][123][124] A nosocomial cluster induced by coinfections with avian influenza A(H7N9) and A(H1N1)pdm09 viruses occurred in two patients at a hospital in China.[75] The implications of such influenza virus coinfections on clinical outcomes are not clear. Because there is potential for virus reassortment, detection of other influenza A virus subtypes as part of an influenza surveillance program is recommended.
Rapid influenza diagnostic tests (antigen tests) lack sensitivity to detect influenza viruses and cannot distinguish between Asian lineage A(H7N9) A virus and other influenza A viruses, and should not be used to diagnose Asian lineage A(H7N9) virus infection.
Commercially available influenza molecular assays have high sensitivity to detect influenza viruses in respiratory specimens, but cannot specifically identify A(H7N9) virus, or distinguish A(H7N9) virus from seasonal influenza A viruses.
Respiratory syncytial virus infection (RSV)
SIGNS / SYMPTOMS
Most common cause of lower respiratory tract infection in children aged <1 year.
Significant and often unrecognized cause of lower respiratory tract infection in both older and immunosuppressed patients.
Gives rise to upper and lower respiratory symptoms that peak in 3 to 5 days and resolve within 7 to 10 days.
INVESTIGATIONS
Rapid assays using antigen-capture technology are the mainstay of diagnosis. Molecular detection methods (PCR) are used increasingly to detect RSV.
Confirmation by diagnostic testing of infection by another respiratory virus does not rule out Asian lineage A(H7N9) virus infection, but coinfection with Asian lineage A(H7N9) virus and other respiratory viruses has not been reported. However, coinfections with other respiratory viruses have been identified in patients infected with A(H1N1)pdm09 and seasonal influenza viruses.
Middle East respiratory syndrome (MERS)
SIGNS / SYMPTOMS
No differentiating signs/symptoms.
Common symptoms are acute, serious respiratory illness with fever, cough, shortness of breath, and breathing difficulties. Most patients have pneumonia, respiratory failure, and acute respiratory distress syndrome. Many also have gastrointestinal symptoms (including diarrhea) while others have kidney failure.
INVESTIGATIONS
RT-PCR for MERS-CoV is not commonly available, but can be found at some international public health laboratories, particularly in regions affected by MERS-CoV infections.
Other novel influenza A virus infections (e.g., avian influenza A(H5N1) or A(H5N6) viruses)
SIGNS / SYMPTOMS
No differentiating signs/symptoms. Most patients have a history of recent direct or close exposure to infected animals (e.g., sick or dead poultry) or to a confirmed case-patient with avian influenza A virus infection.
INVESTIGATIONS
Rapid influenza diagnostic tests (antigen tests) lack sensitivity to detect influenza viruses and cannot distinguish between Asian lineage A(H7N9) virus and seasonal influenza A viruses or other novel influenza A viruses associated with severe disease in infected humans (e.g., H5N1, H5N6). Commercially available influenza molecular assays have high sensitivity to detect influenza viruses in respiratory specimens, but cannot specifically identify A(H7N9) virus, or distinguish A(H7N9) virus from seasonal influenza A viruses or other novel influenza A viruses (e.g., H5N1, H5N6). If an influenza A virus is identified, and cannot be subtyped as a seasonal influenza A virus subtype (e.g., not H1 or H3), then specific testing of respiratory specimens by RT-PCR for H5 and H7 or other avian influenza A virus subtypes based upon epidemiologic factors (e.g., recent poultry exposures) should be undertaken at a public health laboratory.
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