Papular urticaria

What is papular urticaria?

Papular urticaria is a common reaction to insect and arachnid bites. It is more common in children than in adults [1]. It presents during the summer or autumn months as crops of very itchy red papules and vesicles.

Despite the name, papular urticaria is not true urticaria, which is characterised by temporary wealing that resolves within hours. Weals in papular urticaria last for days to weeks. Papular urticaria is not associated with an internal complaint.

It is also called a persistent insect bite reaction.

Papular urticaria

Papular urticaria

Papular urticaria

Who gets papular urticaria?

Papular urticaria most often occurs in children. This is because desensitisation to insect bites has not yet developed [1].

It may also occur in adults, especially in travellers to new environments. 

What is the cause of papular urticaria?

Papular urticaria is thought to be an immunological reaction to insect bites. The reaction dies down after a few months or years, as the person becomes desensitised to them. The initial bite is rarely noticed.

Fleas and mites that live on cats and dogs are most often responsible [4].

• Fleas are easily seen with the naked eye but can be difficult to get rid of. Fleas produce many eggs, which become larvae and pupae. The average cat has only twenty fleas, but may be surrounded by 20,000 of them.
• Mites are too small to see but are an equally common cause of papular urticaria.
• Animals get repeatedly infested and must be treated every few weeks with a leave-on insecticide.  

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 mosquitoes, gnats, bird mites, carpet beetles, caterpillars and other insects [1].

A similar disorder, prurigo simplex, is sometimes called ‘chronic papular urticaria’ and affects adults and children. The cause is unknown. It may be a variant of atopic dermatitis.

What are the clinical features of papular urticaria?

Papular urticaria presents with very itchy clusters of red bumps.

• Most often they arise on legs and other uncovered areas such as forearms and face.
• Sometimes papules are scattered in small groups all over the body [3].
• They appear every few days during the summer or autumn months.
• They range from 0.2–2 cm in diameter.
• Each papule has a central punctum.
• Papular urticaria may present as crops of fluid-filled blisters.
• New lesions develop just as old ones start to clear.

Scratching papular urticaria causes the spots to become crusted.

• Secondary bacterial infection causes painful pustules and scabs (impetiginisation).
• Sometimes one new spot provokes old ones to come up again and itch intensely [3].

The spots remain for a few days to a few weeks and can leave persistent hyperpigmented marks (postinflammatory pigmentation) or hypopigmented scars, especially if they have been scratched deeply.

What are the complications of papular urticaria?

Papular urticaria is not dangerous. 

Infected insect bites can lead to cellulitis and rarely, to bacteraemia (sepsis). Localised cutaneous vasculitis may also occur.

What is the differential diagnosis of papular urticaria?

The differential diagnosis of papular urticaria includes:

• Scabies
• Atopic eczema
• Other forms of prurigo
• Id reaction
• Pityriasis lichenoides
• Lymphomatoid papulosis.

How is papular urticaria diagnosed?

Papular urticaria is usually a clinical diagnosis. A biopsy may support the diagnosis, as insect bites have a characteristic microscopic appearance.

The histopathology of papular urticaria includes mild dermal oedema, extravasation of erythrocytes, interstitial eosinophils, and exocytosis of lymphocytes. Vasculitic features may be noted.

What is the treatment for papular urticaria?

• Topical steroid cream – this should be applied as soon as the itchy spots appear.
• Antihistamine tablets – these may reduce the size and severity of the spots and reduce itching.
• Antiseptic cream can reduce or avoid secondary infection.
• Wear fully covering clothing.
• Insect repellents can be applied to exposed skin to prevent insect bites when outdoors.
• Insecticides can rid the house, workplace or school, of insects. Obtain professional help from a pest control company if necessary.

Seek veterinary advice regarding management of the infested animal(s).

• Keep pets outside.
• Use a pyrethroid kennel and carpet spray – this should be followed by vacuuming.
• Apply long-lasting insect growth regulator to the neck of cats and dogs.

What is the outcome of papular urticaria?

Papular urticaria is normally self-limiting. The immunological basis of this reaction means that it may take months or even years for children to become desensitised to the offending insect [2]. Papular urticaria may clear up on holiday or on moving house.

Occasionally the eruption can clear for years and then recur unexpectedly.