22.3 Viral Infections of the Respiratory Tract

Viral Respiratory Infections

Viral respiratory infections range from the common cold to severe influenza, affecting millions of people each year. Understanding how these viruses differ in structure, transmission, and disease severity is central to microbiology and to clinical prevention and treatment strategies.

Characteristics of Viral Respiratory Infections

Several virus families target the respiratory tract, but they vary widely in severity and the populations they hit hardest.

Common cold (rhinovirus, coronavirus)

• Symptoms include runny nose, sneezing, sore throat, cough, and mild fever
• Self-limiting infection typically lasting 7–14 days
• Rhinoviruses alone account for roughly 50% of colds; they replicate best at the slightly cooler temperatures of the nasal passages (~33°C)

Influenza (influenza viruses)

• Symptoms include high fever, chills, headache, myalgia (muscle aches), cough, sore throat, and fatigue
• Distinguished from the common cold by its sudden onset and more severe systemic symptoms
• Can lead to secondary bacterial pneumonia, especially in high-risk groups (elderly, immunocompromised, pregnant individuals)

Respiratory syncytial virus (RSV)

• The leading cause of bronchiolitis and pneumonia in infants and young children
• Symptoms include runny nose, decreased appetite, coughing, sneezing, fever, and wheezing
• RSV forms syncytia (fused giant cells) in respiratory epithelium, which is how it got its name

Parainfluenza viruses

• Primary cause of croup in children, characterized by a distinctive barking cough, hoarseness, and inspiratory stridor (difficulty breathing in)
• In adults, infection usually resembles a mild cold because of partial immunity from prior exposures

Adenoviruses

• Symptoms include fever, sore throat, cough, conjunctivitis (pink eye), and sometimes diarrhea
• Notable because they are non-enveloped viruses, making them more resistant to disinfectants and environmental conditions than enveloped respiratory viruses
• Can cause severe pneumonia in immunocompromised individuals

Comparison of Influenza Virus Types

Influenza viruses are classified into types A, B, and C. The key differences come down to how much their surface antigens change over time and which hosts they infect.

Influenza A

• Most severe type; responsible for both seasonal epidemics and pandemics
• Undergoes antigenic drift (gradual point mutations in hemagglutinin and neuraminidase genes) and antigenic shift (major reassortment of genome segments producing a novel virus)
• Antigenic shift is why pandemics occur: the population has little to no pre-existing immunity to the new strain
• Infects humans and animals (birds, pigs), and co-infection in a single host allows genetic reassortment between strains
• Transmitted through respiratory droplets and contact with contaminated surfaces

Influenza B

• Less severe than A but still causes significant seasonal illness
• Undergoes antigenic drift only, not antigenic shift, so it does not cause pandemics
• Primarily infects humans (no large animal reservoir)

Influenza C

• Mildest form; rarely causes significant illness
• Antigenically stable (does not undergo meaningful drift or shift)
• Infects humans and pigs
• Transmission is likely similar to A and B but is not as well characterized

Systemic Effects of Respiratory Viruses

Not all respiratory viruses stay confined to the airways. Some cause viremia, meaning the virus enters the bloodstream and can spread to other organs, including the skin. Skin rashes are a clinically useful sign for narrowing down which virus is responsible.

Measles (rubeola)

• Caused by measles virus (family Paramyxoviridae)
• Produces a characteristic red, maculopapular rash that appears 3–5 days after initial respiratory symptoms (cough, coryza, conjunctivitis)
• Koplik spots (small white spots on the buccal mucosa) are pathognomonic and appear before the rash

Rubella (German measles)

• Caused by rubella virus (family Togaviridae)
• Fine pink maculopapular rash starting on the face and spreading downward to the trunk and extremities
• Generally milder than measles, but infection during pregnancy can cause severe congenital rubella syndrome

Varicella (chickenpox)

• Caused by varicella-zoster virus (family Herpesviridae)
• Itchy vesicular rash that appears in successive "crops," so lesions at different stages (macules, vesicles, crusts) are present simultaneously
• The virus establishes latency in dorsal root ganglia and can reactivate later as shingles (herpes zoster)

Viral Infection Process

Understanding the general steps of respiratory viral infection helps connect the dots between exposure and symptoms.

1. Entry: The virus enters through the upper respiratory tract (nose, pharynx, larynx), typically via inhaled droplets or contact with mucous membranes.
2. Attachment and penetration: Viral surface proteins bind to specific receptors on respiratory epithelial cells, and the virus enters the host cell.
3. Replication: The virus hijacks host cell machinery to replicate its genome and produce new viral particles. This damages and eventually lyses the host cell.
4. Spread: In mild infections, the virus stays in the upper respiratory tract. In more severe cases, it spreads to the lower respiratory tract (trachea, bronchi, lungs), causing bronchitis or pneumonia.
5. Immune response: The innate immune system triggers inflammation (causing symptoms like fever, swelling, and mucus production). The adaptive immune system then produces specific antibodies and activates cytotoxic T cells to clear infected cells.
6. Resolution or complications: Most immunocompetent individuals clear the virus within days to weeks. Complications arise when the immune response is insufficient or when secondary bacterial infections take hold in damaged tissue.

The incubation period varies by virus: rhinoviruses typically incubate for 1–3 days, influenza for 1–4 days, and measles for 10–14 days. Knowing these timelines helps in tracing exposures and predicting symptom onset.