Orthomyxoviridae
Taxonomy
Orthomyxoviridae is a family of RNA viruses that includes five genera: Influenza virus A, Influenza virus B, Influenza virus C, Isavirus, and Thogotovirus. Each genus includes viruses that cause disease in birds and mammals, including humans.
Structure and Composition
Orthomyxoviridae viruses are enveloped, with a spherical or filamentous morphology. The envelope is derived from the host cell membrane and contains two major virus-encoded glycoproteins, hemagglutinin (HA) and neuraminidase (NA). Inside the envelope is the viral core, which contains eight segments of linear, single-stranded, negative-sense RNA. Each RNA segment is associated with multiple copies of the viral nucleoprotein (NP) and a single copy of the viral RNA-dependent RNA polymerase, forming a ribonucleoprotein (RNP) complex.
Replication Cycle
The replication cycle of Orthomyxoviridae viruses begins with the attachment of the virus to the host cell via the HA protein. This is followed by endocytosis of the virus into the cell and fusion of the viral envelope with the endosomal membrane, releasing the RNP complexes into the cytoplasm. The RNPs are then transported into the nucleus where transcription and replication of the viral RNA occur. Newly synthesized RNPs are exported from the nucleus and assemble with the HA and NA proteins at the cell membrane to form new virus particles, which are then released from the cell by budding.
Pathogenesis and Immunity
Orthomyxoviridae viruses cause a range of diseases in their hosts. In humans, Influenza virus A and B are responsible for the annual influenza epidemics, while Influenza virus C causes a mild respiratory illness. Isavirus causes infectious salmon anemia in farmed Atlantic salmon, and Thogotovirus can cause febrile illness in humans and encephalitis in horses.
The immune response to Orthomyxoviridae infection involves both the innate and adaptive immune systems. The innate immune response is triggered immediately upon infection and includes the production of interferons and other cytokines, which inhibit viral replication and activate immune cells. The adaptive immune response, which includes both B and T cell responses, is activated a few days after infection and provides long-term immunity.
Epidemiology
Orthomyxoviridae viruses are found worldwide and can infect a wide range of hosts. Influenza viruses A and B are responsible for annual influenza epidemics in humans, which result in 3 to 5 million cases of severe illness and 290,000 to 650,000 deaths each year globally. Influenza virus A can also cause pandemics, which occur when a new subtype of the virus emerges to which most people have no immunity.
Prevention and Control
Vaccination is the most effective method for preventing infection with Orthomyxoviridae viruses. Antiviral drugs are also available for the treatment and prevention of influenza. In addition, infection control measures such as hand hygiene and respiratory etiquette can reduce the spread of the virus.