Why Does My Horse Have Hives or Raised Lumps?

Hives in horses can appear suddenly and look dramatic. Here is what causes them, what else causes lumps, and when to call your vet at once.

Quick Answer

Hives (urticaria, or nettle rash) are raised patches or soft lumps on the skin caused by an allergic reaction, and they usually appear suddenly, often over the neck and body.
Phone your vet immediately if your horse is distressed or has laboured breathing, very swollen eyelids or muzzle, or any sudden or extreme reaction.
Not every raised lump is hives: sarcoids, warts, melanomas and nodular skin disease can look similar, and only a vet can say what a lump is.
Do not give your horse any medicine or treatment for hives without asking your vet first.
Contact your vet if the lumps are itchy, have not improved after 24 hours, or come back.

Hives in horses are raised patches or soft lumps on the skin, caused by the immune system overreacting to something it normally ignores. Vets call this urticaria, and many owners know it as nettle rash.

Most of the time the horse is otherwise well. However, an allergic reaction can sometimes affect the airways, so it helps to know the emergency signs before you need them.

Woman fitting an anti-fly mask on a white horse in a yard – insect bites are one possible trigger for hives in horses

What Do Hives in Horses Look Like?

The lumps usually appear suddenly, often across the neck and body. They can range in size from pea to palm. Hives can also show on the back, flanks, eyelids and legs.

They may start as small bumps that grow and join up into doughy swellings. Some are doughnut-shaped. In severe cases, they can ooze a clear fluid.

Hives may or may not be itchy. A horse that is otherwise well is often unaware of its dramatic appearance. Even so, although serious harm is unlikely, an affected horse can look very uncomfortable.

Common Triggers for Hives in Horses

An allergen is a usually harmless substance that triggers the reaction. Allergies can develop at any point in a horse's life, usually after repeated or continued exposure. Often, the exact cause cannot be pinned down.

Insect bites and stings

Fly bites and stings are a well-known trigger. Horses with insect bite hypersensitivity (sweet itch) can get hives straight after a bite, or itching and crusting a day or two later. See Sweet Itch in Horses for more.

Plants, pollen and nettles

Pollen and contact with nettles can cause hives. A horse that falls or rolls into stinging nettles can also look unwell. It may be lame, weak or wobbly, or keep turning to look at its flanks as if it had colic. This should settle quickly, but if signs persist, contact your vet.

Woman holding the headcollar of a white horse under a corrugated roof – checking the skin regularly helps you spot hives in horses early

Dust, mould and feed

The British Horse Society lists mould and fungi, dust, plants, biting insects, food or supplements, some medications and direct contact as common allergens. Diet changes, often cereal-based feeds, are another possibility. Food allergies are rare but have been reported to cause hives.

Medicines, vaccinations and products

Allergic reactions can also follow medications or vaccination. Insect repellents and shampoos are named as possible triggers too. If lumps appear after any of these, tell your vet.

Physical triggers

Not every case is an allergy. Pressure, heat, temperature extremes, severe exercise, stress and sunlight can all trigger hives. Ringworm, autoimmune disease and inflammation of blood vessels can also cause them.

Recent changes in diet, surroundings or management are often to blame, according to an article by a group of UK equine vet practices.

Other Causes of Raised Lumps

Not all lumps are hives. For example, sarcoids, warts, melanomas and nodular skin disease can all look like lumps. Because skin tumours are so diverse, identifying them should be left to a vet.

Sarcoids

Sarcoids are the most common skin tumour of horses. They can be flat, scaly, wart-like or nodular, and they do not usually clear up. Any persistent lump needs a vet. Read more in Sarcoids in Horses.

Juvenile warts

Warts are caused by a horse-specific papilloma virus. They are typically seen in horses aged about six months to four years, mostly on the muzzle and lips, as grey to white warty lumps. Most resolve on their own within about three months, but the virus spreads by direct and indirect contact and can survive for years in the environment.

Melanomas

Melanomas are dark nodules, most often in grey or white horses. They commonly appear near the tail base and increase over time.

Nodular skin disease

This is described as the most common nodular skin disease in horses. The lumps are round or irregular and not itchy, mainly on the neck, trunk and back. Insect bite reactions are the most likely cause.

What to Check and Note Before You Ring Your Vet

A few simple notes help your vet work out the cause. Keep them brief and factual.

When the lumps appeared, and whether they are spreading or changing.
Whether your horse is itchy, irritated or rubbing.
Any recent change of feed, bedding, paddock, rug or product used on the skin.
Any recent bites or stings, medicines or vaccinations.
Whether the horse has had lumps before.

Do not put antiseptics or disinfectants on sensitive skin without veterinary advice, because they can do more harm than good. Skin problems are also easier to identify before creams and lotions change their appearance.

