influenza

influenza
Name	Influenza
Caption	Colorized transmission electron micrograph of an influenza virus particle.
Field	Infectious disease (medical specialty), Virology
Symptoms	Fever, cough, sore throat, muscle pain, headache
Complications	Pneumonia, Acute respiratory distress syndrome, Myocarditis, Encephalitis
Duration	1–2 weeks
Causes	Influenza virus
Prevention	Influenza vaccine, Hand washing, Face masks
Treatment	Supportive care, Antivirals
Medication	Oseltamivir, Zanamivir, Peramivir, Baloxavir marboxil
Frequency	3–5 million severe cases yearly
Deaths	290,000–650,000 yearly

influenza. It is an infectious disease caused by viruses within the family Orthomyxoviridae, primarily affecting the respiratory system. The illness ranges from mild to severe and can lead to hospitalization or death, particularly among high-risk groups like the elderly, young children, and individuals with certain chronic medical conditions. Annual seasonal epidemics and occasional pandemics pose a significant ongoing public health challenge worldwide.

Signs and symptoms

Classic symptoms often include the abrupt onset of high fever, non-productive cough, sore throat, myalgia, headache, and profound fatigue. Gastrointestinal symptoms like nausea and vomiting are more common in children than adults. Uncomplicated illness typically resolves within a week, though cough and malaise can persist. Severe disease can progress to viral pneumonia or secondary bacterial pneumonia, a common and serious complication. Other rare but severe complications include the exacerbation of underlying chronic conditions, Myocarditis, Encephalitis, and Reye's syndrome, the latter associated with Aspirin use in children.

Causes

The disease is caused by infection with RNA viruses from the family Orthomyxoviridae, categorized into types A, B, C, and D. Influenza A virus and Influenza B virus are responsible for the seasonal human epidemics, with type A being the most variable and historically causing pandemics. The surface proteins Hemagglutinin and Neuraminidase are critical for viral entry and release from host cells and are the basis for subtype classification, such as H1N1 or H3N2. Influenza C virus generally causes mild illness, while Influenza D virus primarily affects cattle and is not known to cause human illness.

Mechanism

Infection begins when the virus enters respiratory epithelial cells via binding of hemagglutinin to sialic acid receptors. The viral RNA genome is transported to the cell nucleus where replication and transcription occur. New viral particles assemble and bud from the cell membrane, utilizing neuraminidase to cleave sialic acid and facilitate release. The immune response involves both innate defenses, like Interferon, and adaptive responses, including the production of neutralizing antibodies by B cells and cytotoxic activity by T cells. This immune response is responsible for both viral clearance and many of the systemic symptoms.

Diagnosis

Clinical diagnosis during a community outbreak is common, but laboratory confirmation is often required for public health surveillance and in severe cases. The gold standard for detection is viral culture, though it is slow. Rapid influenza diagnostic tests (RIDTs) provide quick results but have variable sensitivity. More accurate molecular methods, such as Reverse transcription polymerase chain reaction (RT-PCR), are increasingly used. Specimens are typically collected via Nasopharyngeal swab or Nasal aspirate. Surveillance networks like the World Health Organization's Global Influenza Surveillance and Response System (GISRS) and the Centers for Disease Control and Prevention in the United States monitor circulating strains.

Prevention

Annual vaccination is the primary and most effective method of prevention. The Influenza vaccine is reformulated each year to target the strains predicted to circulate, as recommended by the World Health Organization. Other key preventive measures include frequent Hand washing with soap, use of alcohol-based hand sanitizers, and respiratory hygiene like covering coughs. In healthcare settings and during severe outbreaks, personal protective equipment such as Face masks and N95 respirators are used. Antiviral drugs like Oseltamivir can be used for post-exposure prophylaxis in certain high-risk individuals.

Treatment

The mainstay of management for uncomplicated cases is Supportive care, including rest, hydration, and medications like Acetaminophen for fever. Specific Antiviral drug treatment, most commonly Neuraminidase inhibitors such as Oseltamivir and Zanamivir, can reduce the duration and severity of symptoms if started early. The Cap-dependent endonuclease inhibitor Baloxavir marboxil is another therapeutic option. These antivirals are particularly important for hospitalized patients, those with severe or progressive illness, and high-risk populations. Antibiotics are only indicated if a secondary bacterial infection, such as pneumonia caused by Streptococcus pneumoniae, is suspected.

Epidemiology

It causes significant annual epidemics worldwide, with an estimated 3 to 5 million cases of severe illness and 290,000 to 650,000 respiratory deaths each year, according to the World Health Organization. In temperate climates, seasonal activity peaks during the winter months. The Centers for Disease Control and Prevention conducts extensive surveillance in the United States through systems like FluView. Pandemics occur when a novel influenza A virus, to which the global population has little immunity, emerges and spreads efficiently. Notable pandemics include the 1918 Spanish flu pandemic caused by an H1N1 virus, the 1957 Asian flu (H2N2), the 1968 Hong Kong flu (H3N2), and the 2009 Swine flu pandemic (H1N1/09 virus).

Category:Influenza Category:Viral diseases Category:Respiratory diseases