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The transmission and epidemiology of Scarlet Fever is are dependent on those of Group A Streptococcal Pharyngitis since the majority of cases are derived from that pharyngeal infection. It occurs equally in both males and females.[1] Children are most commonly infected, typically between 5-15 years old. Although streptococcal infections can happen at any time of year, infection rates peak in the winter and spring months, typically in colder climates.[2] Transmission occurs between humans in close contact, via respiratory droplets (for example, saliva or nasal discharge.[2] A person in close contact with another person infected with Group A Streptococcal Pharyngitis has a 35% of becoming infected.[3] 1 in 10 children who are infected with Group A Streptococcal Pharyngitis will develop Scarlet Fever.[1]
The amount of scarlet fever cases has decreased over time along with the morbidity and mortality caused by the disease. The improvement in prognosis can be attributed to the use of penicillin in the treatment of this disease.[4] The cause for the change in overall frequency of the disease however is unclear as strains of group A streptococci that are capable of causing scarlet fever are still present.[5]
Note that the following signs and symptoms will usually be absent:[4]
(The presence of these symptoms indicates that the illness is more likely due to a virus rather than a bacterial infection)
The rash begins 1-2 days following the onset of symptoms caused by the strep pharyngitis (sore throat, fever, fatigue).[2] This characteristic rash has been denoted as "scarlatiniform" and it appears as a diffuse redness of the skin with small papules, or bumps, that resemble goose pimples.[7][5] These bumps are what give the characteristic sand paper texture to the rash. The reddened skin will blanch when you apply pressure to it. It is possible for the skin to be itchy however it will not be painful.[7] It usually first appears on the trunk and then gradually spreads out to the arms and legs. [5] The palms, soles and face are usually left uninvolved by the rash. The face is however is usually flushed, most prominent in cheeks, with a ring of paleness around the mouth.[10] After the rash spreads, it becomes more pronounced in creases in the skin, such as the skin folds in the inguinal and axillary regions of the body.[9] Also in those areas it is possible for there to be Pastia’s Lines which are petechiae arranged in a linear pattern.[9] Within 1 week of onset the rash beings to fade followed by a longer process of desquamation, or shedding of the outer layer of skin, that lasts several weeks.[1] The desquamation process usually begins on the face and progresses downward on the body.[7] After the desquamation the skin will be left with a sunburned appearance.[2]
The streptococcal pharyngitis that is the usual presentation of scarlet fever in combination with the characteristic rash commonly involves the tonsils. The tonsils will appear swollen and reddened. The palate and uvula are also commonly affected by the infection. The involvement of the soft palate can be seen as tiny red and round spots known as Forscheimer spots. [6]
The clinical features of scarlet fever can differ depending on the age and race of the person. Children less than 5 years old can have atypical presentations. Children less than 3 years old can present with nasal congestion and lower grade fever than older children.[3] Infants can potentially only present with increased irritability and decreased appetite.[3]
Children who have darker skin can have a different clinical presentation in that the redness of the skin involved in the rash and the ring of paleness around the mouth can be less obvious or visible.[7] Clinical suspicion based off accompanying symptoms and diagnostic studies are important in these cases.
After the Streptococcal infection is transmitted to the individual, it takes between 12 hours to 7 days for the infection to cause the abrupt onset of the first clinical signs of Strep Pharyngitis including fever, fatigue, and sore throat. The characteristic scarlatiniform rash then comes 12-48 hours later. During the first few days of the rash development and rapid generalization, the Pastia's Lines and strawberry tongue will also present.[7] The rash starts fading within 3-4 days followed by the desquamation of the rash that will last several weeks to a month.[2][6]. If the case of scarlet fever is uncomplicated, recovery forth fever and clinical symptoms other than than the process of desquamation occurs in 5-10 days.[11]
The complications that can arise from scarlet fever when left untreated or inadequately treated can be divided into two categories: suppurative and nonsuppurative.
Suppurative complications: These are rare complications that either arise from direct spread to structures that are close to the primary site of infection, which in most cases of Scarlet Fever is the pharynx. Possible problems from this method of spread include peritonsillar or retropharyngeal abscesses, cellulitis, mastoiditis or sinusitis. It is also possible for the streptococcal infection to spread through the lymphatic system or the blood to areas of the body further away from the pharynx. A few examples of the many complications that can arise from those methods of spread include endocarditis, pneumonia, or meningitis.[10]
Nonsuppurative complications: These complications arise from certain subtypes of the group A streptococci that cause an autoimmune response in the body through what has been termed molecular mimicry. The antibodies that the person’s immune system developed to attack the Group A Streptococci are in these cases also able to attack the person's own tissues. The following complications result depending on which tissues in the person's body are targeted by those antibodies. [5]
The rash of Scarlet Fever, which is what differentiates this disease from an isolated Group A Strep pharyngitis (or strep throat), is caused by specific strains of Group A Streptococcus that produce a pyrogenic exotoxin.[2] These toxin producing strains cause scarlet fever in people who do not already have antitoxin antibodies. Streptococcal Pyrogenic Exotoxins A, B, and C (speA, speB, and speC) have been identified. The pyrogenic exotoxins are also called erythrogenic toxins and cause the erythematous rash of Scarlet Fever.[2] The strains of Group A Streptococcus that cause Scarlet Fever need specific bacteriophages in order for there to be pyrogenic exotoxin production. Specifically, Bacteriophage T12 is responsible for the production of speA.[12]
Streptococcal Pyrogenic Exotoxin A, speA, is the one that is most commonly associated with cases of Scarlet Fever that are complicated by the immune mediated sequelae Acute Rheumatic Fever and Poststreptococcal Glomerulonephritis.[6]
These toxins are also known as “superantigens” because they are able to cause an extensive immune response within the body through activation of some of the main cells responsible for the persons immune system.[11] The body responds to these toxins by making antibodies to those specific toxins. However, those antibodies do not completely protect the person from future Group A Streptococcal infections since there are 12 different pyrogenic exotoxins possible.[2]
Although the classic presentation of Scarlet Fever can be clinically diagnosed, a large amount of variation and severity of presentation exist so further testing is required to distinguish it from other illnesses.[7] Also, history of a recent exposure to someone with a Group A Strep Pharyngitis can aid in a clinical diagnosis.[2] There are two microbiologic tests used to confirm clinical suspicion of scarlet fever.[3]
- Rapid Antigen Detection Test
- Throat Culture
The Rapid Antigen Detection Test is a very specific test but not very sensitive. This means that if the result is positive (indicating that the Group A Strep Antigen was detected and therefore confirming that the patient has a Group A Strep Pharyngitis) then it is appropriate to treat them with antibiotics. However, if the Rapid Antigen Detection Test is negative (indicating that they do not have Group A Strep Pharyngitis), then a throat culture is required to confirm since it could be a false negative result.[13] The throat culture is the current gold standard for diagnosis.[3]
Choosing whether or not to test a patient for Strep Throat when they present with a sore throat is a common choice faced in many primary care doctor's offices and emergency rooms. A list of criteria, called the Centor criteria were created to make this decision easier and to reduce the amount of unnecessary testing done.
Serologic testing looks for the antibodies that the body produces against the streptococcal infection including antistreptolysin-O and antideoxyribonuclease B. It takes the body 2-3 weeks to make these antibodies so this type of testing is not useful for diagnosing a current infection. However, it is useful when assessing a patient who may have one of the complications from a previous streptococcal infection.[1][3]
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