influenza virus

influenza virus
Name	Influenza virus
Taxon	Orthomyxoviridae
Subdivision ranks	Genera
Subdivision	Alphainfluenzavirus, Betainfluenzavirus, Gammainfluenzavirus, Deltainfluenzavirus

influenza virus is a member of the family Orthomyxoviridae and a significant global pathogen responsible for seasonal epidemics and occasional pandemics of respiratory illness. It is characterized by a segmented, single-stranded RNA genome and a lipid envelope studded with glycoprotein spikes. The virus's ability to undergo antigenic drift and antigenic shift contributes to its persistence and periodic emergence of novel strains with pandemic potential, posing a continuous challenge to public health systems like the World Health Organization.

Structure and classification

The virion is roughly spherical, approximately 80-120 nanometers in diameter, and possesses a lipid envelope derived from the host cell membrane. This envelope is embedded with two major glycoproteins: hemagglutinin (HA), which facilitates viral entry, and neuraminidase (NA), which enables progeny virion release. Beneath the envelope lies a layer of matrix protein (M1), surrounding the viral core which contains the ribonucleoprotein (RNP) complexes. These RNPs consist of the eight segments of negative-sense viral RNA, each wrapped around multiple copies of nucleoprotein (NP) and associated with a heterotrimeric RNA polymerase complex. The family Orthomyxoviridae is divided into four genera: Alphainfluenzavirus (Influenza A virus), Betainfluenzavirus (Influenza B virus), Gammainfluenzavirus (Influenza C virus), and Deltainfluenzavirus (Influenza D virus). Influenza A virus is further subtyped based on the antigenic properties of its HA and NA proteins, leading to designations such as H1N1 and H3N2, which are monitored by agencies like the Centers for Disease Control and Prevention and the European Centre for Disease Prevention and Control.

Replication cycle

Infection begins when the HA protein binds to sialic acid receptors on the surface of epithelial cells in the human respiratory tract. The virus enters the cell via endocytosis, and the acidic environment of the endosome triggers a conformational change in HA, leading to fusion of the viral and endosomal membranes. The viral RNPs are released into the cytoplasm and transported into the cell nucleus, a unique feature among RNA viruses. Inside the nucleus, the viral RNA polymerase transcribes the genomic segments into messenger RNA and replicates the viral genome. Newly synthesized viral proteins, such as NP and polymerase components, are imported back into the nucleus to assemble new RNPs. These RNPs are exported to the cytoplasm and travel to the cell membrane, where they bud from the apical surface, incorporating host lipids and the newly synthesized HA and NA glycoproteins. The NA protein cleaves sialic acid residues to prevent viral self-aggregation and facilitate release from the host cell, a process targeted by drugs like oseltamivir.

Transmission and epidemiology

The primary mode of transmission among humans is through respiratory droplets generated by coughing or sneezing, with additional spread possible via direct contact with contaminated surfaces. Influenza viruses circulate globally, causing annual seasonal epidemics that result in significant morbidity and mortality, particularly among high-risk groups like the elderly, young children, and individuals with chronic conditions such as asthma or diabetes. Influenza A virus, with its broad host range including birds and swine, is the source of pandemics when novel subtypes emerge through antigenic shift, as occurred in 1918, 1957, 1968, and 2009. Surveillance networks like the Global Influenza Surveillance and Response System track circulating strains to inform vaccine composition. Avian influenza viruses, such as H5N1 and H7N9, occasionally cause severe zoonotic infections, raising concerns at organizations like the Food and Agriculture Organization.

Symptoms and diagnosis

Infection typically causes an acute onset of fever, cough, sore throat, myalgia, headache, and fatigue. Complications can include pneumonia, exacerbation of underlying medical conditions, and in severe cases, acute respiratory distress syndrome (ARDS) or death. Clinical diagnosis during community outbreaks is common, but laboratory confirmation is essential for surveillance and severe cases. Diagnostic methods include reverse transcription polymerase chain reaction (RT-PCR) assays, which are highly sensitive and specific, rapid influenza diagnostic tests (RIDTs) that detect viral antigens, and viral culture in facilities like the National Institutes of Health. Serological testing for antibody rise can be used for epidemiological studies. Differential diagnosis is important to distinguish it from other respiratory pathogens like rhinovirus or respiratory syncytial virus.

Prevention and treatment

Annual vaccination is the cornerstone of prevention, with vaccines formulated to target the HA and NA proteins of strains predicted to circulate each season, as recommended by the World Health Organization. Antiviral drugs are used for treatment and prophylaxis; the adamantanes (amantadine, rimantadine) target the M2 ion channel of some Influenza A virus strains, while neuraminidase inhibitors (oseltamivir, zanamivir) are effective against both Influenza A virus and Influenza B virus. Supportive care, including hydration and management of fever, is standard. In hospital settings, infection control measures like isolation and use of personal protective equipment are critical to prevent nosocomial transmission, as guided by protocols from the Centers for Disease Control and Prevention. Research into universal vaccines targeting conserved viral regions is ongoing at institutions like the National Institute of Allergy and Infectious Diseases.

History and impact

Descriptions of influenza-like illness date back to Hippocrates, but the causative agent was first identified during the 1918 influenza pandemic, one of the deadliest events in human history. Subsequent pandemics, including the Asian flu of 1957 and the Hong Kong flu of 1968, underscored the virus's persistent threat. The isolation of the virus in ferrets by Wilson Smith, Christopher Andrewes, and Patrick Laidlaw in 1933 was a pivotal moment in virology. The development of inactivated vaccines began in the 1940s under researchers like Thomas Francis Jr., and the establishment of global surveillance was accelerated after the 1957 pandemic. The virus has had profound societal impacts, influencing public health policy, economic productivity, and even military operations, as seen during World War I. Contemporary events like the 2009 swine flu pandemic highlighted the challenges of rapid response and vaccine deployment in a globalized world.

Category:Orthomyxoviridae Category:Viral respiratory diseases