Influenza Quiz

Influenza: Symptoms, Types, Virology, and Life Cycle

Influenza, also known as the flu, is an infectious disease caused by influenza viruses that can range from mild to severe and can cause complications such as pneumonia, acute respiratory distress syndrome, meningitis, and encephalitis.
There are four types of influenza virus: A, B, C, and D, with A and B causing seasonal epidemics and occasional pandemics. Influenza viruses are primarily transmitted through respiratory droplets produced from coughing and sneezing.
Frequent hand washing and covering one's mouth and nose when coughing and sneezing reduce transmission. Annual vaccination can help to provide protection against influenza, but vaccines must be updated regularly to match which influenza strains are in circulation.
In a typical year, 5-15% of the population contracts influenza, with up to 650,000 respiratory-related deaths globally each year. Deaths most commonly occur in high-risk groups, including young children, the elderly, and people with chronic health conditions.
Symptoms of influenza include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. Diarrhea and vomiting can occur, particularly in children. Symptoms usually last for about 2-8 days and may progress to pneumonia, which can be caused by the virus or by a subsequent bacterial infection.
Influenza viruses have a negative-sense, single-stranded RNA genome that is segmented, with IAV and IBV having eight genome segments that encode 10 major proteins and ICV and IDV having seven genome segments that encode nine major proteins.
Influenza viruses bind to cells that contain sialic acid receptors on the surface of the cell membrane. After binding, the virus is internalized into the cell by an endosome and then uncoated in the cytosol. RNPs are imported into the nucleus, and viral RNA polymerase transcribes mRNA using the genomic negative-sense strand as a template. The polymerase snatches 5' caps for viral mRNA from cellular RNA to prime mRNA synthesis and the 3'-end of mRNA is polyadenylated at the end of transcription. Once viral mRNA is transcribed, it is exported out of the nucleus and translated by host ribosomes to synthesize viral proteins. RdRp also synthesizes complementary positive-sense strands of the viral genome, which are then used as templates by viral polymerases to synthesize copies of the negative-sense genome.
Two key processes that influenza viruses evolve through are antigenic drift and antigenic shift. Antigenic drift is the accumulation of random mutations in the genes that encode the virus's surface proteins, HA and NA, which can lead to the virus becoming less susceptible to the immune system. Antigenic shift occurs when different strains of influenza viruses infect the same cell and exchange genetic material, leading to the emergence of a new strain that can cause a pandemic.
IAV is responsible for most cases of severe illness and seasonal epidemics, infecting people of all ages but disproportionately causing severe illness in the elderly, the very young, and those who have chronic health issues. IBV mainly infects humans but has never been associated with a pandemic. ICV infection primarily affects children and is usually asymptomatic or has mild cold-like symptoms, while IDV is not known to cause illness in humans.
InfluenzaInfluenza: Mechanisms, Transmission, Pathophysiology, Immunology, and Prevention
Antigenic drift is a gradual change in an influenza virus's antigens due to mutations in the antigen's gene that can evade pre-existing immunity, especially for the HA protein, and occurs in all influenza species but is slower in B than A and slowest in C and D.
Antigenic shift is a sudden, drastic change in an influenza virus's antigen, usually HA, due to genome segment reassortment that only occurs among influenza viruses of the same genus, most commonly occurs among IAVs, and can cause pandemics.
Influenza is transmitted through respiratory droplets and aerosols that contain virus particles, and transmission through contact with a person, bodily fluids, or intermediate objects can also occur.
Influenza viruses first infect epithelial cells in the respiratory tract, causing lung inflammation and compromise, and severe respiratory illness can be caused by multiple mechanisms, including obstruction of the airways, loss of alveolar structure, and alveolar cell infection.
B cells produce antibodies that bind to influenza antigens HA and NA and other proteins, blocking virions from binding to cellular receptors, and providing some protection to related strains.
Annual vaccination is the primary and most effective way to prevent influenza and influenza-associated complications, especially for high-risk groups, and vaccination recommendations vary by country.
Chemoprophylaxis is most useful for individuals at high-risk of developing complications and those who cannot receive the flu vaccine due to contraindications or lack of effectiveness.
Hand hygiene, respiratory etiquette, and mask wearing are commonly recommended methods of infection control, and protective clothing is recommended in health care settings.
Prevention of transmission from animals is important, and measures include water treatment, indoor raising of animals, quarantining sick animals, vaccination, and biosecurity.
Closure of live poultry markets appears to be the most effective measure to control the spread of influenza viruses, and other biosecurity measures include cleaning and disinfecting facilities and vehicles.
Rapid detection and stamping out via quarantining, decontamination, and culling may be necessary to prevent the virus from becoming endemic if a novel influenza virus breaches biosecurity measures.
Vaccines exist for avian H5, H7, and H9 subtypes that are used in some countries.Influenza: Diagnosis, Treatment, Prognosis, and Epidemiology

