Scarlet fever is a mainly pediatric infectious disease, of the exanthematic type, i.e. associated with a widespread skin rash (exanthema). Although it is a characteristic pathology of the pediatric age, it can also affect adults. Scarlet fever is the only childhood infectious exanthematous disease caused by bacteria; all the others are of viral origin.
The bacterium responsible for the disease is group A beta-haemolytic Streptococcus (Streptococcus pyogenes), which can also cause skin infections. Usually, scarlet fever presents in a harmless form, with a benign course. But it is a highly contagious disease, which spreads very rapidly in community facilities.
Being a disease of bacterial origin, it can be successfully treated through the administration of antibiotics. However, until their discovery, which radically changed the history of the disease, the number of deaths from scarlet fever was quite high.
The marked sensitivity of beta-haemolytic Streptococcus towards penicillin explains the effectiveness of today’s therapeutic tools in controlling the spread of the disease. Although the incidence of scarlet fever is much lower than in the past, outbreaks still occur.
Recently, the threat of antibiotic resistance has also affected the use of antimicrobials for the treatment of scarlet fever. In fact, forms of beta-haemolytic Streptococcus have emerged that are resistant both to penicillin and to other classes of antibiotics, the macrolides.
Scarlet fever does not confer definitive immunity, because there are numerous strains of beta-haemolytic Streptococcus. Although rare, the disease can recur in the same person. Scarlet fever is one of the diseases requiring notification to the Ministry of Health.
Scarlet fever – what is it
Scarlet fever is a pediatric infectious disease belonging to the category of exanthematous diseases. In other words, it produces an exanthema, i.e. a widespread skin rash.
Although it is a typically pediatric pathology, it can also affect adults.
The contagion of scarlet fever occurs mainly in autumn and winter, not for issues directly related to the temperature, but because it is during these seasons that community life takes place in closed, often overcrowded and poorly ventilated environments. All conditions that favor the transmission of the bacterium. In fact, the environment most at risk, from this point of view, is the school.
It is possible to contract scarlet fever a second and a third time: although rare, an infection after some time from the first is possible. In fact, there are several strains of beta-haemolytic Streptococcus and, therefore, the immunity produced by the disease is not definitive.
Scarlet fever manifests its greatest incidence in the age groups between 5 and 8 years and between 18 and 20 years.
Its frequency is practically nil in the first 6 months of the child’s life and is in any case very limited up to 2 years of age. Scarlet fever has almost zero incidence in newborns, probably because up to 6 months of life the child benefits from the immunization coverage received from the mother.
Furthermore, cases of scarlet fever under the age of 2 are very rare. In fact, it is with life in the community, in nursery and childhood schools, that the child experiences the risk of contagion and the consequent development of symptoms.
In the event of a rash or even mild symptoms which overall make one think of scarlet fever or any other exanthematous disease, it is naturally necessary to promptly consult the pediatrician.
Scarlet fever: transmission
Generally, it is not dangerous, even if it is a highly contagious disease that tends to generate outbreaks.
But when the subject is exposed to a source of contagion, the bacteria present in the droplets emitted by coughing or sneezing enter the oral cavity. Thus, they adhere to the pharyngeal mucosa through the action of the virulence factors of the bacterium.
At this point, there are three perspectives and the situation is influenced by several factors, including the immune status of the exposed person.
In the first case, the microorganism colonizes the mucous membrane and remains resident in the subject’s throat, without producing the disease.
In the second case, the bacterium grows and produces toxins, which facilitate tissue destruction and the extension of the infection.
Instead, in the third case, the microorganism spreads through the bloodstream and causes the most serious complication of scarlet fever, which is represented by septicemia.
But this occurs in particular circumstances, when the patient is debilitated or his immune system is compromised (by a pathology or by a therapeutic treatment which involves the suppression of the defences, such as organ transplantation).
