applied cases

Streptococcus pyogenes: Pharyngitis

A 12 year old girl with high fever reports to hospital ENT OPD with complaint of severe sore throat and difficulty in swallowing food. On clinical examination the pharynx was inflamed with yellowish discharge were over the tonsils.

What is your diagnosis?
It is a case of acute pharyngitis or sore throat.

What are the etiological agents of pharyngitis?
Pharyngitis can be caused by bacteria such as Group A Streptococci, Staphylococcus aureus, Corynebacterium diphtheriae, fusobacterium-treponemes or by viruses such as Herpes Simplex, Epstein Barr virus or even fungus such as Candia albicans.

How is the specimen collected?
Two sets of throat swabs are collected by rubbing over the inflamed area in throat and tonsils. Care should be taken not to touch the tongue, cheeks or palate.

Which transport medium can be used in case of delay?
Pike's medium

How is the specimen processed?
One swab is used for preparing a smear and stained by Gram stain. This will give information on possible etiology. Presence of Corynebacterium diphtheriae can be ruled out by Albert's stain. The other swab is used to inoculate on blood agar and incubated at 37oC aerobically or in candle jar overnight. Rapid detection of Streptococcus antigens from the throat swab extracts is now possible using latex agglutination, co-agglutination, Enzyme immunoassay  and immunoblot techniques.

How do you identify the growth?
Small, circular, non-pigmented greyish colonies with wide zone of beta hemolysis are seen on Blood agar. Gram stained smear will demonstrate gram positive cocci in chains. A negative catalase test will confirm it as Streptococci. Further identification of the species can be made by Lancefield typing of the isolate. Streptococcus pyogenes reacts with group A antiserum. S. pyogenes is also identified by positive PYR test.

Which are the possible complications of this condition?
Infection may spread by extension locally and result in tonsillar abscess, retropharyngeal abscess , mastoiditis, sinusitis, or otitis media. It may also lead to post streptococcal sequelae.

How would you treat this condition?
S. pyogenes is susceptible to penicillin and cepahlosporins. Patients allergic to penicllin may be treated by other antibiotics such as erythromycin.

Which are the other lesions produced by beta hemolytic streptococci?
Beta hemolytic streptococci (Group A) are responsible for scarlet fever, pyoderma (impetigo), erysipelas, cellulitis, myositis, puerperal sepsis, wound infections, etc. Important post-streptococcal sequelae include Acute rheumatic fever (ARF) and acute glomerulonephritis (AGN).

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. 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. Certain rheumatogenic strains belonging to M serotypes 1, 3, 5, 6 and 18 are frequently associated with ARF.

How is the diagnosis of ARF made using laboratory tests?
A culture from throat swab is not useful as this condition sets in after an episode of pharyngitis. 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.

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

What is the pathogenesis of AGN?
AGN may follow either pharyngitis or pyoderma by Group A Streptococci. The antibodies that are formed in the body towards streptococcal antigens form antigen-antibody complexes. These complexes get deposited in the glomerular basement membrane of the kidney and elicit complement-mediated attack. Complement levels are usually low in AGN. Certain nephritogenic strains belonging to serotype 1, 4, 6, 12, 25 are associated with pharyngitis-associated AGN whereas serotypes 2, 49, 53, 55, 56, 57, 60 are associated with pyoderma-associated AGN.

How is the diagnosis of AGN made with laboratory tests?
ASO tests are usually not positive in most cases of AGN. Streptodornase test (anti-DNase B) and anti-hyaluronidase tests are more helpful. A titre of 300-350 units or higher of anti-DNase B is considered significant. Streptozyme, a passive hemagglutination test using crude extracts of Streptococci is positive in both AGN and ARF.