Initial Stage: Adhesion and Tropism
The port of entry for all ARI pathogens is the mucous membrane of the upper respiratory tract. Disease pathogenesis is initiated when a virus attaches to a susceptible cell—a mechanism known as adhesion.
The ability of a virus to bind to specific target cells (tropism) is driven by the interaction of its active sites with specific host cell surface receptors. Each respiratory virus family uses a unique 'key' for penetration:
- Rhinoviruses: Capsid proteins bind to the intercellular adhesion molecule 1 (ICAM-1), allowing them to infect fibroblasts and other susceptible cells.
- Parainfluenza viruses: Envelop surface proteins interact with cell membrane glycosides.
- Coronaviruses: Bind to specific glycoprotein receptors.
- Adenoviruses: Use integrins for attachment.
Pathogenesis and Morphological Changes
Following successful adhesion, active replication of viral particles begins, occurring predominantly locally—directly within the cells of the respiratory tract. The release of virus into the systemic circulation (viremia) is typically transient. Certain infections feature distinct localization patterns: for example, adenovirus spread is often limited to regional lymph nodes.
The development of characteristic local symptoms is linked not only to the virus itself, but also to the host immune response. Inflammation is accompanied by the active release of mediators, notably bradykinins, which trigger local edema and catarrhal phenomena.
Cytopathic Effect of Viruses
Pathogens affect infected cells differently:
- Rhinoviruses cause only minor, superficial damage to the nasal mucosa.
- Adenoviruses exhibit pronounced cytotoxicity, provoking a rapid cytopathic effect and subsequent shedding of affected cells.
- Respiratory syncytial virus (RSV) causes potent destructive effects leading to epithelial necrosis.
Overall, the pathomorphological picture of ARI consists of tissue edema, massive cellular infiltration, and desquamation (sloughing) of the superficial epithelium. These mucosal defects are critical for understanding the pathogenesis of complications: they form new 'portals of entry' and create ideal conditions for secondary bacterial colonization.
Clinical Presentation
Despite the variety of etiologic agents, the clinical presentation of ARI shares many common features. They are characterized by a short incubation period and a short-lived course: immunocompetent individuals recover in about a week without intensive therapy. Intoxication is usually mild to moderate, and body temperature often remains normal or reaches only low-grade fever.
Leading manifestations include respiratory tract syndromes: catarrh (in the form of laryngitis, pharyngitis, or tracheitis) and rhinitis with prominent rhinorrhea. However, certain viruses exhibit specific clinical 'markers':
- Rhinovirus infection: Presents primarily as isolated rhinitis accompanied by a dry cough.
- Adenovirus infection: Classically combines pharyngoconjunctivitis and lymphadenopathy.
- RSV infection: Poses a particular danger to children. It runs a severe course by affecting the lower respiratory tract, causing bronchiolitis, acute pneumonia, and marked asthmatic syndrome.
Immunologically, organism sensitization frequently develops during the infectious process.
Complications and Severe Forms
A worsening ARI course and the development of complications are based on two main mechanisms: transient post-infectious immunodeficiency and secondary bacterial superinfection. Bacterial flora colonizes the damaged epithelium, causing sinusitis, otitis media, and purulent bronchitis, which significantly prolongs the illness.
The most formidable 'respiratory' complication is viral-bacterial pneumonia. It features an extremely aggressive course: massive destruction of the pulmonary epithelium occurs, multiple hemorrhages develop, and lung abscesses form. Such pneumonias carry a high mortality risk.
In addition to respiratory tract involvement, systemic (extrapulmonary) complications can affect the nervous and cardiovascular systems, liver, kidneys, and gastrointestinal tract. Such lesions arise both from the direct cytopathic effect of certain viruses and from the pronounced toxic effects of breakdown products from massively dying infected cells.