- Published on
Infectious Disease and Microbiology: Influenza
Influenza is an acute viral respiratory infection caused by influenza viruses, typically presenting as a self-limited illness but capable of causing severe or life-threatening complications in vulnerable populations. While most healthy individuals recover fully, high-risk groups—including the elderly, young children, pregnant women, and those with chronic medical conditions—are more likely to develop complications such as pneumonia, respiratory failure, or death.
Influenza is highly prevalent worldwide, affecting approximately 5–20% of the population annually. Seasonal outbreaks occur predominantly during the winter months, often referred to as “flu season.” In the United States, influenza is responsible for tens of thousands of deaths and hundreds of thousands of hospitalizations each year. Occasionally, pandemics arise when new subtypes of influenza A viruses emerge, often due to major genetic changes, allowing rapid global spread with little preexisting immunity in the population.
Transmission occurs through respiratory droplets, direct contact with infected individuals, or contaminated surfaces. Viral shedding begins early—often within the first day of infection—and continues for about 5–10 days, though children may shed the virus longer. Preventive measures include good hand hygiene, respiratory etiquette, and especially annual vaccination. Vaccination is recommended for all individuals older than 6 months, with particular emphasis on high-risk groups. Two main vaccine types are available: inactivated vaccines given intramuscularly and live attenuated vaccines administered intranasally.
Influenza viruses are RNA viruses belonging to the Orthomyxoviridae family and are classified into types A, B, and C. Types A and B are responsible for most seasonal epidemics, while type C typically causes mild disease. Influenza A viruses are further subtyped based on hemagglutinin (HA) and neuraminidase (NA) surface proteins. Antigenic drift (small mutations) leads to seasonal variation, whereas antigenic shift (major genetic reassortment) can result in pandemics. The virus infects respiratory epithelial cells, and symptoms are largely due to the host immune response, including the release of proinflammatory cytokines, which may lead to severe complications such as cytokine storm and multiorgan failure.
Clinically, influenza is characterized by abrupt onset of fever, cough, nasal congestion, myalgia, headache, and malaise. The sudden onset of fever and cough during influenza season is highly suggestive of the disease. In some cases, especially in high-risk individuals, complications such as bacterial pneumonia may develop several days after initial symptoms, often indicated by recurrence of fever and worsening respiratory symptoms. Severe cases may involve exacerbation of underlying conditions like asthma or heart failure, as well as rare complications such as encephalitis, myocarditis, rhabdomyolysis, or shock.
Diagnosis is confirmed through laboratory testing of respiratory specimens. Reverse transcription polymerase chain reaction (RT-PCR) is the most sensitive and specific test and provides rapid results. Rapid antigen tests are also available but are less sensitive, especially in adults. Imaging such as chest X-ray may be necessary in severe cases to assess for pneumonia or other complications.
Treatment includes antiviral medications and supportive care. Neuraminidase inhibitors such as Oseltamivir and Zanamivir are effective against both influenza A and B and are most beneficial when started within 48 hours of symptom onset. They are especially recommended for high-risk patients or those with severe disease. Older drugs such as amantadine and rimantadine are less commonly used due to resistance. Supportive care includes rest, hydration, and management of symptoms.
In hospitalized patients, severe influenza may require intensive care support, including oxygen therapy, mechanical ventilation, or extracorporeal membrane oxygenation in cases of respiratory failure. Early antiviral treatment is recommended for hospitalized patients regardless of the duration of symptoms.
The prognosis for most individuals is good, with complete recovery expected. However, influenza can lead to serious complications, particularly in high-risk groups. These include bacterial pneumonia (commonly due to Streptococcus pneumoniae), acute respiratory distress syndrome (ARDS), myocarditis, and neurologic complications such as encephalopathy or Guillain–Barré syndrome. Vaccination and early treatment remain key strategies in reducing morbidity and mortality associated with influenza.
0 Comments
