''Influenza C virus'' is the only species in the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

''Gammainfluenzavirus'', in the

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

family ''

Orthomyxoviridae ''Orthomyxoviridae'' (from Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense RNA viruses. It includes seven genera: ''Alphainfluenzavirus'', ''Betainfluenzavirus'', '' Gammainfluenzavirus'', ...

'', 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 symptom ...

. Influenza C viruses are known to infect

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...

s and pigs. 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 patients among a given population within an area in a short period of time. Epidemics of infectious ...

s. Type C has 7 RNA segments and encodes 9

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

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 or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune respon ...

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

influenza A virus '' A virus'' (''IAV'') causes influenza in birds and some mammals, and is the only species of the genus ''Alphainfluenzavirus'' of the virus family ''Orthomyxoviridae''. Strains of all subtypes of influenza A virus have been isolated from wild ...

is the most severe,

influenza B virus ''Influenza B virus'' is the only species in the genus ''Betainfluenzavirus'' in the virus family ''Orthomyxoviridae''. Influenza B virus is known only to infect humans and seals. This limited host range is apparently responsible for the lack o ...

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 hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

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. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal comm ...

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 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 ...

(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 re ...

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 6 lineages, which were estimated to have emerged around 1896 AD. 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 radiation, and when symptoms and signs are first apparent. In a typical infectious disease, the in ...

(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 one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellu ...

of the

respiratory tract The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa. Air is breathed in through the nose to ...

. Influenza virus C tends to cause mild upper

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

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

rhinorrhea Rhinorrhea, rhinorrhoea, or informally runny nose is the free discharge of a thin mucus fluid from the nose; it is a common condition. It is a common symptom of allergies ( hay fever) or certain viral infections, such as the common cold or CO ...

(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 inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severit ...

. After an individual becomes infected, the immune system develops

antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of ...

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 or card. Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a ...

RNA virus An RNA virus is a virusother than a retrovirusthat has ribonucleic acid ( RNA) as its genetic material. The nucleic acid is usually single-stranded RNA (ssRNA) but it may be double-stranded (dsRNA). Notable human diseases caused by RNA virus ...

es with single stranded

genome In the fields of molecular biology and genetics, 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 ...

s. The antigens, matrix protein (M1) and nucleoprotein (NP), are used to determine if an influenza 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 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 as glyco ...

s: hemagglutinin (HA) and neuraminidase (NA). 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 An esterase is a hydrolase enzyme that splits esters into an acid and an alcohol in a chemical reaction with water called hydrolysis. A wide range of different esterases exist that differ in their substrate specificity, their protein structure ...

. 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, m ...

s (antigenic drift) or reassortment in which a new 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 and other body fluids. In practice, the term usually refers to the diagnostic identification of antibodies in the serum. Such antibodies are typically formed in response to an infection (against a given mic ...

, 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 uses a solid-phase type of enzyme immunoassay (EIA) to detect the presen ...

) 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 spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. A widespread endemic disease with a stable number of in ...

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