Pityriasis rosea or “PR”, is common enough that you or someone you know has probably had it. It is a rash that covers most of the body with scaly oval shaped patches. Often a patient’s back will show a “Christmas tree pattern” in which the oval lesions on their back are lined up in a pattern that reminds one of a Christmas tree. Patients often come in thinking they have ringworm because an individual lesion of PR is round and scaly much like ringworm, but PR is not an infection, as far as we know.
Or is it an infection?
The truth is that doctors have long suspected that PR is caused by a virus, but so far no virus has been identified. We suspect a virus for two reasons: 1 – The rash looks “viral”. When somebody has a florid case of PR their whole back and trunk is covered (similar to other viruses like the measles or chicken pox). 2 – We tend to see cases of PR in clusters. I will sometimes see ten cases in two weeks and then nothing for months. The strange thing, though, is that family members and household contacts of these patients do not seem to “catch” pityriasis rosea.
The recent history of medicine is littered with examples of other diseases that existed for decades before an infectious cause was discovered. A few examples come to mind: cervical cancer was determined to be caused by human papilloma virus, many stomach ulcers are now thought to be caused by a bacteria called helicobacter pylori, and it was confirmed several years ago that a skin disease called Kaposi’s Sarcoma is caused by human herpesvirus- 8. But for now, pityriasis rosea is still considered to be an “inflammatory” skin rash and not an “infection”.
Pityriasis rosea tends to last 4 to 6 weeks in most patients. It is not dangerous. It resolves without treatment. We will often prescribe a cortisone lotion to help with itching if there is any, but it does not affect the duration of the rash. PR goes away when it wants to. However, a little sunlight (10 to 15 minutes a day) can sometimes help to clear it up faster. Sometimes we will do a blood test for syphilis because it can look very similar to PR. As a dermatologist, we love to treat pityriasis rosea because patients come in thinking they have something like “the plague”, and we can take one look at this fairly distinct rash and assure them that they have nothing to worry about.