Following the establishment of this framework, which leads to the spread of the pathogen throughout the body, progressive damage to all organs can occur caused by an abnormal inflammatory response. This condition is referred to as sepsis. Therefore, the exceptional activation of defense systems by the organism ends, in fact, with damage to the same. In fact, sepsis includes events that involve all organs and systems of the body, such as the formation of thrombi in the bloodstream, which can damage vital areas (such as the brain and the heart) in an irreversible manner and multiple organ failure.
However, any infection can lead to septicemia and sepsis. Septicemia and sepsis are not synonymous, but the latter is one of the possible consequences of the former and represents its clinical evolution.
Enzymes and toxins
Among the toxins produced by the bacterium, streptolysins contribute to the destruction of cells and open a breach for the bacterium to spread in the tissues.
Streptolysin O is a highly immunogenic molecule (i.e. capable of triggering a strong immune reaction). Instead, streptolysin S is not immunogenic and has a more intense lytic activity than O: it is one of the most potent cytotoxins known.
The erythrogenic toxin is at the basis of the appearance of the rash.
Enzymes also participate in the destructive action of beta-haemolytic Streptococcus:
- Streptokinase, dissolving blood clots.
- DNA-ase, attacking the DNA of cells in tissues already in necrosis.
- Peptidase, by inactivating the complement, a series of 27 factors that are activated in cascade and represent an important resource of the immune system.
- Hyaluronidase, hydrolyzing the hyaluronic acid of the extracellular matrix.
Scarlet fever can manifest itself in forms in which the rash is very modest. These are clinical pictures that are configured with conditions defined as scarlet fever (or fourth disease).
Scarlet fever: causes
Scarlet fever is an exanthematous disease caused by a bacterium, group A beta-haemolytic Streptococcus (Streptococcus pyogenes), also indicated by the acronym SBEGA. It is the only exanthematous disease originating from a bacterium.
The classification of streptococci is twofold.
These bacteria can be studied on the basis of their haemolytic activity (i.e. destruction of red blood cells), observable in the laboratory, and divided into three categories:
- Alpha-haemolytics: cause incomplete hemolysis of red blood cells.
- Beta-haemolytics: cause complete hemolysis of red blood cells.
- Gamma-haemolytics: do not cause haemolysis.
From the point of view of serological groups, streptococci can be classified according to the type of polysaccharide present in the cell wall, which is represented by a letter of the alphabet (Lancefield classification). From a clinical point of view, the significant groups are A and B.
It is a coco, i.e. a round, Gram positive bacterium; Group A beta-haemolytic streptococcus is the most significant pathogenic streptococcus.
Furthermore, this germ can also be responsible for other diseases such as pharyngitis (a clinical component also present in scarlet fever) and some skin diseases, such as impetigo (also known as pyoderma).
Scarlet fever: how the infection occurs
Being characterized by little possibility of surviving in the environment, outside of a host organism, the transmission of this pathogen takes place almost exclusively by direct contact with infected individuals and not through objects.
Its main reservoir in nature is man, where it lives settled in the mucous membrane of the oropharynx (the upper part of the pharynx) forming colonies, even in the absence of disease.
Its virulence factors are represented by:
Toxins are the toxic substances responsible for the clinical picture of the disease and the pathogenic power of bacteria. They cause actual damage in tissues and organs of the host organism. In general, they can be classified as:
- Exotoxins: proteins released into the surrounding cellular environment after secretion by the bacterium.
- Endotoxins: toxic substances, mainly lipids, which remain linked to the cellular structure of the pathogen and which are released in the host organism only after the destruction of the bacterium.
But while exotoxins can be produced by both Gram+ and Gram- bacteria and can be neutralized by the specific antibody, endotoxins are typical of Gram- and do not undergo the neutralizing action of any antibody. Thus, endotoxins are not detoxifiable, representing a more potent virulence factor than exotoxins.
The appearance of the typical rash of scarlet fever is due to an exotoxin secreted by the…