Acute upper respiratory tract infection (URTI), also known as upper respiratory infection, is acute inflammation of the nasal cavity, sinuses, pharynx, or larynx. The primary pathogens are viruses, with bacteria causing a minority of cases; mixed infections can also occur. URTIs are typically mild, self-limiting, and have a short course, with a good prognosis. However, due to their high incidence, they can disrupt work and daily life and may sometimes be accompanied by severe complications, especially in vulnerable populations such as those with underlying conditions, infants, pregnant women, and older adults. URTIs are also somewhat contagious, necessitating active prevention and treatment.
Epidemiology
URTIs are among the most common infectious diseases in humans, occurring more frequently in winter and spring and typically presenting as sporadic cases. They are primarily transmitted via virus-containing droplets or through contact with contaminated hands and objects. Most pathogens that cause URTIs are viruses widely present in nature, and healthy individuals can also carry them. Immunity acquired after infection is weak and short-lived, with no cross-immunity among different viruses, leading to recurrent infections.
Etiology and Pathogenesis
Approximately 200 types of viruses can cause URTIs. On average, adults experience 2-4 episodes per year, while preschool-aged children experience 4-8 episodes annually. About 70-80% of URTIs are caused by viruses, with rhinoviruses being the most common. Other viral pathogens include coronaviruses, adenoviruses, influenza and parainfluenza viruses, respiratory syncytial virus, echoviruses, and coxsackieviruses. The remaining 20-30% of URTIs are caused by bacteria, either as primary infections or secondary to viral infections. The most common bacterial pathogen is group A beta-hemolytic streptococcus, followed by Haemophilus influenzae, Streptococcus pneumoniae, and Staphylococcus species. Occasionally, gram-negative bacilli are implicated. Whether an individual becomes ill after exposure to these pathogens depends on their susceptibility. Factors such as getting wet, exposure to cold, sudden weather changes, overexertion, and smoking can weaken the respiratory tract's local defenses, allowing viruses or bacteria to multiply rapidly. Vulnerable groups, such as older adults, infants, individuals with weakened immune systems, and those with chronic respiratory diseases (e.g., asthma, COPD, sinusitis, or tonsillitis), are more prone to developing URTIs.
Pathology
Histologically, there may be no significant pathological changes, or epithelial cell damage may be observed. Inflammatory mediators can contribute to the condition, causing congestion of the upper respiratory tract mucosa, infiltration of mononuclear cells, and serous or mucous inflammatory exudates. Secondary bacterial infections may lead to neutrophil infiltration and purulent secretions. Mucosal congestion can result in clinical symptoms such as nasal obstruction and sore throat, while edema of the eustachian tube can cause hearing impairment or lead to otitis media. Damage to the respiratory epithelium and the release of inflammatory mediators into the bloodstream can cause systemic symptoms such as fever and generalized muscle aches.
Clinical Manifestations
The clinical manifestations of URTIs can be classified into the following types:
Common Cold
Common cold, also known as acute rhinitis or acute coryza, is caused by viral infections and is colloquially referred to as a cold. It has an abrupt onset, with primary symptoms involving the nasal passages, such as sneezing, nasal congestion, and clear nasal discharge. Other symptoms may include cough, dry or itchy throat, and a burning sensation, sometimes accompanied by postnasal drip. After 2-3 days, the nasal discharge may become thicker. Additional symptoms can include sore throat, headache, lacrimation, diminished taste, dyspnea, and hoarseness. In some cases, inflammation of the eustachian tube may lead to reduced hearing. Severe cases may involve fever, mild chills, and headache. Physical examination may reveal congested and edematous nasal mucosa with secretions, as well as mild pharyngeal congestion. Recovery typically occurs within 5-7 days, although complications can prolong the course of illness.
Acute Viral Pharyngitis and Laryngitis
These conditions are caused by viruses such as rhinoviruses, adenoviruses, influenza and parainfluenza viruses, enteroviruses, and respiratory syncytial viruses. Symptoms include throat itching and a burning sensation, with mild throat pain and rare coughing. Acute laryngitis is often caused by influenza, parainfluenza, or adenoviruses, with clinical features including pronounced hoarseness, difficulty speaking, fever, sore throat, and sometimes cough, which can exacerbate throat pain. Physical examination may reveal laryngeal congestion and edema, mild swelling and tenderness of regional lymph nodes, and occasionally stridor.
Acute Herpetic Pharyngitis
This condition is more common in summer, primarily affecting children but occasionally occurring in adults. It is caused by coxsackievirus A and is characterized by severe throat pain and fever, with a course of about one week. Examination may show pharyngeal congestion, with grayish-white vesicles and superficial ulcers on the soft palate, uvula, pharynx, and tonsils, surrounded by a reddish halo.
Acute Pharyngoconjunctival Fever
This condition is common in summer and often linked to swimming. It primarily affects children and is caused by adenoviruses or coxsackieviruses. Symptoms include fever, sore throat, photophobia, lacrimation, and marked congestion of the pharynx and conjunctiva. The course of illness lasts 4-6 days.
Acute Pharyngotonsillitis
The primary pathogens are group A beta-hemolytic streptococci, followed by Haemophilus influenzae, Streptococcus pneumoniae, and Staphylococcus species. Onset is abrupt, with severe throat pain, fever, and chills, with temperatures often exceeding 39°C. Examination reveals significant pharyngeal congestion, swollen and inflamed tonsils with yellowish purulent exudates on their surface, and sometimes enlarged, tender submandibular lymph nodes.
