What is causing this mysterious allergic rash in my child?

Sometimes children break out in unexplained rashes that appear to have been caused by an allergy to something. Allergic rashes certainly vary from person to person, but the classic allergic rash (called urticaria) usually is:

  • Very itchy
  • Pink
  • Puffy (often called hives)
  • Migratory (that is, it seems to come and go, disappearing and reappearing on different parts of the body)

What could have caused this rash?
There are lots of possibilities. Theoretically, kids could be exposed to a thousand different substances in a week – between school meals (a new fruit dessert), playing outside after school (poison ivy or poison oak), sleeping over at Grandma’s house (new detergents on sheets and towels), and making crafts in Sunday School (glues and paints), and so on. The most common causes of allergic (urticarial) rashes include things like:

  • Common foods, like fish and shellfish, nuts, peanuts, eggs, wheat, and soy
  • Meats, like beef, chicken and pork
  • Fruits, like apple, apricots, lemons, limes, strawberry, and watermelon
  • Vegetables like ,asparagus, beans, leafy greens (cabbage, lettuce, and endives), savory vegetables (pickles, onions, garlic, parsnip, parsley), tomatoes, mushrooms
  • Medicines, like aspirin, ibuprofen, antibiotics (like penicillin or sulfa), antifungal medicines (like Lotrimin), and many medications for seizures or behavioral/psychological problems
  • Essential oils in cosmetics, shampoo, fragrances, lotions, soaps
  • Chemicals in detergents and fabric softeners
  • Additives and preservatives like Yellow #5, Yellow #6, Red #3, annato, sodium benzoate, aspartame (NutraSweet)
  • Latex, from gloves, balloons, or other toys
  • Plants and animals, like pollen, poison ivy, and cat or dog dander.
  • Insects.  While bug bites often cause rashes, even just touching some insects can cause rashes.  For example, the white-marked tussock moth caterpillar (Orgyia leucostigma) is a common caterpillar seen in Tennessee in the summer months.  Curious children often touch its brightly colored body.  Many people develop a rash after touching the caterpillar or its cocoon hairs.

Although not allergies per se , we can see allergic type rashes also from:

  • Heat: Hot food, hot objects, hot bath/showers, sunlight, and sweat
  • Pressure: Friction or prolonged pressure, like tight elastic waistbands, belts, bra straps
  • Cold: Ice, cold air or cold water — worse with sudden changes in temperature

Many viruses also cause allergic-type rashes. These are harmless, brief, and require no treatment.

My child may have eaten a few strawberries — but this rash has been going on for days now!

Sometimes the reaction is quite bad. Often the allergy seems out of proportion to the original exposure, either in how much the rash itches, how much of the body it covers, or how long the rash lasts. Until your child’s body completely gets rid of the substance he’s allergic to — and isn’t exposed to it again in the meantime — it may take several days before the rash finally goes away, even with treatment.

Just before my child broke out in this rash, she had some walnuts — but she’s never had a problem with nuts before.

It still could be the nuts. Many times children (and adults) develop allergies to things that they’ve never been sensitive to before. We frequently see kids who, at age 10, suddenly become allergic to an antibiotic that they’ve had several times before in their life without any problems. The same holds true for other things: foods, preservatives, animals, and other things in our environment.

Alternatively, it’s possible that your child might be allergic to something else, and the nuts were just a coincidence!

Will allergy testing help?

Allergy testing is most useful when we have a short “list of suspects.” That is, if we’ve narrowed the likely causes of the allergic rash to, for example, walnuts, strawberries, and cats, we can do simple blood allergy testing for those three specific items.

On the other hand, if we really have no idea what might be causing the rash — it could be any number of, literally, hundreds of things. Running hundreds of tests to eliminate all the possibilities is much more difficult and expensive — and requires a lot more blood!

Most of the time, we don’t need to do allergy testing if the allergic rash disappears with treatment and doesn’t come back. In most cases of mysterious allergic rashes, 80-90% go away with treatment in a few days and never come back. However, if the rash keeps coming back every few weeks or months, or is quite prolonged or severe, it’s probably worth doing some testing.