Microbiology · Year 2 · from Microbiology

Case 2: Streptococcal Pharyngitis and Complications

Presentation

A 7-year-old girl is brought to the pediatrician by her mother with a 2-day history of sore throat, fever, and refusal to eat. She attends school where several classmates have recently been sick with sore throats. She has no cough, runny nose, or hoarseness.

Physical examination reveals temperature 38.8°C, pharyngeal erythema with tonsillar exudates, and tender bilateral anterior cervical lymphadenopathy. She has a fine, sandpaper-textured erythematous rash on her trunk with accentuation in the skin folds of her elbows (Pastia lines). Her tongue initially shows a white coating with prominent papillae, consistent with early "strawberry tongue."

A rapid streptococcal antigen test is positive.

Clinical Image

Laboratory identification of streptococci including hemolysis patterns on blood agar - essential for differentiating streptococcal species.

Image Source: Lecture image - streptococcal identification

Questions

  1. What is the diagnosis, and what virulence factor is responsible for the rash?
  1. Why is antibiotic treatment of Group A streptococcal pharyngitis important beyond symptomatic relief?
  1. Three weeks later, the patient returns with periorbital edema, dark urine, and blood pressure 130/85 mmHg. What complication has developed?
  1. What is the mechanism of this complication, and could it have been prevented with antibiotic treatment?

Answers

  1. Diagnosis and virulence factor: The diagnosis is scarlet fever - Group A streptococcal (Streptococcus pyogenes) pharyngitis with an associated toxin-mediated rash.

The rash is caused by streptococcal pyrogenic exotoxins (Spe proteins, particularly SpeA, SpeB, or SpeC). These exotoxins are:

  • Superantigens: Similar mechanism to staphylococcal TSST-1
  • Erythrogenic toxins: Cause the characteristic erythematous rash
  • The rash is fine and sandpaper-textured, sparing the palms and soles
  • Pastia lines: Linear accentuation in skin folds from capillary fragility
  • Strawberry tongue: Initially white-coated with prominent papillae, later becoming red ("raspberry tongue")

Note: Scarlet fever is not more severe than strep pharyngitis alone - it simply indicates the strain produces erythrogenic toxin and the patient lacks preexisting antibodies to that toxin.

  1. Importance of antibiotic treatment:

Primary reasons for treating GAS pharyngitis:

  • Prevention of acute rheumatic fever (ARF): The most important reason. Antibiotic treatment within 9 days of symptom onset prevents ARF
  • Faster symptom resolution: Antibiotics shorten illness duration by 1-2 days
  • Reduced transmission: Treatment reduces contagiousness, allowing earlier return to school
  • Prevention of suppurative complications: Peritonsillar abscess, cervical lymphadenitis, mastoiditis

Treatment of choice: Penicillin V orally for 10 days or amoxicillin once daily for 10 days. No penicillin-resistant GAS strains have ever been documented. For penicillin-allergic patients: azithromycin, cephalosporins (if not anaphylactic allergy), or clindamycin.

  1. Complication: Post-streptococcal glomerulonephritis (PSGN).

Clinical features:

  • Periorbital edema (fluid retention)
  • Dark/cola-colored urine (hematuria)
  • Hypertension
  • Onset 1-3 weeks after GAS pharyngitis or 3-6 weeks after skin infection

Laboratory findings would show:

  • Urinalysis: Hematuria with RBC casts, proteinuria
  • Low serum C3 complement (consumed during immune complex deposition)
  • Elevated ASO titer (evidence of recent strep infection)
  • Mild elevation of creatinine
  1. Mechanism and prevention:

Mechanism (immune complex-mediated disease):

  • PSGN is caused by deposition of immune complexes (streptococcal antigen-antibody complexes) in glomerular capillary walls
  • This activates complement (explaining low C3) and triggers inflammatory response
  • Results in proliferative glomerulonephritis with neutrophil infiltration
  • Only certain "nephritogenic" strains of GAS cause PSGN (M types 1, 2, 4, 12, 25, 49)

Could antibiotics have prevented this?:

  • NO - Unlike acute rheumatic fever, PSGN is NOT prevented by antibiotic treatment
  • By the time pharyngitis symptoms appear, immune complex formation has already begun
  • The immune response causing glomerular injury is already underway before diagnosis
  • Treatment is supportive (salt and fluid restriction, antihypertensives, diuretics)

Prognosis: Excellent in children - >95% recover completely. Adults may have more persistent abnormalities.

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