Can equine urticaria be cured?

Introduction

Equine veterinary practitioners and horse owners are familiar with hives as common skin eruptions in horses. Although urticaria is reported in other species, it is more frequently reported in horses (1). Urticarial eruptions are not life threatening, except in the presence of a severe angioedema obstructing the upper respiratory tract, which is rare in horses (2,3). Although fatalities are rare in horses suffering from urticaria, this skin reaction pattern has a major negative impact on the animal, owner, and veterinarian. This dermatosis is often chronic, demands time and patience to find the underlying cause, and requires long-term management. The veterinarian might be frustrated by not finding the cause of the hives, as well as the owner having to spend money for laboratory tests, veterinary fees, and therapeutics. Both veterinarian and owner may be affected by compassion fatigue due to the chronic management of this skin problem. The quality of life of the horse might also be affected if the severity of the disease or the treatments administered necessitate stopping the horse’s activities (such as competitions, shows, training, riding), or cause adverse effects.

Finding an easy way to manage hives in horses could alleviate frustrations for both veterinarian and owner, and improve the quality of life of the animal. The question is: can we cure urticaria in horses?

Understanding urticaria

To properly answer this question, it’s important to understand that urticaria is a cutaneous reaction pattern that is not specifically associated with one etiology. Indeed, urticaria is not a dermatological disease by itself, rather a clinical manifestation initiated by various etiopathological processes (1–4). Hives have been commonly associated with immunologic reactions against allergens in contact with the skin, inhaled, ingested or injected parenterally (4,5). The classical immunologic hypersensitivity reaction leading to urticarial lesions is the type 1 hypersensitivity reaction involving the formation of type E immunoglobulins (IgE). Type E immunoglobulins are molecules that attach to cutaneous mast cells and stimulate the release of mast cell granules, particularly the vasoactive amines, in the presence of an allergen. The release of cytokines from basophils is also involved, usually within hours of mast cell degranulation. Type 3 hypersensitivity reactions, which involve the formation and deposition of immune complexes (antigens-antibodies), have also been described in the pathogenesis of urticaria in horses (1–3,6,7).

The ultimate results of these hypersensitivity reactions is the release of a storm of inflammatory mediators, which increase the permeability of blood vessels and thereby extravasation of plasma. Clinically, this inflammatory wave initiates the formation of edematous papules or plaques and/or angioedema (2,4).

A part of the complexity of this disease resides in various mechanisms causing mast cell degranulation. Immunological and non-immunological (physical, genetic, drug, or chemical triggers) mechanisms have been described as potent causes for the release of mast cell granules (1,3,4). Unfortunately, it is impossible to determine whether the reaction is immunological or not, by relying solely on examination of the affected horse. The answer to the initial question is partially explained by the complexity of the pathogenesis of equine urticaria.

Many causes reported

The dermatological challenge is not in making the diagnosis of urticaria. In most cases, the diagnosis is straightforward, because the clinical findings are usually highly suggestive of urticaria (4). Different patterns of urticaria have been described in horses, including localized or generalized papules or plaques (Figure 1), and giant, linear, exudative, and gyrate (arciform, serpiginous, annular) patterns (Figures 2, 3) (1–9). A common finding in these patterns is pitting edema and blanching of the skin upon pressure (1,2,8). Hives are variably pruritic. They are often acute and short-lived (24 to 48 h). However, if hives persist beyond 6 to 8 wk the condition is considered chronic. When chronic, it is particularly important to try to find the putative trigger since chronic urticaria represents a major therapeutic challenge (1,2,9).

Figure 1.

Papules and plaques representing classical appearance of hives.

Figure 2.

Generalized gyrate pattern of urticaria.

Figure 3.

Serpiginous and annular hives. Erythema multiforme may mimic this pattern of urticaria.

We may assume that by finding the putative trigger it will be possible to cure urticaria; this is partially true. In fact, if a specific etiology can be found and removed from or controlled within the environment of the horse, then a cure is possible. The potential causes reported for equine urticaria may be divided into 5 main categories. Authors have reported etiologies in different manners, but essentially, they are classified as immunological causes, physical insults, infectious and parasitic diseases, stress or psychogenic disorders, and chronic idiopathic cases (1–4,8).

