S. pneumoniae- Epidemiology, Pathogenicity, Laboratory Diagnosis, Prophylaxis, Treatment

Epidemiology

• ·         S. pneumoniae occurs in the throats of 50% of human population, any time 
•        Source of infection is the respiratory tract of carriers, and at times, of patients.
• ·         Transmitted by fingers or by inhalation of droplets
• ·         Dissemination is facilitated by crowding
• ·         Infection usually leads to pharyngeal carriage-- disease results when host resistance is lowered by viral infection, pulmonary congestion, stress, malnutrition, immunodeficiency or alcoholism

  Pathogenicity

• ·         Experimental infection in mice & rabbits by intraperitoneal inoculation - fatal infection – pneumococci demonstrable in peritoneal exudate and heart blood–death in 1-3 days

• ·         Colonise the human Nasopharynx – cause infection of middle ear, paranasal sinuses and respiratory tract – direct infection
• ·         Can spread through blood and lymphatic system– cause Meningitis – distant infections in the heart, peritoneum or joints
• ·         Infection is generally, endogenous in nature, can be Exogenous also

Common Pneumococcal infections 

• otitis media and sinusitis – prior respiratory infection or allergy can lead to these conditions

• common cause of lobar and bronchopneumonia –also cause acute tracheobronchitis and empyema (collection of pus in the pleural cavity)

•  Normal mucosal defense mechanisms (entrapment, cough, ciliary defense) prevent establishment of infection. But if these defense mechanisms are compromised by infections, anesthesia or chilling etc, Pneumococci can multiply and spread through lung and lymphatics- Bacteremia is common in lobar pneumonia

• Diffusion of capsular polysaccharide into the blood and tissues- cause toxemia.
•  SSS neutralized by anti-capsular antibodies produced by the patient- cause fall in temperature and general relief of symptoms
•  In adults, 50% fatalities are due to Pneumococcal bacteremia.

• Bronchopneumonia is always a secondary infection – damage to the respiratory epithelium and excessive bronchial secretions caused by the primary infection, lead to the invasion of Pneumococci to the bronchial tree- terminal event in aged and debilitated patients 

• Meningitis – most serious Pneumococcal infection
• Is a secondary infection after the primary infections like  pneumonia, otitis media, sinusitis or conjunctivitis - Occur at all ages 
• If untreated, highly fatal 
• 25% fatality even with antibiotic therapy

•  Pneumocci also cause suppurative lesions in different parts of the body- empyema, pericarditis, otitis media, sinusitis, conjunctivitis, suppurative arthritis, peritonitis , keratitis, dacryocystitis (lacrymal sacs infection)

Laboratory Diagnosis

• Clinical diagnosis of Pneumonia is easy -etiological diagnosis needs laboratory assistance- many organisms involved
• Specimen – Sputum, CSF, Blood and Urine
• Microscopy - In acute lobar pneumonia, rusty sputum contain Pneumococci in large numbers -hardly any other bacterium- Gram stain.

(Rusty sputum- sputum produced in pneumonia caused by S pneumoniae - contain bacteria, hemorrhage, mucus,  necrotic lung tissue.)

• In the pre-antibiotic era, direct serotyping with Quellung reaction followed by treatment with specific antiserum-routine process

• Culture – The sputum after homogenization, inoculated onto blood agar plates, incubated at 37^oC, under 5-10 % CO₂, overnight. In infants, laryngeal swabs may be used for culture. Gentamycin can be added to blood agar to facilitate isolation of Pneumococci.

• In acute pneumonia, the organism may be obtained from Blood culture in Glucose broth – isolation of pneumococci in blood indicates bad disease condition.

• Animal inoculation – Intraperitoneal inoculation in mice, if specimen contains scanty numbers of the organism and even if cultures are negative. Pneumococci may be demonstrated in the peritoneal exudates and heart blood. Inoculated mice die in 1-3 days.
• Test maybe negative if the strains are avirulent for mice (type 14 strain)

• In otitis media, Pneumococci demonstrated in fluid aspirated from the middle ear

• Meningitis- presumptive diagnosis from Gram stained films of CSF-Gram positive diplocoocci visible inside polymorphs and extracellularly. Diagnosis confirmed by culture. If negative in culture, SSS demonstrated in CSF by immunoprecipitation (with antisera)

• Antigen Detection  -SSS/Capsular polysaccharide -in blood, urine, CSF by Precipitation, counterimmunoelectrophoresis
• Antibody demonstrated by agglutination, precipitation, indirect haemagglutination, radioimmunoassay etc

Prophylaxis

• Immunity is type specific – antibodies against  capsular polysaccharide
• ` 90 serotypes- so complete polyvalent vaccine not practical
• A polyvalent polysaccharide vaccine with  capsular antigens of the 23 most prevalent serotypes gives 80 – 90 % protection 
• not meant for general use but for people at increased risk such as absent or dysfunctional spleen,  Sickle cell disease, chronic heart, renal, lung and celiac disease, Diabetes mellitus, HIV etc
• not recommended for children under two years of age and those with immunosuppressive therapies.

Treatment 

• Parenteral administration of Penicillin in serious cases and  Amoxycillin in mild cases (if the isolate is sensitive to penicillin)  
• Many Penicillin resistant (alteration in Penicillin Binding Protein)- strains are resistant to erythromycin and tetracycline
• For resistant isolates- Third generation cephalosporins and in life threatening illnesses with highly resistant strains, Vancomycin