The following points highlight the top four types of respiratory tract infection. The types are: 1. Infections of Throat and Pharynx 2. Infection of Middle Ear and Sinuses 3. Infection of Trachea and Bronchus 4. Infections of Lungs.
Respiratory Tract Infection: Type # 1.
Infections of Throat and Pharynx:
Sore Throat:
It is an acute pharyngitis and/or tonsillitis; causes:
A. Sore throat Syndrome
B. Acute Glomerulonephritis (AGN):
C. Diphtheria
D. Candidiasis
E. Vincent’s Angina
Viruses are most common causes of sore throat (about two-thirds of all cases are of viral origin). Besides, there are non-infective causes and infection due to Candida sp.
A. Sore Throat Syndrome (STS):
Streptococci and viruses cause similar signs and symptoms of sore throat, i.e., pain on swallowing, congested tonsils, pharynx, enlarged tonsillar lymph nodes and pyrexia. Aetiology: Bacteria, virus and mycoplasma. Bacteria Group A-haemolytic streptococcal, Str. pyogenes (most common). Group C and G are rarely involved, sometime Staph, aureus, H-influenzae.
Streptococcal Sore Throat:
Source of Infection: Carrier or Case:
Children of 5-8 years of age are commonly affected with this disease. In mild cases, there is redness of tonsils and pharynx. The actual illness is characterised by infection and oedema involving the fauces and soft palate with exudate-acute follicular tonsillitis. Incubation period is 3-5 days.
If untreated, streptococcal tonsillitis may give rise to septic complications (peritonsillar abscess, sinusitis, otitis media or to immune complex diseases (glomerulonephritis, rheumatic fever). When the infecting strain of str. pyrogenes is erythrogenic toxin producing (lysogenic), the throat infection is accompanied by erythematous rash (scarlet fever).
Laboratory Diagnosis (Sore Throat):
Specimen:
Throat swab from fauces inoculated in blood agar incubated aerobically and anaerobically.
Identification:
(i) Colonies are small, dry, beta haemolytic.
(ii) Lancefield grouping
Rheumatic fever (RF)
Latent period, 2-3 or up to 6 weeks after throat infection. Rheumatogenic serotypes M types 5, 18, 24 are mostly responsible.
Serology:
ASO titre 200 (significant) or more.
Treatment:
Rheumatic fever is due to repeated attacks. Therefore long term basis penicillin therapy is necessary to eliminate streptococci.
B. Acute Glomerulonephritis (AGN):
It is an immune complex hypersensitivity reaction:
i. Latent period:
1-3 weeks after streptococcal sore throat or skin infection (impetigo).
ii. Nephritogenic serotypes:
Acute glomerulonephritis is liable to follow infection with certain serotypes—mostly M type 12,1, 25, 4, 3 in throat infection, M types 49, 52-55, 57-61 in skin infection.
Serology:
Serum ASO titre is usually not elevated Anti-DNAse B is raised and C3 reduced.
Treatment:
AGN does not recover. Antibodies are of no use in established AGN. If skin infection is treated with penicillin or erythromycin, AGN may be prevented.
C. Diphtheria:
Corynebacterium diphtheriae gravis intermedius, mitis causes diphtheria, mitis type is common in India.
Type:
Faucial diphtheria is common, fauces covered with patches, sero-cellular exudate forming a greyish-white pseudo-membrane. Nasal diphtheria (mild), laryngeal diphtheria is serious because of laryngeal obstruction.
Incubation period:
2-5 days.
Source:
Throat and nose of carriers.
Pathogenesis:
Bacilli remain localised, multiply, produce powerful exotoxin. This exotoxin diffuses through the body, affects myocardium, adrenal glands, and nerve endings.
Treatment:
(a) Large dose of antitoxin should be injected without delay,
(b) Antibiotics (penicillin, erythromycin) can eliminate C. diphtheriae from throat without any effect on toxin already produced,
(c) In case of obstructive respiration, tracheostomy is necessary.
D. Candidiasis:
Oral candidiasis (oral thrush) in neonates is caused by Candida albicans and is characterised by loosely adherent white patches superimposed on red, raw mucous membrane of tongue, cheek and gums,
Source:
Neonates contact the infection from mother’s genital tract or by cross-infection in nurseries.
Though C. albicans is commensal of mucous membrane of mouth and may become an opportunist pathogen of respiratory tract.
Treatment:
Local application of Nystatin or Amphotericin.
E. Vincent’s Angina:
Acute ulceromembranous gingivitis (AUMG) is synergic infection with fusobacterium and spirochaete.