Immunological etiologies are associated with a large number of horses suffering from urticaria. This group of disorders includes atopic dermatitis, insect hypersensitivity (e.g., Culicoides spp., flies, chiggers), adverse food reaction (vitamin and mineral supplements, hay, cereals, pastures), adverse drug reactions (e.g., antibiotics, phenylbutazone, flunixin, narcotics, anthelmintics), vaccine or bacterin reactions, serotherapy or blood transfusion adverse reactions, vasculopathies, sweat hypersensitivity, and snake bites (1–4,10–12). Some urticariogenic substances (e.g., morphine, atropine, dextran, polymyxin B, strawberries, cobra venoms) may also induce hives. In such cases, the substance ingested, injected, or in contact with the animal induces wheals due to the chemical properties of the agent (12). Less commonly, physical reactions may be the culprit. Physical urticaria has been associated with temperature variations (such as cold pack compress or hot shower), skin pressure (dermographism), exercise, or following sun exposure (1–4,12). Many cutaneous infections (viral, fungal, protozoal) and parasitic disorders (endoparasites and ectoparasites) have been reported as potential causes of equine urticaria (1–3,12). Always difficult to diagnose and to clearly explain, hives have been associated with psychogenic stresses such as the arrival of a new horse at the stable or a close pet friend leaving, or just before a horse race (1–3,12).

The numerous etiologies of equine urticaria reflect the complexity of its pathogenesis. It becomes clearer that equine urticaria might be cured, or at least controlled, but not in all cases.

Is there a therapy to cure urticaria?

The short answer to the question “is there a therapy to cure urticaria in horses?” is no. In cases of mild acute eruptions, observation of the patient may be sufficient (benign neglect). However, if the horse shows severe manifestations, such as facial angioedema, even if the episode subsides within 24 to 48 h, treatment with glucocorticoids, antihistamines, and/or epinephrine is recommended (1–3).

In chronic cases, if the underlying cause can be identified, elimination of the trigger is the treatment of choice, and the sole treatment that cures the disease. A thorough approach is key in order to identify the underlying cause. First, the patient’s history is an important step in the dermatological investigation. The trigger might be known, but not yet identified. Several questions regarding the history of the skin disease and general health status (age of onset, seasonality, other horses affected, pruritus, response to previous treatments) must be asked. For example, perennial (year-round) hives that were initially seasonal might be the result of atopy, or hives first noted recently and concomitantly to guttural pouch infection might be secondary to an infectious disease or its treatment. It is also important to question the owner on the management of the stable, the history of the drugs and vaccines administered, the deworming program, and travel. Such questions may be helpful in identifying an adverse drug reaction or a disease transmitted during travel. Stress caused by travelling should also be kept in mind. Adverse drug reactions can occur quickly following the administration of the offending drug only if the horse has already been sensitized to this drug in the past. If the drug hasn’t been given in the past, most reactions will occur 1 to 3 wk after starting the treatment (3). If a vasoactive or urticariogenic agent is administered, the eruptions may occur following the first administration (12).

Simple tests may help define the origin of hives when a physical cause is suspected based on the history. Dermatographism may be induced within 15 min after drawing any pattern on the skin of the horse with the blunt tip of a pencil. Wheals may also be induced by the application of ice or heat on the skin for 5 to 15 min.

When an infectious disease or a parasitic infestation is identified concomitant with urticarial eruptions, the next step is to treat this infection or infestation properly (1,3,4,10).

Obviously, equine urticaria may be cured if its origin is one of the etiologies discussed, and removal or treatment of the offending trigger is possible.

Hives associated with stress or psychogenic causes pose a challenge, and some of these cases are probably classified as idiopathic. Consulting with an animal behaviorist may be beneficial in recognizing subtle behaviors that reveal anxiety. If the anxiety-provoking situation can be managed, then a cure to the urticaria may be expected.

