Pathophysiology

Influenza is a communicable viral disease that affects the upper respiratory tract, including upper and lower respiratory passages. A wide spectrum of influenza viruses causes it. Some of these viruses can infect humans, and some are specific to different species. Following transmission, the virus attaches to and penetrates the epithelial cells in the nasopharynx, trachea, and bronchi. The virus replicates in the upper and lower respiratory passages starting from the time of inoculation and peaking after 48 hours, on average (Boktor S., Hafner J. 2019). Viral replication results in the destruction of these epithelial cells. Epithelial necrosis initiates the inflammatory response that results in the flu symptoms of fever, chills, muscle ache, headache, prostration, and anorexia. Regeneration of epithelium takes about 3 to 4 weeks (Hall. n.d.)

If the virus infects the lungs, loss of alveolar epithelium allows plasma to leak from capillaries into the alveolar space. This is especially problematic in patients predisposed to pulmonary edema. Pulmonary edema can be caused by any of the following underlying conditions: (1) increased capillary hydrostatic pressure, (2) increased capillary permeability, (3) decreased plasma oncotic pressure, and (4) lymphatic obstruction.

Loss of alveolar epithelium and the resulting pulmonary edema predispose the underlying tissue to colonization by bacteria. Secondary bacterial infection can lead to pneumonia, which accounts for most influenza related deaths (CDC, 2020).

Complications in at-risk populations

"People most at risk of developing serious influenza-related complications include people age 65 years and older, people with chronic medical conditions (e.g., heart disease or diabetes), pregnant women, and young children, especially those younger than age 2 years. More common complications of influenza include secondary bacterial pneumonia (e.g., Streptococcus pneumoniae, Haemophilus influenzae, or Staphylococcus aureus), exacerbations of underlying respiratory conditions, otitis media, laryngotracheobronchitis, and bronchitis.

Other complications may include primary pneumonia, encephalitis, aseptic meningitis, transverse myelitis, myocarditis, pericarditis, Guillain-Barré syndrome, and Reye Syndrome. Reye syndrome is a complication that occurs almost exclusively in children taking aspirin, primarily in association with influenza B virus (or varicella zoster virus), and presents with severe vomiting and confusion, which may progress to coma due to swelling of the brain.
Most deaths due to influenza typically occur among persons age 65 years and older(CDC-Hall, n.d.).

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References

Boktor S, Hafner JW. (2019) StatPearls. Influenza. Treasure Island (FL): StatPearls Publishing.

Centers for Disease Control and Prevention. (2020). Clinical signs and symptoms of influenza. Centers for Disease Control and Prevention. Retrieved August 1, 2022, from https://www.cdc.gov/flu/professionals/acip/clinical.htm

CDC, & Hall, E. (n.d.). Influenza. CDC-Epidemiology and Prevention of Vaccine-Preventable Diseases. https://www.cdc.gov/vaccines/pubs/pinkbook/flu.html

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