Infections of the Respiratory System


General Concepts

1. Upper Respiratory Infections:
Common Cold, Sinusitis, Pharyngitis, Epiglottitis and Laryngotracheitis:

Etiology:
Most upper respiratory infections are of viral etiology. Epiglottitis and laryngotracheitis are exceptions with severe cases likely caused by Haemophilus influenzae type b. Bacterial pharyngitis is often caused by Streptococcus pyogenes

Pathogenesis:
Organisms gain entry to the respiratory tract by inhalation of droplets and invade the mucosa. Epithelial destruction may ensue, along with redness, edema, hemorrhage and sometimes an exudate.

Clinical Manifestations:
Initial symptoms of a cold are runny, stuffy nose and sneezing, usually without fever. Other upper respiratory infections may have fever. Children with epiglottitis may have difficulty in breathing, muffled speech, drooling and stridor. Children with serious laryngotracheitis (croup) may also have tachypnea, stridor and cyanosis.

Microbiologic Diagnosis:
Common colds can usually be recognized clinically. Bacterial and viral cultures of throat swab specimens are used for pharyngitis, epiglottitis and laryngotracheitis. Blood cultures are also obtained in cases of epiglottitis.

Prevention and Treatment:
Viral infections are treated symptomatically. Streptococcal pharyngitis and epiglottitis caused by H influenzae are treated with antibacterials. Haemophilus influenzae type b vaccine is commercially available and is now a basic component of childhood immunization program.

2. Lower Respiratory Infections:
Bronchitis, Bronchiolitis and Pneumonia:

Etiology:
Causative agents of lower respiratory infections are viral or bacterial. Viruses cause most cases of bronchitis and bronchiolitis. In community-acquired pneumonias, the most common bacterial agent is Streptococcus pneumoniae. Atypical pneumonias are cause by such agents as Mycoplasma pneumoniae, Chlamydia spp, Legionella, Coxiella burnetti and viruses. Nosocomial pneumonias and pneumonias in immunosuppressed patients have protean etiology with gram-negative organisms and staphylococci as predominant organisms.

Pathogenesis:
Organisms enter the distal airway by inhalation, aspiration or by hematogenous seeding. The pathogen multiplies in or on the epithelium, causing inflammation, increased mucus secretion, and impaired mucociliary function; other lung functions may also be affected. In severe bronchiolitis, inflammation and necrosis of the epithelium may block small airways leading to airway obstruction.

Clinical Manifestations:
Symptoms include cough, fever, chest pain, tachypnea and sputum production. Patients with pneumonia may also exhibit non-respiratory symptoms such as confusion, headache, myalgia, abdominal pain, nausea, vomiting and diarrhea.

Microbiologic Diagnosis:
Sputum specimens are cultured for bacteria, fungi and viruses. Culture of nasal washings is usually sufficient in infants with bronchiolitis. Fluorescent staining technic can be used for legionellosis. Blood cultures and/or serologic methods are used for viruses, rickettsiae, fungi and many bacteria. Enzyme-linked immunoassay methods can be used for detections of microbial antigens as well as antibodies. Detection of nucleotide fragments specific for the microbial antigen in question by DNA probe or polymerase chain reaction can offer a rapid diagnosis.

Prevention and Treatment:
Symptomatic treatment is used for most viral infections. Bacterial pneumonias are treated with antibacterials. A polysaccharide vaccine against 23 serotypes of Streptococcus pneumoniae is recommended for individuals at high risk.