Influenza
Influenza is an acute respiratory illness that produced four global pandemics in the last century,
the worst of which occurred in 1918. Known as “Spanish Flu”, the 1918 pandemic was
considered as a global disaster where 500 million people were believed to be infected and an
estimated death toll reached at least 50 million worldwide.
What is Influenza?
Influenza, one of the most common infectious diseases, is a highly contagious airborne disease
that occurs in seasonal epidemics and manifests as an acute febrile illness with variable degrees
of systemic symptoms.
● Although the seasonal strains of influenza virus that circulate in the annual influenza
cycle constitute a substantial public health concern, far more lethal influenza strains
than these have emerged periodically.
● Besides humans, influenza also infects a variety of animal species; some of these
influenza strains are species-specific, but new strains may spread from other animals
to humans.
Pathophysiology
Influenza viruses are enveloped, negative-sense, single-stranded RNA viruses of the family
Orthomyxoviridae.
● The core nucleoproteins are used to distinguish the 3 types of influenza viruses:
influenza A, B, and C (IAV, IBV, and ICV).
● Hemagglutinin and neuraminidase are critical for virulence, and they are major
targets for the neutralizing antibodies of acquired immunity to influenza.
● Hemagglutinin binds to respiratory epithelial cells, allowing cellular infection.
● Neuraminidase cleaves the bond that holds newly replicated virions to the cell
surface, permitting the infection to spread.
● The species specificity of influenza strains is partly due to the ability of a given
hemagglutinin to bind to different sialic acid receptors on respiratory tract epithelial
cells.
Statistics and Incidences
In tropical areas, influenza occurs throughout the year.
, ● The WHO estimates that worldwide, annual influenza epidemics result in about 3-5
million cases of severe illness and about 250,000 to 500,000 deaths.
● The US Centers for Disease Control and Prevention (CDC) notes that the often-cited
figure of 36,000 annual flu-related deaths was derived from years when the
predominant virus subtype was H3N2, which tends to be more lethal than H1N1.
● For the 2019-2020 influenza season, 55,000 influenza-related hospitalizations had
been reported as of early January 2020, and 2900 attributable deaths had been
reported at that time, including 27 pediatric deaths.
Causes
Influenza results from infection with 1 of 3 basic types of influenza virus: A, B, or C.
● Direct contact. Transmission of influenza from poultry or pigs to humans appears to
occur predominantly as a result of direct contact with infected animals.
● Unhygienic food preparation. The risk is especially high during slaughter and
preparation for consumption; eating properly cooked meat poses nor risk.
● Aerosol transmission. Influenza viruses spread from human to human via aerosols
created when an infected individual coughs or sneezes; infection occurs after an
immunologically susceptible person inhales the aerosol; if not neutralized by
secretory antibodies, the virus invades airway and respiratory tract cells.
● Contact with contaminated objects. Contact with excrement from infected birds or
contaminated surfaces or water are also considered mechanisms of infection.
Clinical Manifestations
The presentation of influenza virus infection varies; however, it usually includes many of the
symptoms described below.
● Cough. Cough and other respiratory symptoms may be initially minimal but
frequently progress as the infection evolves; patients may report nonproductive
cough, cough-related pleuritic chest pain, and dyspnea.
● Fever. Fever may vary widely among patients, with some having low fevers and
other developing fevers as high as 1040F; some patients report feeling feverish and
feeling chills.
● Sore throat. Sore throat may be severe and may last 3-5 days; the sore throat may
be a significant reason why patients seek medical attention.
● Myalgia. Myalgias are common and range from mild to severe.
Influenza is an acute respiratory illness that produced four global pandemics in the last century,
the worst of which occurred in 1918. Known as “Spanish Flu”, the 1918 pandemic was
considered as a global disaster where 500 million people were believed to be infected and an
estimated death toll reached at least 50 million worldwide.
What is Influenza?
Influenza, one of the most common infectious diseases, is a highly contagious airborne disease
that occurs in seasonal epidemics and manifests as an acute febrile illness with variable degrees
of systemic symptoms.
● Although the seasonal strains of influenza virus that circulate in the annual influenza
cycle constitute a substantial public health concern, far more lethal influenza strains
than these have emerged periodically.
● Besides humans, influenza also infects a variety of animal species; some of these
influenza strains are species-specific, but new strains may spread from other animals
to humans.
Pathophysiology
Influenza viruses are enveloped, negative-sense, single-stranded RNA viruses of the family
Orthomyxoviridae.
● The core nucleoproteins are used to distinguish the 3 types of influenza viruses:
influenza A, B, and C (IAV, IBV, and ICV).
● Hemagglutinin and neuraminidase are critical for virulence, and they are major
targets for the neutralizing antibodies of acquired immunity to influenza.
● Hemagglutinin binds to respiratory epithelial cells, allowing cellular infection.
● Neuraminidase cleaves the bond that holds newly replicated virions to the cell
surface, permitting the infection to spread.
● The species specificity of influenza strains is partly due to the ability of a given
hemagglutinin to bind to different sialic acid receptors on respiratory tract epithelial
cells.
Statistics and Incidences
In tropical areas, influenza occurs throughout the year.
, ● The WHO estimates that worldwide, annual influenza epidemics result in about 3-5
million cases of severe illness and about 250,000 to 500,000 deaths.
● The US Centers for Disease Control and Prevention (CDC) notes that the often-cited
figure of 36,000 annual flu-related deaths was derived from years when the
predominant virus subtype was H3N2, which tends to be more lethal than H1N1.
● For the 2019-2020 influenza season, 55,000 influenza-related hospitalizations had
been reported as of early January 2020, and 2900 attributable deaths had been
reported at that time, including 27 pediatric deaths.
Causes
Influenza results from infection with 1 of 3 basic types of influenza virus: A, B, or C.
● Direct contact. Transmission of influenza from poultry or pigs to humans appears to
occur predominantly as a result of direct contact with infected animals.
● Unhygienic food preparation. The risk is especially high during slaughter and
preparation for consumption; eating properly cooked meat poses nor risk.
● Aerosol transmission. Influenza viruses spread from human to human via aerosols
created when an infected individual coughs or sneezes; infection occurs after an
immunologically susceptible person inhales the aerosol; if not neutralized by
secretory antibodies, the virus invades airway and respiratory tract cells.
● Contact with contaminated objects. Contact with excrement from infected birds or
contaminated surfaces or water are also considered mechanisms of infection.
Clinical Manifestations
The presentation of influenza virus infection varies; however, it usually includes many of the
symptoms described below.
● Cough. Cough and other respiratory symptoms may be initially minimal but
frequently progress as the infection evolves; patients may report nonproductive
cough, cough-related pleuritic chest pain, and dyspnea.
● Fever. Fever may vary widely among patients, with some having low fevers and
other developing fevers as high as 1040F; some patients report feeling feverish and
feeling chills.
● Sore throat. Sore throat may be severe and may last 3-5 days; the sore throat may
be a significant reason why patients seek medical attention.
● Myalgia. Myalgias are common and range from mild to severe.
