Pityriasis rosea is a skin rash which also goes by the name of the Christmas tree rash because it typically occurs on the back in the shape of a Christmas tree.

But despite the name, the rash has nothing to do with Christmas trees. Dr Deborah Lee of Dr Fox Online Pharmacy explained: “It affects 0.5-2 percent of the population. It is most common between the ages of 15 and 30 but can occur at any age including babies and the elderly.

“The cause is not known but as it quite often first appears after an upper respiratory tract infection, it’s thought to be caused by a bacterium such as streptococcus, or a virus, but this has not been proven.

“Herpes viruses type 6, and type-7 have both been implicated. The rash has also been documented after vaccinations for influenza, diphtheria, smallpox, hepatitis B BCG and COVID-19.”

A Christmas tree rash is not to be confused with a Christmas tree allergy. Dr Lee said: “ This means you are allergic to your Christmas tree – either to the sap from a pine tree (which contains rosin), or to the dust and mould on the tree. This causes symptoms including wheezing, shortness of breath and/or a skin rash – which is urticaria (hives). But this rash is different to that of pityriasis rosea.”

What are the symptoms of Christmas tree rash?

A pityriasis rosea usually starts with a large patch known as the herald patch, which can measure anything from 2-10 cm in diameter, on the trunk (chest or abdomen) or back, followed by the onset of a more widely disseminated rash over the next two weeks.

Dr Lee advised: “Characteristically, the rash consists of numerous small – around 1.5 cm in diameter – oval-shaped, reddish, slightly raised patches, with scaling around the edges, on the trunk and the back.

“The patches run along Langer’s lines – skin tension lines that are parallel to the direction of collagen fibres in the skin – and 25 percent of cases are intensely itchy.

“And 20 percent of cases are atypical, meaning there are some unusual features.”

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Can it be serious?

The condition is more common in those who are fit and healthy and in pregnancy.

Dr Lee added: “It’s not a serious condition, but it can result in hypopigmentation of the skin at the site of the rash, which can be especially distressing in those with dark skin.

“If it occurs in the first 15 weeks of pregnancy it is associated with an increased risk of premature delivery and stillbirth.

“There is a wide differential diagnosis including secondary syphilis and cutaneous T-cell lymphoma which are serious, life threatening diseases, so it’s very important to have an accurate diagnosis.”

When should you see a GP?

You should always see your GP if you have any unexplained rash. Or if the rash is failing to clear up. In severe cases you may need a referral to a consultant dermatologist.

How is it treated?

Pityriasis rosea is self limiting – meaning it goes away on its own.

Dr Lee said: “The rash can last for 8-10 weeks. If the skin is dry and scaly, and itchy, emollients and antihistamines may be used. Sometimes topical steroids are prescribed. Erythromycin or acyclovir may be offered. Ultraviolet B therapy may be used.

“It’s advised not to sunbathe with pityriasis rosea as this can lead to scarring and hyperpigmentation.”