There’s an old medical proverb that new doctors get reminded of when they’ve been watching too much House, MD: “if you see hoofprints, think horses, not zebras.” In other words, when you see a group of symptoms that could be something wild and exotic – a “zebra” – but could equally be something fairly tame and familiar – a “horse” – it’s usually going to be the less exciting option. But the key word there is “usually.” Sometimes, it really is a zebra. And sometimes, it’s a caterpillar.

A case report published in BMJ Case Reports this week follows a patient with what was assumed to be a patch of eczema on his wrist. But after months of unsuccessfully treating the problem with topical steroid cream and oral antihistamines, a closer look revealed the true culprit: hairs from a spongy moth caterpillar, which had become embedded in his skin.

He was diagnosed with eczema five months prior, after complaining of redness and sharp pain on his left wrist. When these symptoms didn't improve following treatment, he was sent to a new doctor and given a pathological examination.

After coming to the Dermatology department in the First Affiliated Hospital of Fujian Medical University, in China, the patient received a biopsy of the affected part of his wrist.

To the doctors' surprise, the results revealed "unique structures" in the deepest layers of the patient's skin. These “structures”, embedded in and underneath the lower layers of the skin, had defined walls with a hollow interior and short barbules, they explain, and they were surrounded by white blood cells – a sure sign that they were an unwelcome intruder in the patient’s body.

After reviewing the medical literature, the doctors came up with a hypothesis: these structures were the setae of a caterpillar – those hair-like bristles you sometimes see covering the bodies of the creepy crawlies.

This prompted the patient to remember his encounter with the insect. Upon taking a detailed patient history, the doctors learned that the patient had climbed a wax apple tree five months earlier, on which there were many spongy moth caterpillars (spongy moths were previously known as “gypsy moths”, but had their common name changed last year to remove the ethnic slur.)

Spongy moth caterpillars are well-known to cause irritation to those who touch their setae, but luckily, it’s rarely serious. Those who find themselves on the sharp end of a caterpillar prickle “can take an antihistamine for the itching,” advised Dr Abigail Waldman, a dermatologist at Brigham and Women's Hospital in Boston, in Country Living. “Or for severe cases, you can get a topical steroid from your doctor.”

Evidently, the patient in the case report needed a little extra help – the setae had caused a build-up of white blood cells to form around them, and the doctors prescribed regular injections of steroids directly into the lesion over a period of another five months. Soon after, the patient recovered.

Since caterpillar setae embedded in the skin isn’t something doctors come across every day, the authors of the case report hope that their paper will help future physicians faced with similar apple-tree-climbing patients. And their chief takeaway from the experience? Make sure you get all the facts.

Patients with allergic contact dermatitis should be asked for a detailed medical history, including occupation, and potential environmental exposures, the team say.

The setae of a caterpillar have unique characteristics that are difficult to ascertain and provide information on for practicing dermatologists, they conclude.