013 Case Study: Sore Throat

Question 1

What type of bacteria is S.pyogenes?

Answer 1

Gram positive; Cocci in chains; Catalase negative; Group A carbohydrate antigens; causing Beta haemolysis

Question 2

What occurs during alpha haemolysis?

Answer 2

Haemoglobin iron is reduced causing in green halo around colonies as it is converted into methemoglobin.

Question 3

What is the proportion of asymptomatic carriage of GAS?

Answer 3

20%

Question 4

Which complement pathways are activated by GAS? How?

Answer 4

Alternative pathway - PAMP

Lectin pathway - lectin-mannose binding

Question 5

What virulence characteristics of S.pyogenes contribute to avoiding host defence mechanisms?

Answer 5

M protein that is found on the peptidoglycan wall binds with H factor which results in degredation of C3 convertase.
By binding to fibrinogen, M protein also blocks deposition of C3b.
C5a peptidase inactivates C5a, which is a powerful chemoattractant for neutrophils and macrophages
Lipoteichoic acid is an adhesin
Hyaluronic acid capsule that is antiphagocytic

Question 6

What is responsible for the beta hemolysis of GAS?

Answer 6

Streptolysins S and O

Question 7

What symptoms will occur during local infection?

Answer 7

Pharyngitis within 12-24 hours - fever, rash, malaise

Pharynx and tonsils may be erythematous with creamy.y or yellow exudates.

Question 8

What symptoms may occur during invasive infections?

Answer 8

Erysipelas - acute skin infection involving the upper dermis which is accompanied by lymphadenopathy, fever, chills, leukocytosis. Legs frequent site.
Cellulitis: similar to erysipelas but also in connective tissue.
Necrotising fascilitis - deep infection of connective tissue that destroys muscle and fat - swelling and appearance of red or dusky blue skin discoloration, often with fluid filled bullae.

Question 9

How does Scarlet fever arise?

Answer 9

Toxin released by S.pyogenes caused bright beefy tongue, fever, and rash that starts on the face and spreads to chest, back and body.

Question 10

What are 2 autoimmune diseases that can arise after a Strep infection?

Answer 10

Acute rheumatic fever and Acute glomerulonephritis - when the antibodies against the M proteins cross react with human tissues, in particular the heart, joints, subcutaneous tissue and CNS. This is due to the coiled nature of M protein.

Question 11

What is the standard course of treatment?

Answer 11

Penicillin (benzylpenicillin) for those not hypersensitive
Erythromycin or cephalosporin for those hypersensitive
Clindamycin and surgery for those with necrotising facilitis

Question 12

What is a standard diagnosis technique?

Answer 12

Serology and culture

Serology - antistreptomycin O titre detection