Emergency: Contact a Vet Immediately If…

Your horse is distressed, or its breathing is laboured, difficult or wheezy.
The eyelids and muzzle are very swollen.
The reaction is sudden or extreme.
There are signs such as severe breathing distress, excessive drooling, very pale gums, cold limbs, a fast weak pulse or seizures. These can mean a rare, life-threatening reaction called anaphylaxis.

Swelling is more serious than hives, and swelling that reaches the airways can be life-threatening. A swollen face is not always present in a severe reaction, so if in doubt about breathing, call your vet or the nearest out-of-hours emergency vet straight away.

When to Contact Your Vet

The lumps have not improved after 24 hours.
Your horse is itchy or irritated by them.
The lumps come with coughing, sneezing or nasal discharge, which can be signs of an allergy affecting the nose and airways.
The lumps keep coming back, because your vet will need to check for a more serious underlying allergy.
You are unsure what a lump is, or it is persisting or growing.
You are worried about an allergic reaction for any other reason.

If in doubt, phone your vet and describe what you see. Please do not give any medicine or treatment before they advise.

How Vets Investigate Hives in Horses

Your vet will start with a detailed history and an examination. A skin biopsy can confirm urticaria but will not identify the allergen.

To pin down an allergy, vets may suggest restriction tests, where possible allergens are removed, or intradermal skin testing. Opinions vary on blood testing. An elimination diet trial can take weeks to months under vet guidance.

Treatment and Reducing the Risk

Treatment depends on the cause, and your vet decides it. For persistent or severe cases, your vet may prescribe anti-inflammatory or immune-calming medicines under veterinary supervision. Allergen-specific immunotherapy also exists, although results take several months.

For a severe reaction, the vet has emergency medicines available. Those are never an owner's job.

Limiting contact with a known allergen is key, and it can reduce the reaction without medical treatment. Where the allergen is unknown, changing the environment, management or diet may allow the problem to settle. Practical steps include:

Using good-quality, low-dust bedding.
Doing stable chores when the horse is not in.
Washing rugs and saddle pads.
Checking feed for mould.
Asking your vet about fly rugs and other ways to reduce bites, if bites are the cause.

Almost every horse must have a passport, and its food-chain declaration decides which medicines may be used. If you compete, keep a record of any veterinary treatment and talk to your vet about it before competing, because competition medication rules apply.

How Pet Med Direct Can Help

Hives with breathing trouble or marked swelling is a situation for your vet first. Please ring them before you do anything else, and do not order or give anything in the meantime.

If your vet later prescribes a medicine for your horse, you can read how pet prescriptions work with us. You can also browse our equine range.

Some pet medicines require a valid veterinary prescription. Pet Med Direct can only supply prescription medicines after the prescription has been checked. Always follow your vet's advice and the directions on the medicine label.

Frequently Asked Questions

Are hives in horses an emergency?

Not always, but they can be. Hives often settle once the trigger has passed, yet they should not be dismissed as harmless. Call your vet immediately if your horse has laboured breathing, is distressed, or has very swollen eyelids or muzzle.

What causes hives in horses?

Allergic reactions to pollen, insect bites, feed, dust, mould, medicines or products on the skin are common causes. Pressure, heat, stress and exercise can also trigger them. Often the cause is never identified.

Is it hives, a wart or a sarcoid?

They can look different, but a lump cannot be reliably identified at home. Hives are soft and doughy, while warts are warty. Only a vet can say what a lump is, so ask your vet about any lump that persists.

Can nettles cause hives?

Yes. A horse that falls or rolls into stinging nettles can show a nettle rash and look unwell. Things should settle quickly, but contact your vet if signs persist.

Can a vaccination or medicine cause hives?

Allergic reactions can be triggered by medications or vaccination. If lumps appear afterwards, tell your vet, and phone straight away if there is any breathing difficulty.

Should I give my horse something for the itch?

No. Ask your vet before giving anything. Treatment depends on the cause, and some products can make skin problems worse or harder for your vet to assess.

Can sweet itch cause hives?

Yes. Horses with insect bite hypersensitivity can show hives after bites, and may also develop itching and crusting later. Our Sweet Itch in Horses guide covers this in detail.

Will the hives come back?

Sources differ. Some horses have a single episode, but others have repeat reactions because the cause can be hard to pin down. If it happens again, your vet should check for a more serious underlying allergy.

Related Equine Health Guides

Sources and Further Reading

This article was researched from the sources listed above. It is general information for horse owners and is not a substitute for advice from your own vet.

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This article is for informational purposes only and does not constitute veterinary advice.
If you are concerned about your horse’s health, please consult a qualified veterinarian.