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Influenzavirus (Orthomyxoviridae)

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Microbiology

Summary

The Orthomyxovirus family includes the influenza A, B, and C viruses, all of which are enveloped, single-stranded, negative-sense RNA viruses. Notably, orthomyxoviruses replicate in the nucleus of host cells rather than the cytoplasm. These viruses feature two significant virulence factors: hemagglutinin (HA) and neuraminidase (NA). HA allows the virus to attach to host cells, while NA facilitates the release of newly formed virions. Disease-causing mutations within this family occur in two forms: antigenic drift (minor point mutations over time) and antigenic shift (a major genetic chance through exchange of genetic segments between different species). This shift is responsible for pandemics, such as the H1N1 swine flu outbreak.

The influenza virus affects humans primarily in the colder months (December to February in the United States), and is spread via respiratory droplets. Complications of infection include pneumonia in severe cases, and rarely, Guillain-Barr≈Ω syndrome, an ascending paralysis originating from the feet. A child suffering from influenza should never be given aspirin as it can lead to Reye's syndrome, a serious condition characterized by signs such as fever, rash, vomiting, and complications such as liver failure and encephalitis. Antiviral drugs, including amantadine, rimantadine, oseltamivir (Tamiflu), and zanamivir can be given to inhibit parts of the viral life cycle. For prevention, vaccines are available in both live attenuated (nasal spray) and killed (injectable) forms.

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FAQs

What is the role of hemagglutinin and neuraminidase in the life cycle of the influenza virus?

Hemagglutinin and neuraminidase are two important surface glycoproteins of the Influenza virus. Hemagglutinin binds to sialic acid receptors on the host cell, enabling the virus to enter the cell and begin replication. Neuraminidase, on the other hand, helps in the release of new virions from the host cell by cleaving these same sialic acids. Hence, both play vital roles in the viral life cycle.

How does antigenic shift and antigenic drift pertain to influenza viruses?

Antigenic drift and shift are two ways the influenza viruses change. Antigenic drift refers to minor mutations occurring over time as the virus replicates. These small changes can accumulate over time and result in viruses that are related but not identical. Antigenic shift is an abrupt, major change that can result in a new subtype of virus to which most people have little to no immunity. It occurs when two different strains of influenza viruses infect the same cell and exchange genetic material.

What are some potential adverse effects associated with the influenza vaccine?

While the influenza vaccine is typically safe, there might be rare cases of side effects. Common minor side effects include soreness, redness, or swelling at the injection site, low-grade fever, and muscle aches. More serious side effects are rare but may include high fever, behavior changes, or allergic reactions. There's a small possible risk of Guillain-Barr≈Ω syndrome, a severe neurological disorder, but it's less common than the risk of severe complications from actual influenza infection.

What is the structure of the influenza virus?

Influenza virus, along with other orthomyxoviruses, are enveloped with a segmented, negative-sense RNA genome. The envelope is embedded with the glycoproteins hemagglutinin and neuraminidase.

How is Reye's syndrome related to influenza?

Reye's syndrome is a rare but serious condition that causes confusion, swelling in the liver, and brain damage. In the past, it has been associated with the use of aspirin to treat viral illnesses such as influenza in children and teenagers. While the precise cause is unknown, studies indicate a strong link between Reye's syndrome and the use of aspirin for treating viral infections. Thus, doctors recommend avoiding aspirin for children or teenagers with viral illnesses.