Influenza (the flu) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death.
In virus classification influenza viruses are RNA viruses/ Subfamily: Orthomyxoviridae.
Influenza virus A
Influenza virus B
Influenza virus C
Influenza (The flu)
Influenza A virus; the most virulent human pathogens among the three influenza types.
Influenza A virus; capable of infecting human as well as animals (ducks, chickens, pigs, whales, horses and seals). Wild aquatic birds are the natural hosts for a large variety of influenza A.
Influenza A virus is the main cause of worldwide pandemics.
Influenza A viruses subtypes e.g., (H1N1), (H5N1),….
Influenza A virus
Influenza B virus
Influenza B virus; it almost exclusively infects humans.
Influenza B virus; less common than influenza A.
Influenza B viruses are not divided into subtypes, but can be further broken down into different strains.
Influenza B virus; mutates at a rate 2–3 times lower than type A. This reduced rate of antigenic change, combined with its limited host range ensures that pandemics of influenza B do not occur.
Influenza C virus
Influenza C virus; infects humans.
Influenza C virus; less common than the other types and usually only causes mild disease in children.
The viral particles of all influenza Viruses are similar in composition. These are made of a viral envelope containing two main types of glycoproteins, wrapped around a central core.
The central core contains the viral RNA genome and other viral proteins that package and protect this RNA.
Influenza viruses A, B and C are very similar in overall structure.
The virus particle is 80–120 nanometres in diameter and usually roughly spherical, although filamentous forms can occur. These filamentous forms are more common in influenza C, which can form cordlike structures up to 500 micrometres long on the surfaces of infected cells.
Segmented, single stranded RNA of negative polarity/must be copied into positive-sense molecules in order to direct the production of proteins.
The Influenza A Capsid
The influenza A capsid contains the antigenic glycoproteins hemagglutinin (HA) and neuraminidase (NA); several hundred molecules of each protein are needed to form the capsid.
The Influenza A Genome
The influenza A genome encoding for 11 proteins: hemagglutinin (HA), neuraminidase (NA), nucleoprotein (NP), M1, M2, NS1, NS2 (NEP/nuclear export protein), PA, PB1 (polymerase basic 1), PB1-F2 and PB2.
Hemagglutinin and Neuraminidase
There are 16 H and 9 N subtypes known, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans.
Hemagglutinin (HA) is a lectin that mediates binding of the virus to target cells and entry of the viral genome into the target cell.
Neuraminidase (NA) is involved in the release of progeny virus from infected cells, by cleaving sugars that bind the mature viral particles.
These proteins are targets for antiviral drugs.
The Influenza A Subtypes
Type A subtypes of the influenza virus are classified by a naming system that includes:
Human influenza virus subtypes that spread widely among humans.
Human flu-causing viruses can belong to any of three major influenza-causing Orthomyxo viruses; Influenza A; B; C virus.
Three known A subtypes of influenza viruses currently circulating among humans (H1N1, H1N2, and H3N2).
Avian influenza is an infectious disease of birds caused by type A strains of the influenza virus.
These viruses occur naturally among wild aquatic birds worldwide and can infect domestic poultry and other bird and animal species. The disease, which was first identified in Italy more than 100 years ago.
Fifteen subtypes of influenza virus are known to infect birds, thus providing an extensive reservoir of influenza viruses potentially circulating in bird populations.
H5N1; the strain of avian flu known as has been behind outbreaks of deadly avian flu.
Avian influenza transmitted by birds usually through feces or saliva.
Avian influenza is not usually passed on to humans, although it has been contracted by people who have handled infected birds or touched surfaces contaminated by the birds.
Migratory water birds, especially wild ducks. They may do not show clinical disease. The virus colonizes the intestinal tract and is spread in the feces . They act as a reservoir for the infection of other species .
Pigs can be infected by bird influenza (as well as by the form of influenza that affects humans) and can pass on the flu to humans.
Low pathogenicity (LPAI) - usually only causing mild respiratory disease in domestic poultry .
High pathogenicity (HPAI) - the more virulent type formerly known as fowl plague which often results in up to a 100% flock mortality.
Swine influenza (swine flu) is a respiratory disease of pigs caused by type A influenza virus that regularly cause outbreaks of influenza in pigs.
Like human influenza viruses, there are different subtypes and strains of swine influenza viruses. The main swine influenza viruses circulating in U.S. pigs in recent years are: H1N1 influenza virus, H3N2 virus, H1N2 virus.
Influenza in swine was first recognized as an epizootic disease in 1918.
Swine influenza virus was first isolated from humans in 1974. Serologic evidence of infections with a swine influenza virus in humans has also been obtained. Viruses of swine may be a potential source of epidemic disease for humans.
Symptoms and Signs/ In pigs
Fever, lethargy, sneezing, coughing, difficulty breathing and decreased appetite.
Although mortality is usually low (around 1–4%), the virus can produce weight loss and poor growth, causing economic loss to farmers.
Influenza A viruses are constantly changing, and they might adapt over time to infect and spread among humans.
All type A influenza viruses, including those that regularly cause seasonal epidemics of influenza in humans, are genetically labile and well adapted to elude host defenses.
The Influenza A
The Influenza A
Influenza viruses lack mechanisms for the “proofreading” and repair of errors that occur during replication. As a result of these uncorrected errors, the genetic composition of the viruses changes as they replicate in humans and animals, and the existing strain is replaced with a new antigenic variant.
Viruses undergo genetic change by several mechanisms
Genetic drift where individual bases in the DNA or RNA mutate to other bases.
Antigenic shift is where there is a major change in the genome of the virus. This occurs as a result of recombination.
Genesis of Human Influenza Viruses
When a virus such as H5N1 comes along, it may merge with a flu that a human body already has and develop (or mutate) into a whole other subtype for which our body has not built immunity. This, then, is what has prompted the concerns of the public health community.
Epidemic (seasonal) influenza which occurs annually and is attributable to minor changes in genes that encode proteins on the surface of circulating influenza viruses. These are known as interpandemic epidemics.
Pandemic influenza which occurs when more significant changes in the influenza A virus arises when human virus strains acquire genes from influenza viruses of other animal species. When this happens, everyone in the world is susceptible to the new virus, and a worldwide epidemic or pandemic can result.
Treatment with oseltamivir (trade name Tamiflu®) or zanamivir (trade name Relenza®) is recommended for all people with suspected or confirmed influenza who require hospitalization.
References & Further Reading
Warren E. Levinson. Medical Microbiology & Immunology. Paperback: 644 pages. McGraw-Hill/Appleton & Lange; 8 edition (June 24, 2004). ISBN-10: 0071431993 . ISBN-13: 978-0071431996. Publication Date: June 24, 2004. Available in paper copy from the publisher