Immunology - Streptococcal Infections

Chapter 14: Streptococcal Infections

Overview of Streptococcal Infections

• Focus on disease processes caused by these infections rather than organism identification.

• Key organism: Streptococcus pyogenes (Landsfield Group A).

• Diseases associated include strep throat, scarlet fever, impetigo, and necrotizing fasciitis.

Streptococcus pyogenes Infections

• Pharyngitis: Most common cause of strep throat.

• Possible complications if untreated:

  • Acute rheumatic fever

  • Post-streptococcal glomerulonephritis

  • Sinusitis

  • Ear infections

  • Septic arthritis

  • Neonatal septicemia

  • Rare cases of pneumonia

• Importance of early diagnosis and antibiotic treatment to prevent complications.

M Protein and Virulence

• M protein: Major virulence factor that inhibits phagocytosis, allowing infection to occur.

• Variability in strains: Some lack M protein and cannot cause infection.

• Antibodies against M protein lend type-specific immunity to Strep pyogenes.

Cellular Structure and Characteristics

• Strep pyogenes is a Gram-positive coccus, appearing in chains.

• Landsfield Group A testing aids in identification; typically causes beta-hemolysis on blood agar.

• Fimbriae: Projects through cell wall for adhesion.

  • M1 antigen is critical for attachment to human epithelial cells.

Extracellular Products and Pathogenesis

• Key enzymes:

  • Streptolysin O: Oxygen-labile; binds to red blood cell membranes causing hemolysis.

  • Streptolysin S: Oxygen-stable; responsible for beta-hemolysis in blood agar, disrupts cell membranes.

• Hyaluronidase: Breaks down hyaluronic acid in connective tissues, facilitating infection spread.

• DNase: Degrades host DNA, aiding in bacterial invasion.

• Streptokinase: Dissolves blood clots, aiding bacterial dispersion.

• Erythrogenic toxin: Associated with scarlet fever rash.

Epidemiology and Transmission

• Primarily seen in school-aged children; spread via respiratory droplets and direct contact.

• High transmissibility among children due to shared habits (e.g., using same items, close contact).

• Importance of hygiene to prevent household transmission.

Symptoms and Diagnosis

• Common Infections: Strep throat, impetigo, scarlet fever, rheumatic fever.

• Symptoms of strep throat: Sore throat, fever, swollen tonsils, white patches.

• Impetigo can cause red, tender skin lesions.

• Complications can lead to systemic issues affecting heart, kidneys, and other organs.

• Diagnostic methods include throat cultures and rapid test kits, highlighting the importance of accurate sampling to avoid false negatives.

Streptococcal Toxic Shock Syndrome (TSS)

• Introduction usually occurs through skin injuries.

• Production of pyogenic exotoxins can lead to fever, shock, and tissue necrosis.

• Symptoms include shock, fever, rash.

• Early recognition essential to prevent severe outcomes.

Treatment

• Importance of hydration, blood pressure maintenance, and use of antibiotics (penicillin and beta-lactam antibiotics) during TSS.

• Surgical intervention may be necessary for infected wounds post-recovery.

Related Organism: Streptococcus agalactiae (Group B Strep)

• Can cause severe disease in adults and neonates, high mortality rates.

• Screening during pregnancy (35-37 weeks) crucial for preventing transmission.

• Diagnostic tests include cultures from the mother’s genital tract, blood tests, and PCR techniques.

Conclusion

• Understanding the virulence and disease processes related to streptococcal infections is essential for early diagnosis and treatment.

• Knowledge of laboratory testing, signs, and complications aids clinicians in effectively managing these infections.

Chapter 14: Streptococcal Infections

Overview of Streptococcal Infections

• Focus on disease processes caused by these infections, rather than solely on organism identification.

• Key organism: Streptococcus pyogenes (Landsfield Group A).

• Diseases associated include:

  • Strep throat: Most common manifestation, often occurs in children ages 5-15.

  • Scarlet fever: Characterized by a distinctive red rash and is accompanied by strep throat.

  • Impetigo: A highly contagious skin infection that can occur in children.

  • Necrotizing fasciitis: A severe, rapidly progressing infection that destroys tissues, often termed flesh-eating disease.

Streptococcus pyogenes Infections

• Pharyngitis: Most common cause of strep throat, presenting with symptoms of sore throat, fever, and swollen lymph nodes.

• Possible complications if untreated include:

  • Acute rheumatic fever: An autoimmune response leading to heart complications and damage.

