Nettle Stings: How and Why They Happen

What are nettles?

Nettles, also known as stinging nettles, are herbaceous plants that have needle-like hairs on their leaves and stem. These hairs contain a mixture of chemicals that cause a stinging, burning sensation when they come into contact with the skin. This reaction, called urticaria, can result in symptoms such as itching, redness, and swelling.

The nettle plant belongs to the Urticaceae family and there are over 30 different species. The most widely known nettle is the ‘stinging or common nettle’ (Urtica dioica L.), which is native to Europe, temperate Asia, North America and North Africa. Due to its hardy nature and ability to adapt to various environments, it is now almost worldwide. In Australia, New Zealand and South America, the stinging nettle is considered invasive.

There are over 30 species of nettle which are members of the Urticaceae family. The most widely known nettle is known as the common nettle or stinging nettle (Urtica dioica L.) which is native to Europe, parts of Asia and North America but is now found worldwide.

The name ‘nettle‘ is derived from the Anglo-Saxon word ‘netel‘, which means ‘to sting‘.

Mechanism of action

The stinging nettle is covered in fine hollow hairs (known as trichomes or spicules) approximately 1 mm long on the surface of the leaves and stems. Inside the trichomes are neurotransmitters histamine, acetylcholine, serotonin and formic acid. When the stinging nettle is disturbed, the silica tip of the trichome breaks and the toxins are injected into the skin causing contact urticaria. The discomfort caused by stinging nettles stems from a combination of mechanical irritation, as the trichomes penetrate the skin, and chemical irritation due to the toxins released.

Acetylcholine

Acetylcholine is a neurotransmitter that activates nicotinic acetylcholine receptors (nAChRs). When acetylcholine interacts with nAChRs on sensory nerve endings in the skin, it triggers the release of histamine, which can hives and itching. Acetylcholine can also increase sensitivity to pain and inflammation in some cases, by activating nociceptors (pain receptors) and promoting the release of pro-inflammatory cytokines.

Histamine

Histamine is a neurotransmitter produced by a number of living organisms that is involved in a number of physiological processes. When histamine pierces the skin, it binds to H1 receptors on nerve endings, it activates a signalling pathway involving the protein phospholipase C (PLC). Inositol triphosphate (IP3) and diacylglycerol (DAG) are secondary messengers that are released, which cause an influx of calcium ions into the cells triggering the release of neuropeptides, such as substance P and calcitonin gene-related peptide (CGRP), which activate sensory nerve fibres, transmitting the sensation of itchiness to the brain. Histamine also causes vasodilation, increased blood flow and inflammation to the affected area, which further contributes to the sensation of itchiness.

Serotonin

Serotonin is a neurotransmitter that is involved in the transmission of signals in the nervous system, and it plays a role in inflammation and pain sensation. When released from the trichome, it binds to receptors on sensory nerve fibres, causing them to fire, which sends signals to the brain, that are interpreted as itching, pain and burning.

Formic acid

Formic acid is a small and highly reactive molecule found in the venom of many insects such as bees and ants, as well as one of the chemicals in the trichomes of stinging nettles. It interacts with nociceptors (pain receptors) located in the skin, triggering the sensation of stinging and burning.

Formic acid also causes inflammation in the skin, leading to redness, swelling and itching.

Understanding the biological reasons why nettles sting

Plants have adapted a number of ways to protect themselves against herbivory. Thorns, spikes and prickles, toxins, tough, waxy or hairy leaves, strong odours, rapid growth and mimicry. The purpose of the stinging sensation is to deter animals and insects from eating the nettle.

Nettles fall into the toxic category. Toxins produced by plants as a defence can range from mild to deadly. While extremely uncomfortable, most cases of nettle exposure are unpleasant, but not serious. However, some people may experience a severe allergic reaction to nettle stings and experience swelling of the face and throat, difficulty breathing, rapid heartbeat and low blood pressure. One person I spoke to recently described being pushed into a crop of nettles as a teenager, experiencing widespread stings which required hospitalisation.