Laboratory Tests
Blood Tests
Since most infections are viral, white blood cell (WBC) counts are usually normal or slightly decreased, with an increased proportion of lymphocytes. In cases of bacterial infection, there may be an elevated WBC count, increased neutrophils, and a left shift in the neutrophil count.
Pathogen Identification
Due to the wide variety of viruses and the limited therapeutic value of determining the specific type, pathogen identification is generally unnecessary. If needed, methods such as nasal swab, throat swab, and nasopharyngeal swab tests using immunofluorescence, enzyme-linked immunosorbent assays (ELISA), serological diagnosis, virus isolation, or PCR molecular detection can be used to identify the virus. Bacterial cultures can determine the type of bacteria and guide clinical antibiotic use through sensitivity testing.
Complications
A small number of patients may develop complications such as acute sinusitis, otitis media, and tracheobronchitis. For URTIs presenting as pharyngitis, some patients may develop rheumatic fever or glomerulonephritis secondary to group A beta-hemolytic streptococcal infection. Rarely, viral myocarditis may occur and should be closely monitored. Patients with underlying conditions such as chronic obstructive pulmonary disease (COPD), asthma, and bronchiectasis may experience acute exacerbations. Those with heart failure may have worsening cardiac function.
Diagnosis and Differential Diagnosis
A clinical diagnosis can be established based on symptoms and signs in the nasopharyngeal region, combined with blood tests and a negative chest X-ray. Etiological diagnosis is generally unnecessary but can be conducted in special cases. Differential diagnosis with other diseases presenting with cold-like symptoms in the early stages is essential:
Allergic Rhinitis
This condition has an acute onset, often presenting with nasal mucosal congestion and increased secretions, accompanied by sudden sneezing, nasal itching, nasal congestion, and a large amount of clear nasal discharge. Fever is absent, and coughing is rare. It is often triggered by allergens such as dust mites, dust, animal fur, or cold temperatures. Symptoms typically resolve within minutes to 1-2 hours after removal of the allergen. On examination, the nasal mucosa appears pale and edematous, and nasal secretions may show increased eosinophils. Skin allergy tests can confirm the allergen.
Influenza
Caused by the influenza virus, it can occur sporadically or in small outbreaks. During viral mutations, it can lead to large-scale epidemics. The onset is sudden, with mild nasopharyngeal symptoms but severe systemic symptoms, including high fever, body aches, and conjunctivitis. Rapid serological and PCR tests for the influenza virus are now available for differentiation.
Acute Tracheobronchitis
This condition presents with cough and sputum production. WBC counts may be elevated, and nasal symptoms are mild. Chest X-rays often reveal enhanced pulmonary markings.
Prodromal Symptoms of Acute Infectious Diseases
Many viral infectious diseases, such as measles, poliomyelitis, encephalitis, hepatitis, and myocarditis, may present with similar early-stage symptoms. Initial signs, such as nasal congestion and headache, should be closely monitored. If respiratory symptoms subside within a week but new symptoms emerge, further laboratory tests are necessary to avoid misdiagnosis.
Treatment
Currently, there are no specific antiviral drugs for most URTIs, so treatment focuses on symptom management. Patients should avoid smoking, get adequate rest, drink plenty of fluids, ensure good indoor ventilation, and prevent secondary bacterial infections.
Symptomatic Treatment
For patients with acute cough, postnasal drip, and dry throat, pseudoephedrine can be used to relieve nasal congestion, either orally or as nasal drops. Antipyretic and anti-inflammatory drugs, such as acetaminophen and ibuprofen, may be added if necessary. Aspirin should be avoided in children with colds due to the risk of Reye's syndrome. Patients with a history of asthma should also avoid aspirin.
Antibiotic Therapy
Antibiotics are not required for the common cold. If there is evidence of bacterial infection, such as elevated WBC counts, purulent pharyngeal exudates, yellow sputum, and nasal discharge, antibiotics may be selected based on local epidemiology and clinical experience. Options include oral penicillins, first-generation cephalosporins, macrolides, and fluoroquinolones. Fluoroquinolones are contraindicated in individuals under 18 years of age.
Antiviral Therapy
During viral outbreaks, high-risk populations such as older adults, individuals with chronic underlying diseases, immunocompromised individuals, and obese individuals (BMI ≥ 28 kg/m2) diagnosed with influenza or COVID-19 should receive early antiviral therapy. Oseltamivir and baloxavir can be used for influenza, while nirmatrelvir, ensitrelvir, or remdesivir can be used for COVID-19. These treatments can reduce the risk of severe disease, hospitalization, and death. For other respiratory viruses (e.g., rhinoviruses, respiratory syncytial virus, adenoviruses), effective antiviral drugs are lacking. To avoid resistance or adverse effects, patients with mild symptoms are not recommended to use drugs without proven efficacy.
Prevention
Prevention is key. Isolating sources of infection can help reduce transmission. Strengthening physical fitness, improving nutrition, maintaining a regular lifestyle, avoiding cold exposure, and preventing overexertion can reduce susceptibility. Annual influenza vaccination is the best method for preventing URTIs. Older and frail individuals should take precautions, such as wearing masks and avoiding crowded public places during URTI outbreaks.