Orthomyxoviridae

Orthomyxoviridae (from Ancient Greek ὀρθός (orthós) 'straight' and μύξα (mýxa) 'mucus') is a family of negative-sense RNA viruses. It includes nine genera: Alphainfluenzavirus, Betainfluenzavirus, Gammainfluenzavirus, Deltainfluenzavirus, Isavirus, Mykissvirus, Quaranjavirus, Sardinovirus, and Thogotovirus. The first four genera contain viruses that cause influenza in birds (see also avian influenza) and mammals, including humans. Isaviruses infect salmon; the thogotoviruses are arboviruses, infecting vertebrates and invertebrates (such as ticks and mosquitoes). The Quaranjaviruses are also arboviruses, infecting vertebrates (birds) and invertebrates (arthropods).

The four genera of Influenza virus that infect vertebrates, which are identified by antigenic differences in their nucleoprotein and matrix protein, are as follows:

The influenzavirus virion is pleomorphic; the viral envelope can occur in spherical and filamentous forms. In general, the virus's morphology is ellipsoidal with particles 100–120 nm in diameter, or filamentous with particles 80–100 nm in diameter and up to 20 μm long. There are approximately 500 distinct spike-like surface projections in the envelope each projecting 10–14 nm from the surface with varying surface densities. The major glycoprotein (HA) spike is interposed irregularly by clusters of neuraminidase (NA) spikes, with a ratio of HA to NA of about 10 to 1.

The viral envelope composed of a lipid bilayer membrane in which the glycoprotein spikes are anchored encloses the nucleocapsids; nucleoproteins of different sizes; the arrangement within the virion is uncertain. The ribonuclear proteins are filamentous and fall in the range of 50–150 nm long with helical symmetry.

Viruses of the family Orthomyxoviridae contain six to eight segments of linear negative-sense single stranded RNA. They have a total genome length that is 10,000–14,600 nucleotides (nt). The influenza A genome, for instance, has eight pieces of segmented negative-sense RNA (13.5 kilobases total).

The best-characterised of the influenzavirus proteins are hemagglutinin and neuraminidase, two large glycoproteins found on the outside of the viral particles. Hemagglutinin is a lectin that mediates binding of the virus to target cells and entry of the viral genome into the target cell. In contrast, neuraminidase is an enzyme involved in the release of progeny virus from infected cells, by cleaving sugars that bind the mature viral particles. The hemagglutinin (H) and neuraminidase (N) proteins are key targets for antibodies and antiviral drugs, and they are used to classify the different serotypes of influenza A viruses, hence the H and N in H5N1.

The genome sequence has terminal repeated sequences, and these are repeated at both ends (i.e., at both the 5’ end and the 3’ end). These terminal repeats at the 5′-end are 12–13 nucleotides long. Nucleotide sequences at the 3′-terminus are identical, are the same in genera of the same family, most on RNA (segments), or on all RNA species. Terminal repeats at the 3′-end are 9–11 nucleotides long. Encapsidated nucleic acid is solely genomic. Each virion may contain defective interfering copies. In Influenza A (specifically, in H1N1) PB1-F2 is produced from an alternative reading frame in PB1. The M and NS genes produce two genes each (4 genes total) via alternative splicing.

Typically, influenza is transmitted from infected mammals through the air by coughs or sneezes, creating aerosols containing the virus, and from infected birds through their droppings. Influenza can also be transmitted by saliva, nasal secretions, feces and blood. Infections occur through contact with these bodily fluids or with contaminated surfaces. On certain surfaces (i.e, outside of a host), flu viruses can remain infectious for about one week at human body temperature, over 30 days at 0 °C (32 °F), and indefinitely at very low temperatures (such as in lakes in northeast Siberia). They can be inactivated easily by disinfectants and detergents.