Influenza Diagnosis and Treatment Guidelines (2025 Edition) Part 1

Influenza (hereinafter referred to as flu) is an acute respiratory infectious disease caused by the influenza virus. Seasonal epidemics of influenza A and B viruses occur annually, with influenza A viruses capable of causing global pandemics. Most influenza cases are self-limiting, but a small number of patients may develop severe or critical illness due to complications such as pneumonia or exacerbation of underlying conditions, potentially leading to death from acute respiratory distress syndrome (ARDS), acute necrotizing encephalopathy, or multiple organ dysfunction. To further improve influenza diagnosis and treatment, reduce severe cases and fatalities, this diagnostic and treatment protocol has been formulated based on the Influenza Diagnosis and Treatment Guidelines (2020 Edition), incorporating domestic and international research findings and China’s clinical experience in influenza management.

I. Etiology

Influenza viruses belong to the family Orthomyxoviridae and are single-stranded, negative-sense, segmented RNA viruses classified into four types: A, B, C, and D. Currently circulating strains in humans include influenza A subtypes H1N1 and H3N2, as well as the influenza B Victoria lineage. Influenza viruses are susceptible to common disinfectants such as ethanol, iodophor, iodine tincture, and sodium hypochlorite. They are also sensitive to ultraviolet light and heat, with inactivation occurring at 56°C for 30 minutes.

II. Epidemiology

(1) Source of Infection

Patients and asymptomatic carriers are the primary sources of infection. Transmission is possible from the late incubation period through the acute phase, with viral shedding typically lasting 3–7 days. Children, immunocompromised individuals, and critically ill patients may shed the virus for over one week.

(2) Transmission Routes

Influenza spreads primarily through airborne droplets generated by sneezing, coughing, or talking, and is more easily transmitted in crowded, enclosed, or poorly ventilated spaces. Indirect infection can occur via contact with virus-contaminated objects followed by exposure to mucous membranes (e.g., eyes, nose, or mouth).

(3) Susceptible Populations

The general population is universally susceptible.

III. Pathogenesis and Pathology

Influenza virus infects respiratory epithelial cells, leading to clumped or diffuse shedding of ciliated epithelial cells in the airways, congestion and edema of the lamina propria mucosal cells, and mononuclear cell infiltration. The virus may also infect alveolar epithelial cells, macrophages, and other cells, causing widespread alveolar epithelial damage. The alveolar spaces become filled with inflammatory exudate, accompanied by interstitial pulmonary edema and extensive inflammatory cell infiltration, resulting in pneumonia. Severe cases may involve diffuse alveolar damage, characterized by necrosis of alveolar epithelial cells and pulmonary capillary endothelial cells, protein-rich exudate and inflammatory cells in the interstitium and alveolar spaces, microvascular congestion, hemorrhage, microthrombosis, hyaline membrane formation, focal or extensive alveolar collapse, disruption of the blood-gas barrier, and progression to acute respiratory distress syndrome (ARDS). In some cases, an overactivated immune response triggers a massive release of cytokines, leading to sepsis/septic shock and multi-organ damage. Cardiac involvement may manifest as interstitial hemorrhage, lymphocyte infiltration, myocardial cell swelling, and necrosis (myocarditis). Encephalopathy may present with diffuse cerebral congestion, edema, and necrosis, with acute necrotizing encephalopathy showing symmetrical necrotic lesions predominantly in the thalamus. Rare complications include acute tubular necrosis, glomerular microthrombosis, and hepatocyte necrosis.

IV. Clinical Manifestations

The incubation period is typically 1–7 days, most commonly 2–4 days.

(1) Clinical Presentation

The onset is marked by fever, headache, and muscle/joint pain, with body temperature reaching 39–40°C. Common symptoms include sore throat, dry cough, nasal congestion, and rhinorrhea, along with systemic manifestations such as chills, rigors, fatigue, and loss of appetite. Some patients experience mild or no symptoms. Influenza infection can exacerbate underlying chronic conditions. Children often exhibit higher fever than adults, and those with influenza B may experience more pronounced gastrointestinal symptoms such as nausea, vomiting, and diarrhea. Neonates may present with lethargy, refusal to feed, or apnea. In the elderly, symptoms may be atypical, with absent or low-grade fever, and prominent cough, sputum production, dyspnea, or chest pain. Anorexia and altered mental status may also occur. Uncomplicated cases are self-limiting, with fever subsiding and systemic symptoms improving within 3–5 days, though cough and fatigue may persist longer.

(2) Complications

Pneumonia is the most common complication. Others include neurological injury, cardiac damage, myositis/rhabdomyolysis, and shock. Children are more prone to complications such as laryngitis, otitis media, and bronchitis.

1. 1. Primary viral pneumonia: Influenza virus may invade the lower respiratory tract, causing direct viral pneumonia. Secondary infections with bacteria (e.g., Staphylococcus aureus, Streptococcus pneumoniae), other viruses, atypical pathogens, or fungi (e.g., invasive pulmonary aspergillosis) can occur, often leading to severe disease and high mortality.
2. 2. Neurological complications: Encephalitis, encephalopathy, myelitis, and Guillain-Barré syndrome may occur, with acute necrotizing encephalopathy being particularly life-threatening. These are more common in children.
3. 3. Cardiac injury: Myocarditis, pericarditis, elevated cardiac biomarkers, ECG/echocardiographic abnormalities, and even heart failure may develop. Influenza infection also increases the risk of myocardial infarction and ischemic heart disease-related hospitalization and death.
4. 4. Myositis and rhabdomyolysis: Manifest as muscle pain, weakness, elevated creatine kinase and myoglobin levels, potentially leading to acute kidney injury.

To be continued...

About Us: For efficient virus isolation, high-quality oropharyngeal swabs are essential. Mantacc produces medical-grade, sterile, and DNase/RNase-free flocked swabs compatible with viral transport media and molecular diagnostics, certified to ISO, CE, and FDA standards. Learn more at Mantacc 93050L Oral Sampling Swabs.

Related Post  

Influenza Diagnosis and Treatment Guidelines (2025 Edition) Part 2

Influenza Diagnosis and Treatment Guidelines (2025 Edition) Part 3

Oral Sampling Swabs: A Promising Alternative for Infectious Disease Diagnosis