Allergic or contact dermatitis: The erythematous appearance of the skin will be in a more localized distribution rather than the diffuse and generalized rash seen in scarlet fever. Drug eruption: These are potential side effects of taking certain drugs such as penicillin. The reddened maculopapular rash which results can be itchy and be accompanied by a fever. Kawasaki disease: Children with this disease also present a strawberry tongue and undergo a desquamative process on their palms and soles. However, these children tend to be younger than 5 years old, their fever lasts longer (at least five days), and they have additional clinical criteria (including signs such as conjunctival redness and cracked lips), which can help distinguish this from scarlet fever. Toxic shock syndrome: Both streptococcal and staphylococcal bacteria can cause this syndrome. Clinical manifestations include diffuse rash and desquamation of the palms and soles. It can be distinguished from scarlet fever by low blood pressure, lack of sandpaper texture for the rash, and multi-organ system involvement. Staphylococcal scalded skin syndrome: This is a disease that occurs primarily in young children due to a toxin-producing strain of the bacteria Staphylococcus aureus. The abrupt start of the fever and diffused sunburned appearance of the rash can resemble scarlet fever. However, this rash is associated with tenderness and large blister formation. These blisters easily pop, followed by causing the skin to peel. Staphylococcal scarlet fever: The rash is identical to the streptococcal scarlet fever in distribution and texture, but the skin affected by the rash will be tender.
Pathophysiology
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 which produce a pyrogenic exotoxin, mainly responsible for the skin manifestation of the infection. 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. The strains of group A streptococcus that cause scarlet fever need specific bacteriophages for there to be pyrogenic exotoxin production. Specifically, bacteriophage T12 is responsible for the production of speA. Streptococcal Pyrogenic Exotoxin A, speA, is the one which is most commonly associated with cases of scarlet fever which are complicated by the immune-mediated sequelae acute rheumatic fever and post-streptococcal glomerulonephritis.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. 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, because there are 12 different pyrogenic exotoxins possible. Microbiology
The disease is caused by secretion of pyrogenic exotoxins by the infecting Streptococcus bacteria. Streptococcal pyrogenic exotoxin A (speA) is probably the best studied of these toxins.