Bacterial Aetiology:
(i) Spirochaete-Borrelia vincenti.
(ii) Anaerobic bacilli-Fusobacterium fusiform (now called Leptotrichia buccalis, the present name of Vincent’s bacillus).
(iii) Oral Bacteroides sp (B. intermedius) are now believed to be responsible for infection.
Source:
Endogenous, the organisms are present in the normal mouth.
Pathogenesis:
AUMG usually develops in patients with poor oral hygiene or dental caries; malnutrition and debility resulting in overgrowth of the offending organisms.
Treatment:
Penicillin and Metronidazole.
Infectious Mononucleosis:
Presenting feature:
Exudative tonsillitis.
Cause:
Epstein-Barr virus (EBV).
Source:
Oropharyngeal secretions (the kissing disease of young adults).
Diagnosis:
i. Atypical lymphocytosis;
ii. Presence of heterophilic antibodies in serum (Paul-Bunnel test);
iii. Isolation of EBV from oropharyngeal washing or blood is laborious.
Treatment:
No specific treatment.
Laboratory Diagnosis of Throat and Pharyngeal Infections:
Specimen:
Throat swabs should be rubbed over the affected area and quickly sent to the laboratory. If delay in transit is anticipated, swabs should be preserved in the refrigerator.
Microscopy:
(i) Gram stain:
C.albicans are Gram-positive oval yeast cells with germ tube. Vincent’s infection shows Gram-negative spirochaete (B. Vincenti) and Gram-negative fusiform bacilli (Fusobacterium sp.).
(ii) Albert staining of smear shows V and L shaped bacilli with metachromatic granules (barred appearance). This is useful for presumptive diagnosis of C. diphtheriae. For confirmatory diagnosis, smear of diphtheriae must be isolated on culture and it should be subjected to toxigenicity test. 
Respiratory Tract Infection: Type # 2.
Infection of Middle Ear and Sinuses:
Aetiology:
The direct spread of pathogen from throat via Eustachian tube may cause the infection of middle ear and sinuses, viruses, mycoplasma, bacteria may be responsible for this infection. The infection may be acute or chronic.
H. influenza and Str. pneumonia are bacterial pathogens in acute infection whereas Ps. aeruginosa and Staph, aureus (aerobic pathogens), Peptococcus, Peptostreptococcus, Bacteroides (B. fragilis, B. melaninogenicus—anaerobic pathogens) cause chronic infection of middle ear and sinuses in pathogenesis, the obstruction is an important features.
Laboratory Diagnosis:
Specimen:
Swabs or pus, antral washings (collected carefully for anaerobic culture by disposable sterile syringe, needle which is bent at right angle immediately after collection).
Microscopy:
Gram stained film is not helpful.
Culture, Blood agar, nutrient agar, RCM incubation-aerobically and anaerobically.
Other tests:
Coagulase test-staph. aureus, oxidase test-Ps. aeruginosa urease test-Proteus sp. Biochemical tests help to distinguish different species.
Treatment:
i. Acute infection:
Ampicillin, amoxycilin or erythromycin.
ii. Chronic infection:
Drug resistant organisms are often encountered.
(a) Treatment should be guided by antibiotic sensitivity test.
(b) Anaerobes are sensitive to metronidazole, many are sensitive to lincomycin, Chloramphenicol, and Cefoxitin.
(c) Topical applications of neomycin, polymyxin, bacitracin is useful. Staph, aureus shows multidrug resistance, particularly in the hospital.
Many strains of Staph, aureus and Bacteroides produce a penicillin destroying enzyme (B-lactamase) that hydrolases the beta lactam ring in the penicillin nucleus.
Respiratory Tract Infection: Type # 3.
Infection of Trachea and Bronchus:
Many infections of trachea (tracheitis), larynx( (laryngitis) and bronchus (bronchitis) are caused by viruses.
A. Acute Laryngo-Tracheo-Bronchitis (Croup):
i. Cause:
Bacteria, H. influenza type 6, virus, par influenza virus 1, 2, 3
ii. Incidence:
Infants and young children (2-7 years)
iii. Symptoms:
Croaking cough
4. Blood culture is positive in septicaemia, pharyngeal swab culture is positive for H. influenza.
Treatment:
Ampicillin or Chloramphenicol.
B. Chronic Bronchitis:
i. Clinical features:
At the beginning cough is dry and painful, later it becomes productive with expectoration of yellowish green sputum, most marked in early morning samples.
ii. Pathogenesis:
It is a relapsing condition. Predisposing factors are smoking, previous lung damage, atmospheric pollution, cold damp weather.
iii. Pathological changes:
(a) Mucous secreting cells in bronchi are increased in number resulting in hyper-secretion of mucus.
