Urticaria (also known as hives, welts or nettle rash) is a raised, red, itchy rash that appears on the skin. The raised marks in the rash are called weals.
Each weal is short-lived, but the rash can move about. The rash normally disappears after a few days, but can sometimes last longer.
Urticaria happens when a trigger (see below) causes a protein called histamine to be released in the skin. Histamine causes redness, swelling and itching.
When urticaria lasts less than six weeks it is described as acute. If it occurs daily, or more or less daily, for six weeks it is called chronic urticaria. The causes for acute and chronic urticaria are different.
Triggers
Acute urticaria can be triggered by many things, including allergens (such as food or latex), irritants (such as nettles), medicines or physical factors, such as exercise or heat (see Uticaria - causes for a full list).
However, in around half of acute urticaria cases, no cause can be identified.
In most cases of chronic urticaria, the cause is not identified. Unlike acute urticaria, external triggers are not responsible. Chronic urticaria may be linked to other diseases, including autoimmune problems (where the immune system attacks its own tissues).
Who is affected
Acute urticaria is a common condition, and it affects one in five people at some point in their life. It is more likely to affect children.
Chronic urticaria is much rarer, and affects one in every 1,000 people.
Outlook
The symptoms of urticaria are usually short-lived and mild. They can be controlled with antihistamines or, if these do not work, corticosteroid tablets.
Urticaria can be one of the first symptoms of a severe allergic reaction known as anaphylaxis (see Uticaria - complications for more information). Anaphylaxis can cause difficulty breathing, a feeling that you are about to faint, and sometimes loss of consciousness. It is a life-threatening condition and needs emergency treatment.