Among the common immunologic disorders, the most challenging ones are atopic dermatitis, insect hypersensitivity, and adverse food reaction. Allergy testing is available, but reliable only in allergies associated with the environment. Intradermal testing or serological tests are not considered diagnostic per se. Diagnosis of atopic dermatitis should be based on medical history, dermatological findings, and the elimination of other causes of urticaria (3,4). Allergy testing is recommended once the elimination process is completed, in order to initiate allergen-specific immunotherapy (desensitization vaccine) (3,13). Once offending allergens are identified, the removal of these allergens represents the ideal therapeutic approach. However, this option is rarely possible given the environment of the horses and the fact that they are often polysensitized. Immunotherapy is considered an effective long-term therapeutic approach, especially for horses with perennial symptoms (1,9). Concomitant symptomatic treatment might be required, especially within the first months after starting immunotherapy. Insect hypersensitivity, especially to Culicoides spp, is the most common cause of equine allergy (2). Since insect desensitization is not always possible, various measures can be taken to minimize exposure to insects (insect repellents, modification of hours on pasture, use of fans and mosquito nets suitable for the stable, face or full protection net for horses) (14). Adverse food reaction represents significant diagnostic and therapeutic challenges. Only a change in the diet over a period of 4 to 6 wk can give a clue to a possible food allergy. A first step would be to remove all treats and oral supplements. If required, the second step would be to change the diet. For example, if the horse’s ration contains a mixture of grains, the horse can be fed with only one type of grain. Likewise, new unmixed hay (for example, hay containing only alfalfa) may be an option to consider. However, dietary changes in horses can be a major source of concern as certain nutrient intakes are essential for horse activities and colic can also occur. Moreover, the food trial might not be strict considering that horses have access to different plants on pasture. If an adverse reaction to a food or supplement can be proven by a provocative challenge following the food trial, a cure may be possible if the triggers can be removed from the environment or the ration of the horse (1–4,14–16).

Idiopathic urticaria is diagnosed when no underlying cause can be found (1–3). Idiopathic cases also encompass horses for which hives are not investigated. Overall, it could potentially represent ~75% of the cases (3).

The therapeutic management of horses suffering from urticaria without identified triggers, or when the known triggers cannot be avoided or treated, relies on symptomatic therapy (1,2). The therapeutic modalities are summarized in Table 1. The therapeutic approach chosen by the veterinary practitioner should be based on the severity of the clinical signs, the chronicity of the condition, the underlying cause, the general health status of the animal, and the potential side effects. The main goal is to relieve pain, discomfort, or pruritus without causing further health issues.

Table 1.

Common symptomatic treatments for urticaria in horses (2,3,4,5,11,12,14,16).

Therapeutic classes	Drugs and dosages
Glucocorticoids (17)	Predniso(lo)ne: 0.5 to 2 mg/kg body weight (BW) PO, q24h
	Dexamethasone: 0.02 to 0.1 mg/kg BW, PO, q24h
	Administer for 3 to 10 d (until resolution of the hives), and then every 48 h [predniso(lo)ne: ≤ 0.5 mg/kg BW] or 72 h (dexamethasone: ≤ 0.02 mg/kg BW) if treatment is required over a longer period.
Antihistamines	Hydroxyzine: 0.5 to 2 mg/kg BW, PO, q8h to q12h
	Chlorpheniramine: 0.2 to −0.5 mg/kg BW, PO, q12h
	Diphenhydramine: 1 to 2 mg/kg BW, PO, q8h to q12h
	Cetirizine: 0.2 to 0.4 mg/kg BW, PO, q12h (18)
	Can be combined with glucocorticoids to reduce the exposure to steroids.
Tricyclic antidepressants	Doxepin: 0.5 to 0.75 mg/kg BW, PO, q12h
	Trimeprazine: 1 to 2 mg/kg BW, PO, q12h
	Amitriptyline: 1 mg/kg BW, PO, q12h
	Used for their antihistaminic potencies.
Methylxanthine derivative	Pentoxifylline: 8 to 10 mg/kg BW, PO, q8h to q12h
Fatty acid supplementation	Flaxseed or flaxseed oil.
	Several commercial products are available, e.g., EO-3TM (KER), Formulas Platinum Performance Equine and Platinum Skin & Allergy (Platinum Performance), Herbs for horses herring oil (Selected BioProducts).

In conclusion, returning to the initial question; unfortunately, in most instances, equine urticaria is an incurable lifelong disease. In a few cases, the underlying cause can be identified and addressed, which leads to a cure for the hives. It is important for veterinary equine practitioners to be aware of this fact. The recommendations can then be appropriate for the horse, and realistic expectations can be set for the owner.