Urticaria: a frustrating problem

Urticaria, or hives, is a relatively common skin problem in horses. It has been estimated that around 60 percent of urticaria cases are a one-off occurrence, which either self-resolve or respond to appropriate symptomatic treatment and do not recur. However, in a significant number of cases, the problem is persistent or recurrent in nature.

Urticarial lesions develop due to the release of histamine from mast cells, causing vasodilation and vascular permeability, which results in tissue oedema. While the classic mechanism of mast cell degranulation is type I hypersensitivity, there are many potential causes aside from cross-linking of surface-bound IgE by allergens. The triggers that have been implicated are listed in Table 1 (Scott and Miller, 2011).

Clinical features of equine urticaria

The clinical features can be very variable, consisting of raised oedematous lesions in the form of papules, wheals or plaques in annular or linear patterns, including doughnut and serpiginous shapes (Figure 1) and sometimes more diffuse angioedema. However, the appearance is not well correlated to an underlying cause.

Urticaria in equids is not necessarily accompanied by pruritus; studies by Stepnik et al. (2011) and Loeffler et al. (2018) reported pruritus in only 39 and 26 percent of allergic horses with urticaria, respectively.

Sometimes lesions will ooze and become crusted, and secondary infection may occur. Classic urticaria lesions “pit” when pressure is applied, allowing for clinical differentiation from lesions caused by cellular infiltration or growth. The differential diagnoses for urticaria include:

• Erythema multiforme
• Cutaneous lymphoma
• Other neoplasms
• Amyloidosis
• Eosinophilic granuloma

Approach to urticaria cases in horses

As many cases will not recur, logical symptomatic treatment is appropriate for the first episode, although not knowing the cause is often frustrating for owners.

When lesions persist or are recurrent for a period longer than six weeks, further investigations are indicated. As with all dermatological conditions, a thorough history is essential and should be followed by clinical examination of the whole horse to check for signs of concurrent or underlying conditions and to characterise the skin lesions. Crucial historical information in cases of urticaria include:

• The onset, duration and seasonality of clinical signs
• Any affected relatives or in-contact animals
• The management practices of the yard/owner, etc
• Concurrent or previous illnesses
• Drug usage, both systemic and topical
• Responses to treatment(s)

Further investigations

At this stage, it should be possible to rule out some of the potential causes and draw up a differential diagnosis list.

Allergens

In most cases of chronic or recurrent urticaria, the underlying cause is allergic in nature, so restriction and provocation testing may help to identify the causal factors.

In most cases of chronic or recurrent urticaria, the underlying cause is allergic in nature

Changing stable bedding is easily undertaken, although many “indoor” allergens, such as dust and forage mites, can thrive in most types of bedding. However, specific bedding materials can be implicated in some cases. A better test for indoor allergens is to turn the horse out completely – but this is inappropriate in animals showing signs of concurrent insect-bite hypersensitivity (sweet itch, Culicoides hypersensitivity, etc) – or to keep completely indoors if pollen allergens are suspected. Also, rugs have been shown to harbour dust mite allergens, which may confound attempts to distinguish between indoor and outdoor triggers.

Food sensitivity

Food sensitivity is commonly suspected in cases of equine urticaria; however, there is only a single well-defined report in the veterinary literature (Miyazawa et al., 1991) and no good case series in the peer-reviewed literature. The urticaria case reported by Miyazawa et al. was associated with the ingestion of a garlic powder supplement.

Where food sensitivity is suspected, an elimination diet should be undertaken. A diet using a novel food source can be difficult to achieve, but the author recommends an exclusively grass-based diet of grass nuts, haylage (in preference to hay) and lucerne (unless previously exposed), with the exclusion of all cereals and supplements. The appropriate duration of elimination diets in the horse has not been established, but a minimum of a month is advised. Any improvement should be followed by dietary provocation to confirm if there will be a flare-up of signs after re-exposure, with subsequent improvement again on a return to the strict exclusion diet.

Serological tests for food allergies are unreliable for urticaria (Dupont et al., 2016).

Environmental factors

Environmental triggers are implicated if all other causes are ruled out, which gives a clinical diagnosis of atopic dermatitis. In these cases, allergen-specific IgE testing can be undertaken. It should be emphasised that IgE tests should not be used to make the diagnosis of atopic dermatitis since unaffected animals can give positive results in both intradermal and serological tests. Intradermal testing is still considered the gold standard (Figure 2), with serological tests showing poor or no correlation between intradermal tests and each other (Lorch et al., 2001).

Over the past two decades, there have been significant improvements in detection reagents employed in serological assays, with the development of species-specific equine monoclonal antibody reagents and measures to eliminate the carbohydrate cross-reacting determinants that can give rise to false positive results. However, there are no published studies comparing intradermal test results with these newer serological tests.

Management of equine urticaria

Results of IgE testing must be interpreted in light of the horse’s history, but the identification of implicated allergens allows avoidance measures to be employed. It also offers the option of allergen-specific immunotherapy (ASIT) as part of the management regime for equine urticaria cases.

Dust and forage mite allergens are implicated in most cases of equine atopic dermatitis in the UK, with moulds and pollens also playing a part in some cases

Dust and forage mite allergens are implicated in most cases of equine atopic dermatitis in the UK, with moulds and pollens also playing a part in some cases (Loeffler et al., 2018; Rendle et al., 2010). Allergen-avoidance measures in the indoor environment can be very effective in reducing thresholds for symptoms (Littlewood et al., 1998).

While there are few publications concerning the efficacy of immunotherapy in equine atopic dermatitis, Loeffler et al. (2018) reported a benefit in 64 percent of horses treated with ASIT, and in an earlier study, 85 percent of owners felt ASIT had been of benefit (Stepnik et al., 2011). It can take many months before a response to immunotherapy is seen, so it is essential that symptomatic therapy is employed in the meantime, and this may need to be ongoing.

While many horses with urticaria are untroubled by the lesions, the condition can be of significant welfare concern where pruritus is present

While many horses with urticaria are untroubled by the lesions, the condition can be of significant welfare concern where pruritus is present. It can also be challenging to control, particularly in competition horses where the drugs that are commonly employed are prohibited. The involvement of a specialist veterinary dermatologist can be invaluable in the management of cases of equine atopic dermatitis, helping to manage owner expectations and leading to better long-term outcomes for patients.