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 every few weeks. Unfortunately flea collars are not very effective. Fleas produce many eggs, which become larvae and pupae. 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.