What does a nettle sting feel like?

I would describe the feeling as a sharp and burning sensation along with intense itching. As an active bushwalker and gardener living in Australia, I’m quite often bitten or scratched by various insects and plants, but nothing has stung as much as I experienced when I accidentally brushed past some nettles during a visit to England a few years ago. While the discomfort didn’t last for more than an hour or two, it was certainly unpleasant.

How to treat a nettle sting

Dock leaves (Rumex spp.) are a common remedy for relieving symptoms associated with stinging nettles. The exact mechanism of action is unknown but may be due to the mucilage and tannins in the leaves that may help to neutralise the irritant compound from nettles. It is also possible that they offer a placebo effect.

Wash the affected area: Use soap and water to wash the affected area and remove any remaining stinging hairs. There are some suggestions that you should wait for ten minutes to allow the chemicals to dry, which will make removing them easier. However, others claim that they should be removed immediately to reduce skin irritation and remove any remaining trichomes.
Apply a cold compress: A cold compress can reduce swelling and relieve pain and itching.
Topical creams: Over-the-counter corticosteroids or antihistamines can reduce inflammation and relieve itching.
Apply a paste of baking soda and water: Mix baking soda with water to make a paste and apply it to the affected area. This can help neutralise the chemicals and relieve symptoms.
Take over-the-counter pain relievers such as ibuprofen or paracetamol to relieve discomfort.

How long do the symptoms last?

Untreated, the symptoms last between 2 – 24 hours. Treating the affected area can reduce the duration of symptoms.

Do nettles have venom?

The jury seems to be out as to whether nettles are venomous or not. Generally, venom is associated with animals such as snakes and spiders which inject their venomous toxin using specialised stingers or barbs. However, the nettle also injects its toxic substance, so is often classed as venomous.

Why do nettles not sting when you eat them?

When nettles are cooked the heat destroys the trichomes along with the chemicals they contain, making nettles to handle and eat. The heat from cooking causes the trichomes to burst and the chemicals are denatured and broken down. This process is known as hydrolysis. Soaking nettles in water before cooking can also remove some of the chemicals and reduce the stinging sensation.

Nettle uses

It’s not all bad news when it comes to nettles, they have a long history of culinary and medicinal uses, which include:

Antihemorrhagic agent: Nettles have been used to control menstrual bleeding and nosebleeds.
Arthritis: The anti-inflammatory properties such as flavonoids, lectins, phenolic acids and triterpenoids may assist with pain and inflammation associated with arthritis. Fresh stinging nettles are applied to the joint in a process known as urtification.
Urinary tract infections: Nettles are believed to have diuretic properties which can assist to flush out the urinary system and prevent urinary tract infections.
Skin conditions: The anti-inflammatory and astringent properties of nettles can be used to treat eczema, psoriasis and acne.
Allergies: Nettles contain antihistamine compounds which include quercetin, kaempferol and lectins that can help to reduce allergic reactions.
Benign prostatic hyperplasia (prostate enlargement): One study¹ involving 287 patients found that the phytosterols, lignans and polysaccharides in nettles were able significantly reduce the International Prostate Symptom Score (IPSS) score, serum PSA and prostate size.

Nettles have been used as a food source for centuries and are a popular ingredient in many traditional dishes, particularly in Europe. Culinary uses of nettle include nettle soup, pesto, tea and smoothies. Nettles are a great source of A, B, C, and K, proteins, iron, calcium, magnesium, linoleic acid, linolenic acid, palmitic acid, stearic acid and oleic acid, amino acid and antioxidants.

The tough fibres in the stalks of nettles have been used for thousands of years to create strong and durable textiles, such as linen and cordage. Nettle fibre is durable and has a unique texture. In recent years there has been a growing interest in the use of nettles as a sustainable and eco-friendly alternative to traditional textiles.

Wear protective clothing when harvesting nettles, even when cut down, they can still sting.

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