  • Post-streptococcal glomerulonephritis: Caused by antigen-antibody complexes leading to kidney dysfunction.

  • Sinusitis: Infection of the sinus cavities can occur as a complication.

  • Ear infections (otitis media): Common in children, particularly following strep throat.

  • Septic arthritis: Infection in joints can result from hematogenous spread.

  • Neonatal septicemia: Life-threatening condition in newborns.

  • Rare cases of pneumonia can arise, particularly in immunocompromised individuals.

• Importance of early diagnosis and antibiotic treatment is critical to prevent complications and reduce spread.

M Protein and Virulence

• M protein: Major virulence factor that plays a crucial role in inhibiting phagocytosis, thereby allowing infection to occur.

  • Displays significant variability among strains; some strains lack M protein and therefore cannot cause infection.

  • Antibodies against M protein provide type-specific immunity against Streptococcus pyogenes infections, explaining the occurrence of repeat infections with different strains.

Cellular Structure and Characteristics

• Streptococcus pyogenes is a Gram-positive coccus, appearing in chains under microscopy.

• Landsfield Group A testing aids in identification; typically causes beta-hemolysis on blood agar, which is a characteristic indicator.

• Fimbriae: These projections through the cell wall aid in adhesion to host tissues, enhancing virulence.

• M1 antigen: Critical for attachment to human epithelial cells, facilitates colonization.

Extracellular Products and Pathogenesis

• Key enzymes produced by S. pyogenes include:

  • Streptolysin O: Oxygen-labile toxin that binds to red blood cell membranes, causing hemolysis; can contribute to tissue damage.

  • Streptolysin S: Oxygen-stable and responsible for the characteristic beta-hemolysis observed in blood agar, disrupts cell membranes of host cells, facilitating invasion.

  • Hyaluronidase: Enzyme that breaks down hyaluronic acid in connective tissues, facilitating the spread of infection.

  • DNase: Degrades host DNA, which aids in bacterial invasion and evasion of the immune response.

  • Streptokinase: Dissolves blood clots, allowing bacteria to disperse from initial infection sites.

  • Erythrogenic toxin: Associated with the rash in scarlet fever and contributes to systemic toxicity.

Epidemiology and Transmission

• Primarily observed in school-aged children, who are significantly at risk due to close contact environments.

• Spread through respiratory droplets and direct contact with infected lesions.

• High transmissibility among children is due to shared habits such as using the same items and close interaction.

• Importance of hygiene practices (i.e., hand washing, keeping surfaces clean) is vital to prevent household transmission.

Symptoms and Diagnosis

• Common infections include strep throat, impetigo, scarlet fever, and rheumatic fever.

• Symptoms of strep throat include: sore throat, fever, swollen tonsils, white patches, and swollen lymph nodes.

• Impetigo presents with red, tender skin lesions often around the mouth and nose, which can be itchy.

• Complications can lead to systemic issues affecting the heart, kidneys, and other organs.

• Diagnostic methods include throat cultures and rapid antigen detection test kits, highlighting the importance of accurate sampling and interpretation to avoid false negatives.

Streptococcal Toxic Shock Syndrome (TSS)

• Introduction of S. pyogenes usually occurs through skin injuries or wounds.

• Production of pyogenic exotoxins can lead to severe systemic symptoms including fever, shock, and tissue necrosis.

• Symptoms of TSS include shock, fever, and widespread rash.

• Early recognition and treatment are essential to prevent severe outcomes and mortality associated with TSS.

Treatment

• Importance of hydration, maintaining blood pressure, and using appropriate antibiotics (primarily penicillin and beta-lactam antibiotics) during TSS management.

• In cases of necrotizing fasciitis, surgical intervention may be necessary for infected wounds to remove necrotic tissue and control infection spread post-recovery.

Related Organism: Streptococcus agalactiae (Group B Strep)

• This organism can cause severe disease in both adults and neonates, with high mortality rates associated with invasive infections.

• Screening during pregnancy (35-37 weeks) is crucial to prevent transmission to newborns, who are particularly vulnerable.

• Diagnostic tests include cultures obtained from the mother’s genital tract, blood tests, and PCR techniques to rapidly identify group B strep.

Conclusion

• Understanding the virulence and disease processes related to streptococcal infections is essential for early diagnosis and treatment.

• Knowledge of laboratory testing, signs and symptoms, and potential complications helps clinicians effectively manage these significant infections in the population.