Acute hives

Hives form when immune cells in your skin (called mast cells) release chemicals including histamine. This causes your blood vessels to open up and leak a little fluid under your skin. Histamine release can be triggered by lots of things and the cause isn't always clear. 

• Infection from a virus is the most common cause of hives in children, especially if they last for more than 24 hours.
• Direct contact with plants or animals may cause hives in just one area of your body. This includes latex from rubber plants which is used in making rubber gloves and party balloons.
• Allergic reactions to food, medicines or insect stings can appear as hives. They usually occur within 1 to 2 hours of exposure and disappear in most cases within 6 to 8 hours.
• Hives can be caused by physical triggers, including cold (eg, cold air, water or ice), heat, sunlight (solar), vibration, rubbing or scratching your skin (dermatographism) and delayed pressure (eg, after carrying heavy bags).
• Rarely, exercise or sweating may cause hives.
• Stress rarely causes hives but may make your symptoms worse.

Identifying the cause of hives is tricky. An allergic reaction could be the cause if there are patterns to when they appear, eg: 

• always within 2 hours of a meal
• always when exercising
• other symptoms occur around the same time (eg, stomach pain, vomiting, difficulty breathing or dizziness).

Ongoing hives lasting days at a time are almost never due to allergy, with the exception of some cases of allergy to medicines.

The key symptoms of hives are weals (raised, itchy swellings) that:

• are round or form rings, a map-like pattern or giant patches
• may change shape
• are a few millimetres or several centimetres in diameter
• coloured white or red
• with or without a red flare
• may last a few minutes or several hours
• can affect any part of your body
• may be widespread across your skin.
  
  

Image credit: Healthify He Puna Waiora

For most cases of acute hives, you don't need to see a healthcare provider.

• Most people with acute hives don't need tests, unless they go on for a long time or you have unusual symptoms around the same time.
• If the suspected cause is allergy, skin or blood tests may be done.

If the reaction is mild, simple measures, eg, a cool bath or shower may be all that’s needed. You can lay a cool flannel over the itchy skin.

• If there's a specific trigger, avoiding these foods or substances will stop more hives coming up and will reduce the chances of them coming back again.
• Since the skin reaction is caused by histamine release, most people benefit from taking antihistamines, eg, cetirizine (best) or loratadine to ease the itch. They can be bought from a pharmacy or prescribed by a doctor or nurse practitioner. Please note these medicines may cause drowsiness, so take care when driving or operating machinery. 
• Aspirin, ibuprofen and other NSAIDs should be avoided as they often make symptoms worse.
• Avoid anything that can make hives worse, eg, tight clothes, heat, spicy foods or alcohol. 
• Special diets aren't useful for managing hives.

If you have taken antihistamines and your hives are still severely itchy make an appointment to see your healthcare provider to discuss a higher dose or extra medicines and to make sure that there is not another cause for your rash.