Influenzavirus
Summary
Influenza virus is a segmented, negative-sense, single-stranded RNA virus of the Orthomyxoviridae family, causing seasonal respiratory illness (fever, myalgia, cough, sore throat). Types A, B, and C exist, with Influenza A being responsible for pandemics due to antigenic shift, while both A and B cause seasonal epidemics via antigenic drift.
Detail
Influenza virus is an enveloped, segmented (8 segments), negative-sense single-stranded RNA virus. Its segmented genome allows for genetic reassortment (antigenic shift), which can produce novel strains capable of causing pandemics, particularly relevant to Influenza A which infects humans, birds, and pigs (reservoir for reassortment). Point mutations causing antigenic drift explain the need for annual vaccine reformulation and seasonal epidemics.
Key surface glycoproteins: Hemagglutinin (HA) mediates viral attachment to sialic acid receptors on respiratory epithelium and is the target of neutralizing antibodies; Neuraminidase (NA) cleaves sialic acid to release budding virions, facilitating spread. HA and NA subtypes define strains (e.g., H1N1, H3N2). Influenza A is classified by HA/NA combinations; Influenza B and C lack this subtyping and cause milder or less prevalent disease (C is often asymptomatic/mild).
Clinical presentation: abrupt onset fever, chills, myalgias, headache, dry cough, sore throat, and fatigue lasting about a week. Complications include secondary bacterial pneumonia (commonly S. pneumoniae, S. aureus, H. influenzae), viral pneumonia, myocarditis, encephalopathy, and Reye syndrome (associated with aspirin use in children with viral infections including influenza and varicella).
Diagnosis: rapid antigen testing, RT-PCR, or viral culture from nasopharyngeal swabs.
Treatment: Neuraminidase inhibitors (oseltamivir, zanamivir) block viral release; most effective within 48 hours of symptom onset. Baloxavir marboxil (cap-dependent endonuclease inhibitor) is a newer option blocking viral RNA transcription.
Prevention: Annual vaccination (inactivated or live attenuated) reformulated based on predicted circulating strains due to antigenic drift. High-risk groups (elderly, immunocompromised, pregnant women, young children) are prioritized for vaccination.
High-yield board points: Remember the mnemonic for Orthomyxoviridae features—segmented genome, helical nucleocapsid, envelope with HA and NA spikes. Differentiate from Paramyxoviridae (non-segmented RNA viruses like measles, mumps, RSV, parainfluenza) which cause different diseases despite morphologic similarities.
Sources
- First Aid for the USMLE Step 1
- Sherris Medical Microbiology
- CDC Influenza Clinical Guidance
- Harrison's Principles of Internal Medicine
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.