Rashes are a common symptom in babies. Most rashes in babies are benign and will go away on their own or with symptomatic treatment with over-the-counter medicines or home remedies. A rash on a baby's leg can indicate an infection, an inflammatory response or a sign of a serious condition.
According to the Merck Manuals, viral illnesses are common causes of rashes in babies. For example, parvovirus, the cause of fifth disease, causes an itchy, red, blotchy rash on the leg, as well as the characteristic slapped-cheek redness on the baby's face. Rashes caused by viruses are known as viral exanthems. Bacterial infections of the skin, or cellulitis, can cause an area of redness on the leg. Ringworm, a fungal skin infection, can appear as a round, scaly patch of skin.
The description and appearance of the rash can help determine the cause of the skin eruption. Flat, red, discrete spots, called macules, usually accompany viral infections. Fluid-filled vesicles are consistent with chickenpox. Urticaria, also known as hives, is a raised patch of redness with clearly defined borders that is consistent with the inflammation of an allergic reaction.
Accompanying symptoms can also help identify the rash. Viruses cause fever, upper-respiratory symptoms of runny nose and cough, ear infections, conjunctivitis or pink eye, and vomiting and diarrhea. Allergic reactions usually cause hives, itching, and, in serious cases, difficulty breathing with swelling of the face, neck and around the eyes. Ringworm can be asymptomatic and may only be noticed on close inspection.
The treatment of a rash on a baby's leg is symptomatic. Viral rashes go away after a short period of time. Bacterial rashes may require oral or topical antibiotics. The treatment for ringworm involves several weeks of topical antifungal ointments. For urticaria that is limited to the leg, a topical antihistamine or steroid may help decrease the swelling, but often, the allergic reaction needs treatment with oral antihistamines or steroids.
There are several rashes on babies that need prompt medical attention. If the rash is a pinpoint violet-colored eruption or a diffuse area of purple-colored rash, like a bruise, the baby may be at risk for meningococcemia, a serious bacterial infection. This rash indicates a problem with the platelets, the cells in charge of clotting off blood. Other possible causes of the rash include leukemia and other conditions that affect the bone marrow. According to the National Heart, Lung and Blood Institute, another cause is idiopathic thrombocytopenic purpura, a common condition in children in which the body temporarily destroys its own platelets.