160
EQUINE VETERINARY EDUCATION / AE / MARCH 2015
Diagnosis of atopic dermatitis is based on clinical signs and
the exclusion of other diagnoses (especially Culicoides allergy). Horses may have a secondary bacterial infection (pyoderma), typified by excess scale, particularly in the shape of a circle (epidermal collarettes). Intradermal tests or serum allergy tests may be performed to identify allergens that may be used for allergen-specific immunotherapy (ASIT; aka hyposensitisation or ‘allergy shots’). Horses with atopic dermatitis and recurrent hives generally have a higher incidence of positive reactions than healthy horses, but the diagnosis (as in other species) cannot solely be made on the basis of the intradermal test or serological testing alone (Lorch et al. 2001a,b,c). The testing should be interpreted in light of the history of the disease and used to determine which allergens might be useful in ASIT, should the owners elect that route of treatment. Medical therapy may include corticosteroids; prednisolone
(1 mg/kg bwt q. 24 h, then taper), or dexamethasone (0.05–0.01 mg/kg bwt q. 24 h, then taper) are the most commonly used. Prednisolone seems to be better absorbed orally in horses than prednisone (Peroni et al. 2002). The injectable dexamethasone solution may be used orally, although the bioavailability is 60–70% of the injectable route. Antihistamines are sometimes quite effective: hydroxyzine pamoate (0.4–0.8 mg/kg bwt t.i.d. or b.i.d.), cetirizine (0.4 mg/kg bwt b.i.d.) or doxepin (a tricyclic antidepressant with antihistaminic effects – 0.6 mg/kg bwt b.i.d.). Antihistamines rarely cause either drowsiness or excitability in horses, although the author has seen this in <5% of horses receiving these drugs. The commonly used antihistamine pyrilamine maleate (Tri-Hist Granules)11 has been shown to have poor bioavailability when given orally in the horse, but detectable in the urine up to one week after a single dose, which may present difficulties with withdrawal times (Dirikolu et al. 2009). Some clinicians have noted improvement when an
essential fatty acid product (Platinum Performance)12 is added to the feed in some atopic horses as an adjunctive treatment. Urticaria, secondary to atopic dermatitis, may be a familial problem in some horses (Rees 2001). Topical corticosteroids products as noted above with Culicoides hypersensitivity are sometimes useful. Atopic dermatitis has been successfully managed with allergen-specific immunotherapy, with horses showing improvement as early as 2 months into treatment (Rees 2001; Stepnik et al. 2012). However, a full 12 months of ASIT should be given before fully evaluating its effectiveness, and veterinarians should maintain consistent communication with the client to monitor the progress of the treatment and to encourage the owner to continue with the injections for the full year. While in other domestic species it is thought that most patients will need to be maintained on the injections for life, this may not be true in the horse, as some horses had the ASIT injections stopped and did not relapse (Stepnik et al. 2012). In general, approximately 70% of atopic horses improve with ASIT (Stepnik et al. 2012); other researchers have reported even better results, albeit in a noncontrolled report (Tallarico and Tallarico 1998).
Food allergies Substantiated food allergies that both resolve on a ‘hypoallergenic diet’ and recur upon re-challenge are very uncommon in the horse (or other herbivores). Reported clinical
© 2015 EVJ Ltd
signs are pruritus and urticaria. Age of onset and distribution of lesions are not well reported. It is uncertain how often cutaneous signs of food allergy in a horse is associated with gastrointestinal signs. Food allergy is best diagnosed by limiting the diet to one foodstuff for at least 4 weeks. Most authors prefer oat hay or beet pulp. Products with barley should be avoided due to a high predilection to induce founder. If the lesions and pruritus resolve during this trial, after documenting recurrence upon re-challenge, various foodstuffs may be added weekly, one at a time, to determine the offending allergen.
Contact allergies These may be deduced via history, and if a liquid is involved, having lesions with gravity-induced ‘drip signs’. A complete history is very important, as owners may not always remember (or care to admit) home remedies they have applied to the skin. Treatment is discontinuing the offending contactant. In addition, corticosteroids or pentoxifylline (10 mg/kg bwt, 2–3×/24 h) may be helpful. Remember that bedding, especially with wood chips, can be a contact allergen (Campagnolo et al. 1995).
Urticaria Among all the species of domestic animals, the horse has the greatest prevalence of urticaria. It should be stressed that urticaria is not a specific disease entity. Rather, it is a skin lesion or reaction pattern that may have differing aetiologies and pathogenesis. The characteristic lesion is a wheal: a flat topped papule/nodule with steep walled sides. It is the result of localised transient oedema of the dermis. Many wheals have a slightly depressed centre giving them an annular configuration. The onset of the lesions is acute to peracute with the wheals developing within a few minutes to a few hours. The size of the individual wheals can vary considerably. The diagnosis of urticaria is usually based on the acute
onset of characteristic multiple lesions. If in doubt about the character of the lesions, the most important criteria is that they ‘pit’ on pressure. Of great importance is trying to determine the cause of the urticaria via the medical history. A complete drug history is mandatory especially regarding medications that have been given in the previous 2 weeks. In addition to drugs, an attempt should be made to associate the onset of urticaria to other events (especially in recurrent/chronic cases) such as exercise, cold weather etc. Treatment of urticaria in the horse is thus based both on
determining the underlying cause, and using the same medications as noted for atopic dermatitis.
Infections: bacterial
Bacterial folliculitis (other than dermatophilosis) is usually caused by a coagulase-positive Staphylococcus spp. Both S. aureus and S. intermedius have been isolated (Scott and Manning 1980; Shimizu et al. 1991; Chiers et al. 2003). In one study, S. aureus accounted for twice as many isolates as S. intermedius; this study also isolated some strains of S. hyicus (Chiers et al. 2003). Interestingly, in another study, lysozymes from equine neutrophils were only slightly bactericidal for S. aureus (Pellegrini et al. 1991). Many isolates are resistant to penicillin G (Chiers et al. 2003). Occurrence of pyoderma has been linked to poor nutrition and husbandry in some cases (Inokuma et al. 2003). More recently, the staphylococcal
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88
