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Orthomyxoviridae family comprises unique classes of viruses known as Orthomyxoviruses. Orthomyxoviruses causes highly contagious respiratory disease known as influenza or viral flu in humans. Orthomyxoviridae family comprise RNA viruses that includes seven viral genera which are: Influenzavirus A, Influenzavirus B, Influenzavirus C, Influenzavirus D, Isavirus, Thogotovirus and Quaranjavirus. Influenzavirus A, Influenzavirus B, Influenzavirus C and Influenzavirus D all cause influenza or respiratory tract infections in humans, birds and other mammals. Isavirus causes infection in salmon while Thogotoviruses are arboviruses, and thus cause infection in both vertebrates and invertebrates such as mosquitoes and ticks. Orthomyxoviruses are zoonotic viral disease agents transmissible from animals to humans. The Orthomyxoviruses are distributed worldwide and they cause respiratory infections in both humans and animals. Orthomyxoviruses have a ss(-)RNA genome, and they have a helical nucleocapsid.

They measure between 80-120 nm in diameter. The virion envelope of Orthomyxoviruses or influenza viruses is usually derived from the cell membrane of the host cell, and they are incorporated with several virus glycoproteins and nonglycosylated proteins during budding from the host cell. They replicate in the nucleus. Orthomyxoviruses are enveloped viruses with a spherical or pleomorphic structure. Though the synthesis of the RNA genomes of RNA viruses occurs in the cytoplasm; that of the Orthomyxoviruses are known to occur in the nucleus of the cell, and this is possible because they contain or harbour an RNA-dependent RNA polymerase (as aforementioned) which convert their ss(-)RNA genome to a ss(+)RNA genome. The genus Influenzavirus A contains viruses that are pathogenic to wild and domestic animals and birds whereas viruses in Influenzavirus B and C circulate continuously in human populations. Human epidemic and pandemic cases of influenza (a highly contagious respiratory tract infection) caused by influenza A viruses typically have no animal involvement beyond the initial incursion; however, Influenza A viruses infrequently are transmitted from their animal hosts to humans.

Orthomyxoviruses are released from the host cell through the cytoplasmic membrane by the process of budding. As aforementioned, there are seven genera of viruses that make up the Orthomyxoviridae family; and three of these viral genera including Influenzavirus A, B and C cause significant human infections. Influenzavirus A (which comprise influenza A), Influenzavirus B (which comprise influenza B) and Influenzavirus C (which comprise influenza C) are the genera of Orthomyxoviridae family that cause infections in humans. Thogotovirus (found in the Thogotovirus genera) and infectious salmon anemia virus (found in Isavirus genera) cause infections in mammals and fishes respectively. The influenza viruses (especially influenza A and B) are important human pathogens that are primarily transmitted through the respiratory tract or route in aerosols of infected persons. Influenza viruses have continued to cause epidemics and pandemics in human population around the globe; and this is due in part to the reassortment of the genetic makeup of the viral pathogen. And this mutation or gene reassortment in influenza viruses has impacted negatively on the development of long-lasting immunity against the pathogen due to the continuous change in the structural and genetic makeup of the organism.

When the RNA genome of two genetically distinct strains of influenza virus that infect the same cell or organism are re-assorted, a biological process known as antigenic shift occurs; and this leads to the production of a new virus that is different from that of the two original virions from which it was initially developed from. Influenza viruses also undergo antigenic drift in their quest to mutate into new virions. Antigenic drift occurs whenthe protein structure of the virus (especially the neuraminidase and haemagglutinins) undergoes mutation to produce newer virions that will not be recognized by protective antibodies produced by the host’s immune system. Antigenic drift and antigenic shift are mainly responsible for the continuous emergence of new epidemics of influenza virus because these processes lead to the formation of novel strains of the virus. Neuraminidase (NA or N) and haemagglutinin (HA or H) arethe two important proteins found on the surface of influenza viruses; and they are used in the typing of the organism especially in the face of outbreak of disease caused by influenza virus. HA and NA are the two major antigens of influenza viruses. While HA is mainly involved in the attachment and fusion of influenza virus to the host cell, NA basically aids in the release of the virion from the infected host cell. There are several serotypes or subtypes of influenza viruses, and these have been classified based on their HA and NA variations. Influenza viruses are mainly reserved in animals inclusive of birds from which human infections have also occurred and disseminated.

The H5N1 influenza virus strain that caused influenza outbreaks in parts of Asia, Europe and Africa is mainly transmitted from birds to humans; and human to human transmission of this particular strain has not been recorded but likely. The first outbreak of the H5N1 influenza virus infection known as “Avian Flu” occurred in poultry farms in Hong Kong in 1997; and human outbreaks of avian flu have been recorded. The virus (i.e., the H5N1 strain) replicate in the mucosa of the nasopharynx where they cause pharyngitis. The incubation period of the disease is between 2-3 days; and the infection can reach the lungs where pulmonary infections also occur. The clinical symptoms of infections caused by the influenza viruses include fever, headache, cough, sore throat and nasal congestion. Secondary bacterial infections can also occur in influenza infections; and these infections which occur mainly in the lungs are caused by Streptococcus and Haemophilus species. However, secondary bacterial infections due to influenza are common amongst the immunocompromised host and people who are seriously ill. Flu is usually the general name used to describe viral infections caused by influenza virus. But the infection is sometimes confused with common cold and other bacterial respiratory diseases which present with similar clinical symptoms. Proper clinical and laboratory diagnosis are critical for the effective diagnosis and treatment of flu caused by influenza viruses. Antiviral therapy such as the use of amantadine (a synthetic amine and a viral uncoating blocker) is available for the treatment of flu caused by influenza virus. And since the organism is a genetically variable viral pathogen (as exemplified by their antigenic drift and antigenic shift phenomenon), effective vaccination and prophylaxis should be administered to vulnerable groups of the human population as a way of protecting susceptible individuals from getting the infection.          

Further reading

Acheson N.H (2011). Fundamentals of Molecular Virology. Second edition. John Wiley and Sons Limited, West Sussex, United Kingdom.

Brian W.J Mahy (2001). A Dictionary of Virology. Third edition. Academic Press, California, USA.

Cann A.J (2011). Principles of Molecular Virology. Fifth edition. Academic Press, San Diego, United States.

Carter J and Saunders V (2013). Virology: Principles and Applications. Second edition. Wiley-Blackwell, New Jersey, United States.

Dimmock N (2015). Introduction to Modern Virology. Seventh edition. Wiley-Blackwell, New Jersey, United States.

Kudesia G and Wreghitt T (2009). Clinical and Diagnostic Virology. Cambridge University Press, New York, USA. 

Marty A.M, Jahrling P.B and Geisbert T.W (2006). Viral hemorrhagic fevers. Clin Lab Med, 26(2):345–386.

Strauss J.H and Straus E.G (2008). Viruses and Human Diseases. 2nd edition. Elsevier Academic Press Publications, Oxford, UK.

Zuckerman A.J, Banatvala J.E, Schoub B.D, Grifiths P.D and Mortimer P (2009). Principles and Practice of Clinical Virology. Sixth edition. John Wiley and Sons Ltd Publication, UK.

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