Influenza C virus is the only species in the

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

''Gammainfluenzavirus'', in the

virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...

family ''

Orthomyxoviridae ''Orthomyxoviridae'' () is a family of negative-sense single-stranded RNA virus, negative-sense RNA viruses. It includes nine genus, genera: ''Influenza A virus, Alphainfluenzavirus'', ''Influenza B virus, Betainfluenzavirus'', ''Influenza C vir ...

'', which like other influenza viruses, causes

influenza Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These sympto ...

. Influenza C viruses are known to infect

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s and

pig The pig (''Sus domesticus''), also called swine (: swine) or hog, is an omnivorous, domesticated, even-toed, hoofed mammal. It is named the domestic pig when distinguishing it from other members of the genus '' Sus''. Some authorities cons ...

s. Flu due to the Type C species is rare compared with Types B or A, but can be severe and can cause local

epidemic An epidemic (from Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of hosts in a given population within a short period of time. For example, in meningococcal infection ...

s. Type C has 7

RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...

segments and encodes 9

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s, while Types A and B have 8 RNA segments and encode at least 10 proteins.

Influenza C virus

Influenza viruses are members of the family ''

''. Influenza viruses A, B, C, and D represent the four

antigen In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

ic types of influenza viruses. Of the four antigenic types,

influenza A virus ''Influenza A virus'' (''Alphainfluenzavirus influenzae'') or IAV is the only species of the genus ''Alphainfluenzavirus'' of the virus family '' Orthomyxoviridae''. It is a pathogen with strains that infect birds and some mammals, as well as c ...

is the most severe,

influenza B virus ''Influenza B virus'' is the Monotypic taxon, only species in the genus ''Betainfluenzavirus'' in the virus family ''Orthomyxoviridae''. Influenza B virus is a Negative-strand RNA virus, negative-sense single-strand RNA virus known only to inf ...

is less severe but can still cause outbreaks, and influenza C virus is usually only associated with minor symptoms. Influenza D virus is 50% similar in

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

composition to influenza C virus, similar to the level of divergence between types A and B, while types C and D have a much greater level of divergence from types A and B. Influenza viruses C and D were estimated to have diverged from a

common ancestor Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. According to modern evolutionary biology, all living beings could be descendants of a unique ancestor commonl ...

over 1,500 years ago, around 482 AD. Influenza viruses A and B are estimated to have diverged from a single ancestor around 4,000 years ago, while the ancestor of influenza viruses A and B and the ancestor of influenza virus C are estimated to have diverged from a common ancestor around 8,000 years ago. Influenza A virus can infect a variety of animals as well as humans, and its

natural reservoir In Infection, infectious disease ecology and epidemiology, a natural reservoir, also known as a disease reservoir or a reservoir of infection, is the population of organisms or the specific environment in which an infectious pathogen naturally li ...

(natural host) is birds, whereas influenza viruses B, C, and D do not have

animal reservoir In infectious disease ecology and epidemiology, a natural reservoir, also known as a disease reservoir or a reservoir of infection, is the population of organisms or the specific environment in which an infectious pathogen naturally lives and r ...

s. Influenza C virus is not as easily isolated so less information is known of this type, but studies show that it occurs worldwide. Influenza C virus currently has six lineages, which were estimated to have emerged around 1896 AD.Metatranscriptomics studies also have identified closely related "Influenza C and D-like" viruses in several amphibian and fish species suggesting the potential for divergent influenza C/D like viruses circulating in aquatic systems. This virus may be spread from person to person through respiratory droplets or by

fomite A fomite () or fomes () is any inanimate object that, when contaminated with or exposed to infectious agents (such as pathogenic bacteria, viruses or fungi), can transfer disease to a new host. Transfer of pathogens by fomites A fomite is any ...

s (non-living material) due to its ability to survive on surfaces for short durations. Influenza viruses have a relatively short

incubation period Incubation period (also known as the latent period or latency period) is the time elapsed between exposure to a pathogenic organism, a chemical, or ionizing radiation, radiation, and when symptoms and signs are first apparent. In a typical infect ...

(lapse of time from exposure to pathogen to the appearance of symptoms) of 18–72 hours and infect the

epithelial cells Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

of the

respiratory tract The respiratory tract is the subdivision of the respiratory system involved with the process of conducting air to the alveoli for the purposes of gas exchange in mammals. The respiratory tract is lined with respiratory epithelium as respirato ...

. Influenza virus C tends to cause mild upper

respiratory infection Respiratory tract infections (RTIs) are infectious diseases involving the lower or upper respiratory tract. An infection of this type usually is further classified as an upper respiratory tract infection (URI or URTI) or a lower respiratory tra ...

s. Cold-like symptoms are associated with the virus including fever (38–40 °C), dry cough,

rhinorrhea Rhinorrhea (American English), also spelled rhinorrhoea or rhinorrhœa (British English), or informally runny nose is the free discharge of a thin mucus fluid from the nose; it is an extremely common condition. It is a common symptom of allergie ...

