Infectious Disease · Respiratory Infections
The facts most likely to be tested
Influenza presents with the abrupt onset of high fever, myalgias, headache, and a non-productive cough.
The gold standard for diagnosis is reverse transcription-polymerase chain reaction (RT-PCR) due to its high sensitivity and specificity.
Oseltamivir is the neuraminidase inhibitor of choice for treatment and post-exposure prophylaxis in high-risk patients.
Antiviral therapy provides the greatest clinical benefit when initiated within 48 hours of symptom onset.
Secondary bacterial pneumonia, most commonly caused by *Staphylococcus aureus* or *Streptococcus pneumoniae*, is the most frequent serious complication.
Annual inactivated influenza vaccine is recommended for all individuals aged 6 months and older.
Antigenic drift refers to minor mutations in surface proteins that necessitate annual vaccine updates, while antigenic shift involves major reassortment leading to pandemics.
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A 68-year-old male with a history of COPD presents to the urgent care clinic in January with a 24-hour history of high-grade fever, severe diffuse myalgias, and a dry, hacking cough. He reports that his symptoms began suddenly yesterday morning. His temperature is 102.4°F (39.1°C), pulse is 105/min, and oxygen saturation is 94% on room air. Lung auscultation reveals scattered wheezing but no focal consolidation.
What is the most appropriate next step in management?
Initiation of oral oseltamivir
The patient presents with classic symptoms of influenza within the 48-hour window for antiviral therapy, and his age and underlying COPD place him in a high-risk category for complications.
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Etiology / Epidemiology
Highly contagious orthomyxovirus transmitted via respiratory droplets; peak incidence in winter months.
Clinical Manifestations
Abrupt onset of systemic symptoms including high fever, myalgias, and non-productive cough.
Diagnosis
RT-PCR is the gold standard for definitive diagnosis with high sensitivity/specificity.
Treatment
Oseltamivir is the first-line treatment; do not delay if patient is high-risk.
Prognosis
Most recover in 7-10 days; secondary bacterial pneumonia is the most common lethal complication.
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Epidemiology & Etiology
Influenza A and B are the primary human pathogens, characterized by frequent antigenic drift. Transmission occurs via large respiratory droplets or contact with contaminated surfaces. High-risk groups include the elderly, pregnant women, and those with chronic cardiopulmonary disease.
Pertinent Anatomy
The virus targets the respiratory epithelium of the upper and lower airways. Viral entry is mediated by hemagglutinin binding to sialic acid receptors, leading to widespread mucosal inflammation.
Pathophysiology
Viral replication causes direct damage to the respiratory epithelium, leading to the classic cytokine storm responsible for systemic symptoms. The virus utilizes neuraminidase to release progeny virions from infected cells. This process facilitates rapid spread throughout the tracheobronchial tree.
Clinical Manifestations
Patients present with the abrupt onset of fever, headache, and severe myalgias. Respiratory symptoms like non-productive cough and sore throat follow. Red flags include dyspnea, hypoxia, or altered mental status, suggesting primary viral pneumonia or secondary bacterial superinfection.
Diagnosis
RT-PCR is the preferred diagnostic test due to superior sensitivity compared to rapid antigen tests. Rapid influenza diagnostic tests (RIDTs) have high specificity but are prone to false negatives. Testing is generally reserved for hospitalized patients or those at high risk for complications.
Treatment
Oseltamivir is the first-line neuraminidase inhibitor, ideally initiated within 48 hours of symptom onset. Contraindications are rare, but caution is advised in patients with severe renal impairment. Supportive care includes hydration and antipyretics; avoid aspirin in children due to Reye syndrome risk.
Prognosis
Most patients recover fully within one week. Secondary bacterial pneumonia (often *S. pneumoniae* or *S. aureus*) is the most common cause of mortality. Monitor for worsening respiratory status after initial improvement, which suggests a secondary infection.
Differential Diagnosis
COVID-19: loss of taste/smell
Common cold: gradual onset, afebrile
Streptococcal pharyngitis: exudates, lack of cough
Bacterial pneumonia: focal consolidation on CXR
RSV: common in infants, wheezing