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Nettle rash (hives)

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Nettle rash (hives)

Reviewed by Dr John Pillinger, GP

What is nettle rash?

Nettle rash (also known as urticaria) is composed of reddish itchy weals or swellings in the skin similar to those resulting from contact with stinging nettle. Urtica is the Latin word for nettle.

The rash may vary in shape and size, can be very itchy and usually goes away within 24 hours. Nettle rash may occur repeatedly or as a single episode.

What causes nettle rash?

In about 90 per cent of cases there is no apparent cause. Sometimes, however, it is possible to identify the cause of the nettle rash and it appears to be most commonly due to an allergic reaction. Any of the following may trigger symptoms:

a particular kind of food that causes an allergy. Eggs, nuts, strawberries and seafood are some of the common culprits.

contact with pollen or fungal spores.

medicines, most commonly, non-steroidal anti-inflammatory drugs (NSAIDs), antibiotics, and ACE-inhibitors.

insect bites.

infections, which may be caused by viruses, parasites or bacteria.

contact with plants such as stinging nettles.

contact with animals or creatures, such as stinging jellyfish.

physical stimuli, such as feeling cold or warm, sunlight, water, sweating or pressure.

substances, such as perfumes, preservatives, colourings, nickel or tar.

During an allergic reaction, a chemical called histamine is released by cells in the skin. Histamine makes the blood vessels dilate and become more permeable (leaky) so that fluid seeps out into the skin.

The rash itself is due to a local build up of an excessive amount of fluid in the skin (oedema). In severe cases this may be followed by swelling in other parts of the body as well.

If swelling occurs around the eyes, lips, tongue, larynx or hands it is called angioneurotic oedema.

What are the symptoms of nettle rash?

Nettle rash usually starts out as an itchy spot on the skin.

After a few minutes the weals develop into areas of reddish blisters of varying sizes.

The rash usually lasts for about two to three hours and can then disappear only to return in a different place.

How do you differentiate between the different types of nettle rash?

Nettle rash is differentiated according to the cause of the rash and whether it has appeared suddenly or represents a chronic condition, such as attacks that last for more than a month.

Acute nettle rash In cases of acute nettle rash, the attack begins within an hour of exposure to the allergen so it is often possible to work out what has caused it. Symptoms will appear every time the individual is exposed to that particular allergen.

There is usually no need to call the doctor for acute cases of nettle rash. The patient is able to find the cause of the rash and follow it up with home treatment. In more severe cases, however, it may be necessary to consult a doctor.

Children and young adults are particularly prone to repeated attacks of nettle rash.

Acute nettle rash can be divided into allergic and non-allergic (physical) nettle rash. Both types present immunological reactions.

Chronic nettle rash

Chronic cases are, by definition, more difficult to treat and often need long-term treatment. With chronic nettle rash, the rash may last for several weeks and it is advisable to contact your GP and if appropriate, get a referral to a skin specialist (dermatologist).

In about 90 per cent of cases, no cause can be found for the symptoms. This kind of nettle rash is called idiopathic urticaria. In the other 10 per cent of cases the substance (such as nickel or perfume) causing the allergy can be identified.

In extremely rare cases, urticaria may be related to a more serious underlying medical condition.

If a thorough examination shows that there is no apparent trigger for the chronic nettle rash, do not be alarmed.

Chronic nettle rash can be treated and in about half of all cases the rash disappears by itself within six months. However, sometimes the condition may be resistant to treatment and persist for years.

Hereditary angioneurotic oedema

This is an inherited condition that can cause nettle rash and swelling around the eyes, mouth and hands. It requires treatment by a specialist.

How does the doctor make a diagnosis?

Often it is enough to tell the doctor when, where and how you got the symptoms. Skin tests, a thorough examination and blood samples can be used to confirm the diagnosis.

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