self-learning slides

Streptococcus pyogenes: Pharyngitis

A 12 year old girl presents with fever, rash, headache, weight loss and fatigue. Her knees, ankles, elbows, and wrists showed warmth, swelling, redness, and tenderness. Firm, painless nodules on the extensor surfaces of the wrists, elbows, and knees were noticed. She displayed rapid, purposeless movements of the face and upper extremities. It was revealed that she had suffered from pharyngitis three weeks earlier and had a history of repeated attacks of sore throat.

What is your diagnosis?
It is a likely case of acute rheumatic fever.

How do you diagnose this condition?
Clinically, diagnosis of acute rheumatic fever is made according to revised Jones criteria by looking for major and minor criteria. Either two major criteria or one major criterion and two minor criteria along with history of strepotococcal throat infection is required to establish rheumatic fever.

What is the pathogenesis of ARF?
Recurrent attacks of pharyngitis by Group A Streptococci may lead to ARF. Some degree of antigenic cross-reactions is known to occur between human antigens and streptococcal antigens. There is similarity between group specific carbohydrate antigen of Streptococcus pyogenes and the glycoprotein of the heart valves. An immune response against the bacterial antigens is thought to mediate an attack on cross-reacting self-antigens. Some amount of genetic predisposition is also known to occur in such patients. Although rheumatic fever is known to follow attacks of pharyngitis, few cases of rheumatic fever has been documented to occur following streptococcal pyoderma in Australia. The exact mechanism of pathogenesis is still not convincingly clear, but the evidences suggest autoimmune phenomenon. Certain rheumatogenic strains belonging to M serotypes 1, 3, 5, 6 and 18 are frequently associated with ARF. These strains are known to produce large amounts of M protein and possess hyaluronic acid capsule.

How is the diagnosis of ARF made with laboratory tests?
A culture from throat swab is not useful as this condition sets in after an episode of pharyngitis, however Group A Streptococcus have been recovered from throat specimens. If cultures are negative, streptococcal antigen detection by enzyme immunoassay or latex agglutination can be attempted on throat specimens. A retrospective diagnosis can be made serologically by detecting anti-streptolysin O antibodies in patient's serum. The ASO test that is frequently used is based on latex agglutination. A titre of 200 units or higher is considered significant. Additionally anti-DNase B or anti-hyaluronidase too can be performed. A rise in antibody titre from acute to convalescent stage is more reliable.

Why is anti-streptolysin S not used?
Streptolysin S is not antigenic; hence there are no anti-streptolysin S antibodies.

How can the onset of ARF be prevented?
It can be prevented by treating patients with antibiotic (e.g.,Penicillin) within nine days of onset of Streptococcal pharyngitis.