Diagnosis:

Diagnosis of influenza requires laboratory confirmation due to its similarity to other respiratory illnesses
Samples for testing include nasal and throat swabs or lower respiratory tract samples
Diagnostic methods include viral cultures, antibody- and antigen-detecting tests, and nucleic acid-based tests
Nucleic acid-based tests, particularly RT-PCR, are considered the gold standard for diagnosis
Rapid influenza diagnostic tests (RIDTs) are commonly used but are less sensitive than nucleic acid-based tests

Treatment:

Treatment of mild to moderate influenza is supportive, including rest, hydration, and anti-fever medication
Aspirin is not recommended for children due to the risk of Reye syndrome
Antiviral drugs are primarily used to treat severely ill patients, particularly those with compromised immune systems
Neuraminidase (NA) inhibitors, such as oseltamivir and zanamivir, and baloxavir marboxil are the primary antiviral drugs used for influenza
Antivirals are most effective when started within 48 hours of symptom onset

Prognosis:

Influenza is self-limiting in healthy individuals and rarely fatal
Complications and mortality occur primarily in high-risk populations and those who are hospitalized
Complications include pneumonia, sinusitis, bronchitis, and exacerbation of chronic illnesses
Neurological complications, such as encephalitis and Guillain–Barré syndrome, can occur rarely
Pregnant women, children under one year of age, and those with chronic illnesses are at an elevated risk of complications

Epidemiology:

Influenza causes seasonal epidemics and sporadic pandemics, primarily caused by IAV and IBV
In a typical year, influenza viruses infect 5-15% of the global population and account for 290,000-650,000 deaths each year due to respiratory illness
Mortality is concentrated in the very young, the elderly, and those with chronic illnesses
In temperate regions, influenza peaks during the winter season, while in tropical and subtropical regions, seasonality is more complex
ICV infection is most common in children under the age of 2 and most people have been exposed to it by adulthood
Novel influenza viruses can cause pandemics, depending on the level of pre-existing immunity in the populationA Brief History of Influenza Pandemics
Influenza pandemics are caused by antigenic shifts involving animal influenza viruses.
Pandemics follow the same pattern of spreading from an origin point over the course of multiple waves in a year.
Pandemic strains are associated with higher rates of pneumonia in otherwise healthy individuals.
From 1700 to 1889, influenza pandemics occurred about once every 50-60 years. Since then, pandemics have occurred about once every 10-50 years.
The first convincing record of an influenza pandemic was chronicled in 1510.
The 1918 Spanish flu pandemic was the most devastating and one of the deadliest pandemics in history, killing tens of millions of people.
Four influenza pandemics have occurred since WWII, with the Asian flu from 1957 to 1958 being the first flu pandemic to occur in the presence of a global surveillance system and laboratories able to study the novel influenza virus.
The most recent flu pandemic was the 2009 swine flu pandemic, which originated in Mexico and resulted in hundreds of thousands of deaths.
Birds are the primary natural reservoir of influenza A viruses.
Highly pathogenic avian influenza was recognized in 1878 and was soon linked to transmission to humans.
Influenza research includes efforts to improve the diagnosis, treatment, and prevention of influenza.
"Broadly-reactive" or "universal" flu vaccines are being researched that can provide protection against many or all influenza viruses.