(b) Inflammation, fibrosis, collapse, dilatation, cyst formation occur in bronchi and alveoli leading to irreversible damage.
iv. Laboratory Diagnosis
Sputum culture.
C. Whooping Cough:
It is an acute tracheo-bronchitis of childhood. Incubation period is 10 days. Patients with catarrhal stages can be the source of infection. Transmission is by airborne or droplet infection.
Clinical Features:
Initial catarrhal stage with cold lasts for 2 weeks. It is followed by a stage of 2 weeks’ paroxysmal coughing. Then a persisting residual cough for another month. Fatality is low; morbidity high; 50 times higher in children under one year than 4 year olds. Whooping cough appears in epidemic preparation.
Immunity:
Immunity by infection or by vaccination is long lasting.
Laboratory Diagnosis:
Specimen:
Perinasal swab passed by gently along the floor of the nose and nasopharyngeal secretions is collected.
(i) Culture:
Specimen is inoculated in Bordet Gengou medium agar or charcoal medium. Alternatively, medium is held in front of mouth of patient during a paroxysm of coughing, plate is incubated for 3-5 days at 35-36°C; colonies are moist “mercury drop” like.
(ii) Agglutination:
Organisms are identified by slide agglutination with specific anti- serum.
(iii) Serology:
Rising antibody titres may be found in serum in children over one year.
Respiratory Tract Infection: Type # 4.
Infections of Lungs:
Pneumonia may be defined as inflammation and consolidation of the lung substance.
It may be caused by:
(a) Microorganisms,
(b) Physical, and
(c) Chemical agents.
Types of Pneumonia:
Infective pneumonia are to three types:
1. Lobar pneumonia is an acute inflammation characterised by homogeneous consolidation of one or more lobes or segments of lung.
2. Bronchopneumonia is a secondary pneumonia preceded by bronchial infection. It is an acute inflammation of bronchi, especially the terminal bronchioles which are filled with pus, the consolidation is scattered and distributed bilaterally in small patches.
3. Atypical pneumonia. There is patchy consolidation of lungs. “Mycoplasma pneumoniae” causes atypical pneumonia in which upper respiratory tract infection progresses to pneumonia. o
4. Nosocomial pneumonia. Patient admitted in hospital for more than 48 hours may develop pneumonia-hospital acquired infection.
The most common causes are:
Str. pneumoniae, Staph, aureus, M. pneumoniae, Legionella pneumophila, E. coli.
Pneumonia in the immuno-compromised patients. Pulmonary infection is common in patients receiving immuno-suppressive drugs or in congenital or acquired immune deficiencies (AIDS).
The common causes are Pneumocystis carinii, Ps. aeruginosa, Staph, aureus, Aspergillus fumigatus, Candida albicans, viral infections (CMV), Herpes and Myco-tuberculosis. Viruses, Respiratory syncytial virus causes most severe infection in young children as bronchiolitis and bronchopneumonia. Influenza and para-influenza viruses in a previously healthy adult is rare.
Laboratory Diagnosis of Lung Disease:
Specimen:
Early morning sputum sample is most purulent. Sample taken properly with minimal salivary contamination should be despatched immediately to the laboratory. In the diagnosis of lobar pneumonia, blood culture should be helpful.
Microscopy:
1. Gram stained smear. Adequate number of pus cells and presence of organisms is due to the probable pathogen.
2. Ziehl-Neelsen or Auramine stained smear. Presence of Acid Fast Bacilli (AFB) is an indication of tuberculosis.
Culture:
Purulent sputum is best for culture.
Blood agar:
After inoculation, the medium is incubated aerobically at 37°C. Isolate later confirmed.
Chocolate agar is a selective medium for H. influenza. It is incubated at 37°C in 5-10% CO2.
Culture for Mycobacteria:
Lowenstein-Jensen medium is inoculated with three samples of sputum collected on three successive days, then incubated aerobically at 37°C for 6-8 weeks (long incubation period) LJ medium is screw capped to avoid evaporation of medium during long incubation period.
Detection of Bacterial Antigens:
A rapid diagnosis of Pneumocystis carinii and Legionella pneumophila can be done by fluorescent antibody staining of bronchial lavage and respiratory secretions, respectively.
Serological Tests:
When the causative organisms are difficult to isolate, serological tests should be undertaken:
