Orthomyxoviridae

Influenzavirus A
Influenzavirus B
Influenzavirus C
Isavirus
''Thogotovirus

The Orthomyxoviridae are a family of RNA viruses which, so far as is known, infect mainly vertebrates (Thogotovirus in ticks, Isavirus in the sea louse). It includes those viruses which cause influenza.

Morphology

The virions have envelopes and occur in pleomorphic and filamentous forms. In general the virus's morphology is spherical with particles 50 to 120 nm in diameter, or filamentous virions 20 nm in diameter and 200 to 300 (-3000) nm long. There are some 500 distinct spike-like surface projections of the envelope each projecting 10 to 14 nm from the surface with some types (i.e. hemagglutininesterase (HEF)) densely dispersed over the surface, and with others (i.e. hemagglutinin (HA)) spaced widely apart.

The major glycoprotein (HA) is interposed irregularly by clusters of neuraminidase (NA), with a ratio of HA to NA of about 4-5 to 1.

Lipoprotein membranes enclose the nucleocapsids; nucleoproteins of different size classes with a loop at each end; the arrangement within the virion is uncertain. The nucleocapsids are filamentous and fall in the range of 50 to 130 nm long and 9 to 15 nm in diameter. They have a helical symmetry.

Nucleic Acid

Viruses of this family contain 7 to 8 segments of linear negative-sense single stranded RNA.

The total genome length is 12000-15000 nucleotides (nt). The largest segment 2300-2500 nt; of second largest 2300-2500 nt; of third 2200-2300 nt; of fourth 1700-1800 nt; of fifth 1500-1600 nt; of sixth 1400-1500 nt; of seventh 1000-1100 nt; of eighth 800-900 nt. Genome sequence has terminal repeated sequences; repeated at both ends. Terminal repeats at the 5'-end 12-13 nucleotides long. Nucleotide sequences of 3'-terminus identical; the same in genera of same family; most on RNA (segments), or on all RNA species. Terminal repeats at the 3'-end 9-11 nucleotides long. Encapsidated nucleic acid is solely genomic. Each virion may contain defective interfering copies.

Genera

Orthomyxoviridae include the following genera and species:

Genus Influenzavirus A; type species: Influenza A virus
Genus Influenzavirus B; type species: Influenza B virus
Genus Influenzavirus C; type species: Influenza C virus
Genus Isavirus; type species: Infectious salmon anemia virus
Genus Thogotovirus; type species: Thogoto virus

Types of influenza virus

There are three genera of influenza virus, identified by antigenic differences in their nucleoprotein and matrix protein:

Influenza A viruses are known to infect humans, other mammals and birds (see also avian influenza),
Influenza B viruses are known to infect humans and seals,
Influenza C viruses are known to infect humans and pigs.

The A type of influenza virus, also called avian flu virus, is the type most likely to cause epidemics and pandemics. This is because the influenza A virus can undergo antigenic shift and present a new, immune target to susceptible people. Populations tend to have more resistance to influenza B and C, because they only undergo antigenic drift, and have more similarity with previous strains.

The avian flu virus is a species of virus. Its genetic code is RNA not DNA. It mutates very fast by comparison to both DNA and non-viruses. Cold preserves it while heat destroys it. It mutates enough so after a year of mutating in birds, humans can catch the new strain and not be protected by their body from last year's strain. Cold preserves it enough so in the winter it can pass from bird to human to human without being destroyed by heat. Sometimes an avian virus strain mutates into a very deadly version. H5N1 did just that.

Influenza A viruses can be further classified, based on the viral surface proteins hemagglutinin (HA or H) and neuraminidase (NA or N) that are essential to the virus' life cycle. Sixteen H subtypes and nine N subtypes have been identified for influenza A virus. Only one H subtype and one N subtype have been identified for influenza B virus. At present, the most common antigenic variants of influenza A virus are H1N1 and H3N2 (Yohannes et al., 2004).

Yet further variation exists; thus, specific influenza strain isolates are identified by a standard nomenclature specifying virus type, geographical location where first isolated, sequential number of isolation, year of isolation, and HA and NA subtypehttp://www.cdc.gov/nip/publications/pink/flu.pdf Epidemiology & Prevention of Vaccine-Preventable Diseases,"The Pink Book", 9th Edition. 2006. National Immunization Program, Centers for Disease Control and Prevention, U.S. Department of Health and Human Services http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/biofacts/avflu_human.html Avian Influenza (Bird Flu): Implications for Human Disease (CIDRAP) - see 8th main bullet point from top.

Examples of the nomenclature are:

A/Moscow/10/99 (H3N2)
B/Hong Kong/330/2001

The term superflu is used to refer to a strain of flu that spreads unusually quickly, is unusually virulent, or for which the host is uncommonly unresponsive to treatment — the kinds of strains which cause epidemics or pandemics. There is no exact scientific definition of a superflu.

Terminology

Avian influenza is not a genus of Orthomyxoviridae. The term "avian influenza" denotes a disease, not a virus. The orthomyxovirus family consists of 5 genera: Influenzavirus A, Influenzavirus B, Influenzavirus C, Isavirus, and Thogotovirus. Influenzavirus A is not the same as "avian influenza": the former is a genus of viruses, the latter is an illness.

In a phylogenetic-based taxonomy the "RNA viruses" includes the "negative-sense ssRNA viruses" which includes the Order "Mononegavirales", and the Family "Orthomyxoviridae" (among others), which includes the Genus "Influenzavirus A" which includes the Type Species "Influenza A virus".

The category "influenza virus" is the subset of orthomyxoviruses that cause influenza. This is not a phylogenetically based taxonomic category.

Sources