Influenza C virus is the species in the genusGammainfluenzavirus, in the virus family Orthomyxoviridae, which like other influenza viruses, causes influenza. Influenza C viruses are known to infect humans and pigs. Flu due to the Type C species is rare compared to Types B or A, but can be severe and can cause local epidemics. Type C has 7 RNA segments and encodes 9 proteins, 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 Orthomyxoviridae. Influenza viruses A, B, C, and D represent the four antigenic types of influenza viruses. Of the four antigenic types, influenza A virus is the most severe, influenza B virus 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 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 single ancestor over 1,500 years ago, around 482 AD. Influenza A virus can infect a variety of animals as well as humans, and its natural host or reservoir is birds, whereas influenza viruses B, C, and D do not have animal reservoirs. 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 fomites due to its ability to survive on surfaces for short durations. Influenza viruses have a relatively short incubation period of 18–72 hours and infect the epithelial cells of the respiratory tract. Influenza virus C tends to cause mild upper respiratory infections. Cold-like symptoms are associated with the virus including fever, dry cough, rhinorrhea, headache, muscle pain, and achiness. The virus may lead to more severe infections such as bronchitis and pneumonia. After an individual becomes infected, the immune system develops antibodies 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 enveloped RNA viruses with single stranded genomes. The antigens, matrix protein and nucleoprotein, 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 glycoproteins: hemagglutinin and neuraminidase. Types C and D have only one glycoprotein: hemagglutinin-esterase fusion. 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. 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 mutations or reassortment in which a new HA or NA is produced. Influenza virus C is only capable of antigenic drift whereas Type A undergoes antigenic shift, as well. When either of these processes occur, the antibodies formed by the immune system no longer protect against these altered glycoproteins. 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, and other tests. Hemagglutination inhibition is one method of serology that detects antibodies for diagnostic purposes. Western blot and enzyme-linked immunosorbent assay are two other methods used to detect proteins 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 antigenic shift, this type of influenza virus is capable of producing pandemics. Influenza viruses A and B also cause seasonal epidemics almost every year due to their ability to antigenic drift. 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.
