The what, where and why of hives

The onset of hives is often unexpected. DRs CIARAN MASTERS & CAITLYN MITTELSTADT shed light on this disturbing ailment.

One of the more common skin conditions we come across in horses is hives. For most horse owners, the appearance of hives is usually a sudden and shocking occurrence. You wake up one morning and your horse is covered in bumps that have seemingly come from nowhere. This can leave even the most experienced owners scratching their heads as to the cause and what to do.

What are hives?

The correct medical term for hives is urticaria; a reaction of the skin characterised by the appearance of a rash or raised, round, itchy welts. These welts can be tiny dots or large bumps, and can be either localised to a specific area or be more generalised. Whilst they are usually a minor affliction, they can cause your horse significant discomfort.

What are the causes?

Hives have many causes and pinpointing the exact causative factor can be like trying to find a needle in a haystack – and just as itchy!

Many different disease processes either immunological or non-immunological can cause hives, but in the paddock, they are commonly the sign of an allergic reaction. They can appear quite dramatically on a horse’s skin with no obvious cause after the immune system has responded to an external foreign stimulus by producing an inflammatory chemical in the body called histamine. Hives often resolve within a day, but ongoing exposure to the stimuli can cause them to reoccur and worsen over time, progressing into seasonal flare-ups or chronic reactions as the horse’s immune response becomes overstimulated.

A common immunological cause of hives in horses is from the bites of flying insects, specifically from proteins in the saliva of midges, mosquitoes and biting flies, which can start a dermatitis (an insect bite hypersensitivity reaction of the skin) sometimes termed Queensland Itch or Sweet Itch. Affected areas can include the neck, face, ears, torso and legs, and can result in hair loss of the mane and tail.

Non-immunological causes, such as chemical or toxic plant exposures, act as skin irritants and may present as hives. Culprits can include some soap products; oil of turpentine and petroleum-based products like crude oil or phenol (carbolic acid); plants such as ragweed, parthenium, stinging trees, and high pollen producers; as well as environmental extremes including heat, cold, exercise or stress, all of which can elicit a hive response.

Whether it be insect bites or stings, medications, chemicals, dust, pollen, mould or other environmental factors, they can all cause a reaction and present as hives. Even certain foods can trigger a reaction if the horse is allergic.

Infections are another common cause. Skin infections whether they are of viral, bacterial or fungal origin will all cause a reaction and can present as hives, although the condition is usually precluded by other signs. Similarly to humans, in some less common cases stress and its associated adrenaline release can cause hives. Changes in routine, travelling or intense training are all known stressors. Of course, in some cases it may be down to genetics, and your poor horse may just be more prone to developing hives than other horses.

What are the symptoms?

The most obvious signs are the noticeable and distinctive lumps (weals). Commonly affected areas are the neck, shoulders and sides but these lumps can be found anywhere on the horse. The weals can be pruritic (itchy) and so your horse is likely to scratch or rub the affected areas. When you’re itchy, irritated and uncomfortable, staying still can be a challenge, and it’s the same with horses – the itchiness is often accompanied by a general restlessness and agitation.

Although hives often self-resolve, in more severe reactions there may be a generalised swelling of areas, particularly around the eyes, face and muzzle; coughing and sneezing; nasal or eye discharge; and in extreme cases, anaphylaxis. Reoccurring symptoms and seasonal allergies have been linked to the development of recurrent airway obstruction (RAO) also known as equine asthma or heaves. If any of these symptoms occur, or your horse is having difficulty breathing, or is in distress, immediately contact your veterinarian for assistance.

After the hives disappear there may also be a swelling underneath your horse’s chest or stomach as the fluid pools there before being redistributed.

How are hives treated?

The most obvious treatment is to remove whatever is causing the hives. However, this is very rarely possible as often the causative factor is either unknown or cannot be removed – as in the case of pollen or flies, for example.

Topical treatments such as soothing creams or cool rinsing can also be useful in keeping the horse comfortable. Although some cases of hives will resolve themselves, very often veterinary intervention is needed and can help speed up resolution. Your vet will examine the horse and take a thorough history in an attempt to ascertain a possible cause. Very commonly they will then give a steroid and/or antihistamine injection to help reduce the symptoms and make the horse more comfortable.

These injections can also aid in rapidly treating allergic reactions and investigating or eliminating infectious causes. In severe chronic cases, allergy testing is an option. Similar to allergy testing in humans, a skin test is carried out. This is done by injecting small amounts of allergens under the skin in a grid pattern and observing the reaction. These results can then sometimes be used to create allergen specific immunotherapy injections (allergy vaccines). This may involve a lengthy period of treatment with the custom-made vaccine, which is administered in order to desensitise the horse’s immune system so that it tolerates allergens instead of overreacting to them. However, these therapies seem to have a variable success rate, with some reports suggesting they are approximately 60 to 70% effective at achieving positive results.

One of the keys to managing hives is managing your horse’s environment. Other than that, how we treat hives is largely dependent on the cause, which as we know, can in many cases prove to be elusive, so a broad treatment approach should be followed. On presentation of acute (sudden) hives, remove skin contaminants by bathing the horse with cold water and an antifungal, antibacterial wash (such as Quit-Itch or Topizole) which will aid in removing contaminants and infectious agents from the hair coat. Dish-washing liquid is also a safe option in a pinch, but keep any soap products away from your horse’s eyes and mouth and wear gloves for self-protection during this first step. Washing with antibacterial antifungal products is important in the prevention of secondary infections while the skin barrier is weak or damaged.

Please keep in mind that in the case of hives, you should always consult an equine veterinary professional to discuss what the best treatment option for your horse might be before administering any type of medication.

Can hives be prevented?

Prevention of hives caused from insect bites generally involves using a daily to twice daily insect repellent and protective clothing such as horse rugs/sheets and fly masks. An important distinction that needs to be made here is the difference between insect killing (pyrethrin) and insect repellent (permethrin) products, as preventing the bite in the first place is the aim. Environment control is important and moving animals away from insect prone regions and eliminating stagnant water sources are good options. Natural products with neem oil can also be used for mild seasonal cases, a common brand is Calafea Horse Oil.

The take home message

Unfortunately, because the exact cause of the reaction is often unknown, prevention can be very difficult. Ensuring the horse doesn’t come into contact with known reactants is an obvious solution. However, this is often impractical due to the variety of ways they might be encountered. Rugs and fly sprays can be beneficial if the reactant is likely of insect origin. Ensuring low stress can help, although that can be difficult for horses in high levels of work, and as mentioned, allergy injections are another alternative.

Whilst hives can be a nuisance, the condition is usually manageable. By understanding the causes, symptoms and prevention techniques you can prepare yourself for a possible outbreak. So next time you see some mysterious lumps and bumps on your horse, don’t panic – and remember, your vet is always there if you have any concerns.

Dr Ciaran Masters and Dr Caitlyn Mittelstadt both work for APIAM Animal Health in Queensland, Ciaran at Samford Valley Veterinary Hospital, and Caitlyn at Clermont Veterinary Surgery.