Manual of Clinical Microbiology


Influenza Viruses

Last Updated:2023

The influenza viruses are members of the family Orthomyxoviridae. Antigenic differences in two major structural proteins, the matrix protein (M) and the nucleoprotein (NP), and phylogenetic analyses of the virus…

Description of the Agents

Orthomyxoviruses are enveloped, single‐stranded RNA viruses with segmented genomes of negative sense. Influenza A and B viruses have eight RNA segments, while influenza C and D viruses have only seven…

Epidemiology and Transmission

Influenza A and B viruses cause annual epidemics in areas with temperate climates, but in tropical climates, seasonality is less apparent and influenza viruses can be isolated throughout the year.…

Clinical Significance

Influenza A and B virus infections typically cause a febrile respiratory illness characterized by fever, cough, upper respiratory tract symptoms (including sore throat, rhinorrhea, and nasal congestion), and systemic symptoms…

Collection, Transport, and Storage of Specimens

Influenza viruses infect the respiratory epithelium and can be found in respiratory secretions of all types. The level of virus shedding parallels the severity of clinical symptoms in uncomplicated influenza…

Direct Detection

Influenza viruses have been detected in clinical specimens by direct and indirect visualization of their typical morphological appearance by electron microscopy (EM). Immune EM has been the most sensitive EM…

Isolation Procedures

Influenza virus isolation procedures should be performed under biosafety level 2 (BSL‐2) conditions. When the clinical sample comes from a patient suspected to be infected with a highly pathogenic avian…

Identification and Typing Systems

A variety of methods are used to identify and characterize influenza virus isolates. The most common are shown in Table 5 and are based upon immunologic or molecular…

Serologic Tests

Influenza virus infections are also identified by serologic methods. Most persons have been infected previously with influenza viruses, so detection of virus‐specific immunoglobulin M or other immunoglobulin subclasses has not…

Antiviral Susceptibilities

Plaque inhibition assays are the gold standard for measuring susceptibility to amantadine and rimantadine, but the assays are cumbersome and time‐consuming to perform. ELISA methods have also been used to…

Evaluation, Interpretation, and Reporting of Results

The results of a diagnostic test must be considered in the context of the overall setting in which the test is ordered. Clinicians play a critical role in assessing the…

Section Editor: Ben Pinsky

Volume Editor: Marie Louise Landry

Editors in Chief: Karen C. Carroll and Michael A. Pfaller

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