Papular urticaria

What is papular urticaria?

Papular urticaria is a common reaction to insect bites. It is more common in children than in adults.

Crops of very itchy red bumps, 0.2 - 2 cm in diameter, appear every few days during the summer or autumn months. Sometimes each spot develops a fluid-filled blister up to one centimetre in diameter. They are most often on the legs and other uncovered areas such as forearms and face but sometimes they are scattered in small groups all over the body.

It is difficult not to scratch the spots, which become crusted and may get infected - they are then pussy and sore. Sometimes one new spot provokes all the old ones to come up again and itch intensely.

The spots seem to remain for a few days to a few weeks and can leave persistent marks or scars, especially if they have been scratched deeply.

Papular urticaria may clear up on holiday or on moving house. One or several members of the family may be affected. Occasionally the eruption can clear up for years and then recur unexpectedly. It is not associated with any internal complaint and is never a serious disease.

Papular urticaria

What is the cause of papular urticaria?

Papular urticaria is thought to be an allergic reaction to insects in the environment. Often after a few years the person becomes desensitized to these insects and the reaction dies down. A bite isn't usually noticed and it is thought that the reaction can occur simply from skin contact with parts of the insect such as its faeces and eggs - this accounts for spots in unexpected places. The most common identified causes are insects that live on cats and dogs, such as fleas and mites.

Fleas are easily seen with the naked eye but can be difficult to get rid of. Mites are too small to see but are equally common. The animal gets repeatedly infested and has to be treated with flea powder or a leave-on preparation such as fipronil every few weeks. Unfortunately flea collars are not very effective. Fleas produce many eggs, which become larvae and pupae. The average cat has only twenty fleas, but is surrounded by 20,000!

Not everyone with papular urticaria has pets, and it can be nearly impossible to work out what a patient is reacting to. There have been reports of allergy to bird mites, carpet beetles, caterpillars and insects that live in masonry disturbed by renovation.

A similar disorder, called ‘chronic papular urticaria’ or ‘prurigo simplex’ in adults is not due to insects; the cause is unknown. It may be a variant of atopic dermatitis.

Treatment

• Topical steroid cream should be applied as soon as the itchy spots appear.
• Antihistamine tablets at night may be useful for severe itching.
• Apply antiseptic cream if the spots get infected.
• Clear the house, and if possible work place or school, of insects.
• Use a kennel and carpet spray containing a pyrethroid, followed by vacuuming. Apply a long lasting insect growth regulator in November each year.
• Keep pets outside.
• Wear fully covering clothing.
• Apply insect repellents to exposed skin to prevent insect bites outdoors.