Introdcution to Upper Respiratory Tract Diseases casued by S. pyogenes

The most serious cases of pharyngitis (inflammation of the throat) are casued by Streptococcus pyogenes, a group A streptococcus. S. pyogenes is a gram positive occus that grows in chains, does nto form endospores, is nonmotile and forms capsules and slim layers. It is a facultative anaerobe that ferments a variety of sugars. 

Untreated streptococcal throat infections can occasionaly result in serious complications. The psot streptococcal conditions can be caused by the presence of an extra toxin (as in scarlet fever) or by the deposition of antigen-antibody complexes in the body (as in glomerulonepthritis). 

About 30% of sore throats may be caused by S. pyogenes, resulting in several million cases each year. Most transmission of S. pyogenes is via respiratory droplets or direct contact with mucus secretions. Thsi bacterium is carried as normal bioa by 15% of the population, but transmsiion fromt his reservoir is less likely than from a person who is experiencing active disease because of the higher number of bacteria prsent in the disease condition. 

Humans are the only significant reservoir of S. pyogenes. 

More than 80 serotypes of S. pyogenes exist, and thus people can experience multiple infections throught their lives becasue immunity is eserotype specific. 

Streptococcus pyogenes is sensitive to a minute concentration of bacitracin. This serves for a rpaid, immunologic test for diagnosis of group A infections., the bacitracin disc test. Any zone of inhibition around the B disc is interpreseted as a presumptive indication of this species. 

The antibiotic choice for S. pyogenes is penicillin. 

Rheumatic Fever

Cause: Rheumatic fever is an illness caused from complications from untreated or inadequately treated Group A Streptococcus.

Who is at Risk? Most outbreaks of scarlet fever occur under conditions of impoverished overcrowding where access to antibiotics is limited. Since children are more at risk for strep throat, they are also more at risk for scarlet fever. The peak age is 6-20 years.

Mechanism/Virulence: Scarlet fever is believed to be caused when the host’s immunologic response (antibodies) to the bacterial antigens cross react with target organs. In particular, Rheumatic fever is thought to be due to an immunologic cross reaction between the streptococcal M protein and heart muslce. The lymphocyte clones activated by the M protein react with an epitope on the heart muscle. 

Streptococci display numerous surface antigens. Specialized polysaccharides on the surface of the cell wall help to protect the bacterium from being dissolved by the lysozyme of the host. Lipoteichoic acid, LTA, contribues to the adherence of S. pyrogens to epithelial cells in the pharynx. A spiky curae protection called M protein contributes to virulence by resisting phagocytosis and possibly by contributing to adherence. A capsule made of hyaluronic acid (HA) is formed by most S. pyogenes strains. It probably contribues to teh bacterium’s adhesiveness. 

Group A streptococci owe some of their virulence to the effects of hemolysins called streptolysins which casue beta-hemolysis of sheep blood agar. These hemolysins (streptolysin O and S) injure many cells and tissues, including leukocytes and liver and heart muscle. 

Symptoms: include fever, arthritis in large joints, electrical changes in the heart, shortness of breath due to heart failure, skin rashes and subcutaneous nodules. Other symptoms include arthritis in multiple joings and appearance of nodules over bondy surfaces just under the skin. 

Treatment: includes oral antibiotics (e.g., penicillin). Prevention of reorrurence is usually necessary (injection every 3 weeks for up to 5 years or even more.

Scarlet Fever

Cause: Scarlet fever is a disease caused by exotoxin released by Streptococcus pyogenes.

Mechanism/Virulence: The S. pyorgenes strain is itself infected with a bacteriophage which gives the streptococcus the ability to produce erythrogenic toxin. 

A key toxin in the development of scarlet fever is erythrogenic toxin which is responsbiel for the bright red rash typical of thsi disease, and also induces fever by acting upon the temperature regulatory center in the brain. 

Symptoms include sore throat, high fever, bright red tongue and a rash which appears 12-28 hours after the fever. The rash is sandpaper like, most often on the neck, chest, elbows, inner surfaces of the thighs. Immune complications include rheumatic fever above.

Who is at Risk? Scalrlet fever most often affects school-age children and was a source of great suffering in the U.S. in the early part of the 20th centure. The disease can have a fatality rate of up to 95%. Although the disease had all but disappeared int he last centruy, a resurgence in cases in the UK and in parts of Asia has been documented.