(nasal discharge), headache, muscle pain, and achiness. The virus may lead to more severe infections such as

bronchitis Bronchitis is inflammation of the bronchi (large and medium-sized airways) in the lungs that causes coughing. Bronchitis usually begins as an infection in the nose, ears, throat, or sinuses. The infection then makes its way down to the bronchi. ...

and

pneumonia Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of Cough#Classification, productive or dry cough, ches ...

. After an individual becomes infected, the immune system develops

antibodies An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...

against that infectious agent. This is the body's main source of protection. Most children between five and ten years old have already produced antibodies for influenza virus C. As with all influenza viruses, type C affects individuals of all ages but is most severe in young children, the elderly and individuals with underlying health problems. Young children have less prior exposure and have not developed the antibodies and the elderly have less effective immune systems. Influenza virus infections have one of the highest preventable mortalities in many countries of the world.

Structure and variation

Influenza viruses, like all viruses in the family ''Orthomyxoviridae'', are

envelope An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter (message), letter or Greeting card, card. Traditional envelopes are made from sheets of paper cut to one o ...

RNA virus An RNA virus is a virus characterized by a ribonucleic acid (RNA) based genome. The genome can be single-stranded RNA (ssRNA) or double-stranded (Double-stranded RNA, dsRNA). Notable human diseases caused by RNA viruses include influenza, SARS, ...

es with single stranded negative sense RNA

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

s. Divergent evolution of the matrix protein (M1) and nucleoprotein (NP), are used to determine if the virus is type A, B, C, or D. The M1 protein is required for virus assembly and NP functions in transcription and replication. These viruses also contain proteins on the surface of the cell membrane called glycoproteins. Type A and B have two

glycoprotein Glycoproteins are proteins which contain oligosaccharide (sugar) chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known a ...

s: hemagglutinin (HA) and neuraminidase (NA). Type A is divided into subtypes based on distinct differences in the types of these glycoproteins. Types C and D have only one glycoprotein: hemagglutinin-esterase fusion (HEF). These glycoproteins allow for attachment and fusion of viral and cellular membranes. Fusion of these membranes allows the viral proteins and genome to be released into the host cell, which then causes the infection. Types C and D are the only influenza viruses to express the enzyme

esterase In biochemistry, an esterase is a class of enzyme that splits esters into an acid and an alcohol in a chemical reaction with water called hydrolysis (and as such, it is a type of hydrolase). A wide range of different esterases exist that differ ...

. This enzyme is similar to the enzyme neuraminidase produced by Types A and B in that they both function in destroying the host cell receptors. Glycoproteins may undergo

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...

s (antigenic drift) or reassortment in which a new type of HA or NA is produced (antigenic shift). Influenza virus C is only capable of

antigenic drift Antigenic drift is a kind of genetic variation in viruses, arising from the accumulation of mutations in the virus genes that code for virus-surface proteins that host antibodies recognize. This results in a new strain of virus particles that is ...

whereas Type A undergoes

antigenic shift Antigenic shift is the process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains. The term is ...

, as well. When either of these processes occur, the antibodies formed by the immune system no longer protect against these altered

s. Because of this, viruses continually cause infections.

Identification

Influenza virus C is different from Types A and B in its growth requirements. Because of this, it is not isolated and identified as frequently. Diagnosis is by virus isolation,

serology Serology is the scientific study of Serum (blood), serum and other body fluids. In practice, the term usually refers to the medical diagnosis, diagnostic identification of Antibody, antibodies in the serum. Such antibodies are typically formed in r ...

, and other tests. Hemagglutination inhibition (HI) is one method of serology that detects antibodies for diagnostic purposes.

Western blot The western blot (sometimes called the protein immunoblot), or western blotting, is a widely used analytical technique in molecular biology and immunogenetics to detect specific proteins in a sample of tissue homogenate or extract. Besides detect ...

(immunoblot assay) and enzyme-linked immunosorbent assay (

ELISA The enzyme-linked immunosorbent assay (ELISA) (, ) is a commonly used analytical biochemistry assay, first described by Eva Engvall and Peter Perlmann in 1971. The assay is a solid-phase type of enzyme immunoassay (EIA) to detect the presence of ...

) are two other methods used to detect proteins (or antigens) in serum. In each of these techniques, the antibodies for the protein of interest are added and the presence of the specific protein is indicated by a color change. ELISA was shown to have higher sensitivity to the HEF than the HI test. Because only Influenza viruses C and D produce esterase, In Situ Esterase Assays provide a quick and inexpensive method of detecting just Types C and D. If more individuals were tested for Influenza virus C as well as the other three types, infections not previously associated with Type C may be recognized.

Vaccination

Because influenza virus A has an animal reservoir that contains all the known subtypes and can undergo

, this type of influenza virus is capable of producing

pandemic A pandemic ( ) is an epidemic of an infectious disease that has a sudden increase in cases and spreads across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. Widespread endemic (epi ...

s. Influenza viruses A and B also cause seasonal

s almost every year due to their ability to

. Influenza virus C does not have this capability and it is not thought to be a significant concern for human health. Therefore, there are no vaccinations against influenza virus C.

References

External links