Summer is Here!
Part 1.
By Marie Leger, MD PhD
I used to host a dermatology show on Doctor Radio. The show was kind of like NPR’s Car Talk, where people called in to act out their cars’ odd noises and quirks for the hosts, the Magliozzi brothers, who would then try to diagnose their mechanical problems. Our show was the skin version of that. One of our most popular call-in segments were seasonal rashes—listeners tried to describe what was going on with their skin and my guests (often including Dr. Levin!) and I would try to help and launch into all kinds of tangential discussions. In honor of those fabulous shows, here are a few of our “favorite” summer rashes. (Not to make light, but dermatologists are skin nerds and find things “fun” and “interesting” that sometimes make our friends and relatives squirm. Sorry in advance!)
Phytophotodermatitis. Diagnosing this rash makes our patients think we are really smart. It comes on suddenly in the summer, often after someone comes back from vacation, and looks like odd geographic patterns on the hands, arms, front of the legs, or around the mouth. When it initially pops up, it may look red and even blister, but as it fades it turns a characteristic well-demarcated brown. This rash occurs when a plant containing furocoumarins (think limes, celery, figs, and some grasses) touches the skin, is exposed to sunlight, and then damages the skin through what is called a “phototoxic” reaction. A friend’s nine-year-old daughter got this while eating celery on the beach. I have had patients with a handprint shaped brown spot on their thigh after wiping off lime juice after making margaritas. I had a friend get this on her mouth after drinking a corona with lime while on vacation a few days before a job interview. Sunscreen and often a prescription anti-inflammatory can help it go away faster—but it can take some time to disappear completely. I am going to try to summarize all of this in a limerick.
There once was a traveling student
Spent time at the beach—wasn’t prudent.
A lime in her beer
Made a weird rash appear--
Saw her doc when she had no improvement.
Tada!
Arthropod assault. Sometime usually in June or July, dermatologists start seeing a lot of patients with welts, usually on their legs and arms, that come and go and itch like crazy. The billable medical code for this is “T63.483A,” in case you were wondering, which stands for “the toxic effect of venom or other arthropod assault.” I have always found this description to be particularly apt, in fact I can’t even get a single mosquito bite anymore without thinking about it as an assault. No one ever believes that their bug bites are bug bites. Understandable, because they can be pretty dramatic and look weird, even out of the blue. Sometimes if they are particularly odd looking, we may order some labwork as exuberant bite reactions have been reported in association with malignancy. Super rare—don’t lose sleep over this, please. A dermatologist can differentiate bug bites from other things that look like them (urticaria, dermal hypersensitivity reactions, drug reactions, etc etc) and also prescribe medications to help with itch and potential pigmentary change that may linger after the bite itself has resolved.
Rhus dermatitis. Also in the summer, we start to see people with a lot of itchy, sometimes blistering rashes often on the lower legs in odd, sometimes linear shapes. Dermatologists will sometimes say that these kinds of rashes look like an “outside job,” meaning that something outside—not intrinsic to your own body—caused it. The most usual culprit for this kind of rash is a member of the rhus family, which includes poison oak, poison ivy, and poison sumac. These plants contain an oily ingredient in their stems and leaves called “urushiol” that is highly allergenic. After repeated exposures, many people develop an allergy to urushiol. Once sensitized, your next encounter with the plant will often yield a very itchy rash, which will usually pop up 24-48 hours after touching it. The oil can linger on shoes, clothing, pets, towels—so it’s important to wash all these things carefully if you know you have been exposed. We treat these rashes depending on their severity—often we use prescription topical or oral steroids, if there are any infections that have developed we treat those, and we often have tips to keep people comfortable for the few weeks it takes for this to clear up. Animal studies have recently shown that rhus dermatitis is mediated by a signaling molecule called IL-33, which also is involved in asthma and eczema. Brian Kim, a dermatologist (yay!) at Washington University also recently found that mast cells are involved in these rashes. Histamine, however, is not involved—which is why antihistamines are not so helpful! (Other than possibly making you so sleepy that you don’t scratch). Using this new molecular knowledge, there are attempts to develop a poison oak/poison ivy vaccine! It worked in guinea pigs and is now in human trials. Stay tuned.