CHAPTER | 7 Hypersensitivity Disorders
Canine Atopy (environmental, pollen allergies)
Diagnosis
1 Seasonal foot licking is the most unique and typical symptom of atopy. If year-round allergens (house dust mites) are causing the allergy, the foot licking may be nonseasonal.
2 Allergy testing (intradermal, serologic): allergy testing can be highly variable according to the method used. Positive reactions to grass, weed, tree, mould, insect, dander, or indoor environmental allergens are seen. False-negative and false-positive reactions may occur.
Treatment and Prognosis
1 Infection prevention: any secondary pyoderma, otitis externa, and Malassezia dermatitis should be treated with appropriate therapies. Controlling and preventing secondary infection is an essential component of managing atopic dogs. Bathing every 3 to 7 days and treating the ears after every bath help to wash off pollens and disinfect the skin and ear canals, preventing secondary infections from recurring.
2 Symptomatic therapy (itch control):
a An integrated flea control program should be instituted to prevent flea bites from aggravating the pruritus.
b Topical therapy with antimicrobial shampoos and anti-itch conditioners, and sprays (i.e., those containing oatmeal, pramoxine, antihistamines, or glucocorticoids) applied every 2 to 7 days or as needed may help reduce clinical symptoms.
c Systemic antihistamine therapy reduces clinical symptoms in many cases (Table 7-1). Antihistamines can be used alone or in combination with glucocorticoids or essential fatty acids for a synergistic effect. One- to 2-week-long therapeutic trials with different antihistamines may be required to determine which is most effective.
d Oral essential fatty acid supplements (180 mg ecosopentanoic acid [EPA]/10 lb) help control pruritus in 20% to 50% of cases, but 8 to 12 weeks of therapy may be needed before beneficial effects are seen. Also, a synergistic effect is often noted when essential fatty acid supplements are administered in combination with glucocorticoids or antihistamines.
e Dextromethorphan, an opioid antagonist, may be a useful adjunct in managing the licking, chewing, and biting behaviors associated with allergic dermatitis in dogs. Dextromethorphan 2 mg/kg PO should be administered every 12 hours. A beneficial effect should be seen within 2 weeks.
f Systemic glucocorticoid therapy is often effective (75%) in controlling pruritus but almost always results in adverse effects ranging from mild (polyuria [PU]/polydipsia [PD]) to severe (immune dysfunction, demodicosis, and calcinosis cutis). It is a therapeutic option if the allergy season is very short but may result in unacceptable adverse effects, especially if used over the long term.
Potent, long-acting injectable steroids are contraindicated for the treatment of allergies because of their comparatively short anti-inflammatory benefits (3 weeks) relative to their prolonged metabolic and immunodepressive effects (6–10 weeks).
Potent, long-acting injectable steroids are contraindicated for the treatment of allergies because of their comparatively short anti-inflammatory benefits (3 weeks) relative to their prolonged metabolic and immunodepressive effects (6–10 weeks).
3 Allergy treatment (immune modulation)
a Exposure to offending allergens should be reduced, if possible, by their removal from the environment. High-efficiency particulate air (HEPA) and charcoal filters should be used to reduce pollens, molds, and dust in the home. For house dust mite–sensitive dogs, household treatments for carpets, mattresses, and upholstery with the acaricide benzyl benzoate once a month for approximately 3 months, then every 3 months thereafter, may effectively eliminate house dust mites from the environment. Old dog beds should be discarded as these accumulate house dust mite antigens. Dehumidifying the house to below 40% relative humidity decreases house dust mite, mould, and flea antigen loads. To achieve this, high-efficiency dehumidifiers that are capable of pulling several liters of water from the air per day are required.
b Cyclosporine (Atopica) helps control pruritus in 75% of atopic dogs. A dose of 5 mg/kg PO should be administered every 24 hours until beneficial effects are seen (approximately 4–6 weeks). Then, dosage frequency should be tapered down to every 48 to 72 hours. For long-term control, approximately 25% of dogs require daily dosing, 50% can be controlled with every-other-day dosing, and approximately 25% can be controlled with twice-weekly dosing. Glucocorticoids can be used initially to speed response. As of this writing, no statistically significant increases in tumor risk or severe infection resulting from the immune effects of cyclosporine have been noted.
4 The prognosis is good, although lifelong therapy for control is needed in most dogs. Relapses (pruritic flare-ups with/without secondary infections) are common, so individualized treatment adjustments to meet patient needs may be required periodically. In dogs that become poorly controlled, one should rule out secondary infection (e.g., that caused by bacteria or Malassezia); sarcoptic mange; demodicosis; and concurrent food, flea bite, and recently acquired hypersensitivity to additional environmental allergens. Because a strong genetic component is present, the breeding of any male or female dog with clinical signs of atopic dermatitis should be discouraged.
TABLE 7-1 Antihistamine Therapy in Dogs*
| Antihistamine | Dose |
|---|---|
| Chlorpheniramine | 0.2–3 mg/kg PO q 8–12 hours |
| Diphenhydramine | 1–4 mg/kg PO q 8 hours |
| Hydroxyzine | 3–7 mg/kg PO q 8 hours |
| Amitriptyline | 1–2 mg/kg PO q 12 hours |
| Cyproheptadine | 0.1–2 mg/kg PO q 8–12 hours |
| Trimeprazine | 0.5–5 mg/kg PO q 8–12 hours |
| Brompheniramine | 0.5–2 mg/kg PO q 12 hours |
| Clemastine | 0.05–1.5 mg/kg PO q 12 hours |
| Terfenadine | 0.25–1.5 mg/kg PO q 12–24 hours |
| Astemizole | 1 mg/kg PO q 12–24 hours |
| Promethazine | 1–2.5 mg/kg PO q 12 hours |
| Loratadine | 0.5 mg/kg PO q 24 hours |
| Cetirizine | 0.5–1 mg/kg PO q 24 hours |
| Doxepin | 0.5–1 mg/kg PO q 8–12 hours |
| Dimenhydrinate | 8 mg/kg PO q 8 hours |
| Tripelennamine | 1 mg/kg PO q 12 hours |
| Clomipramine | 1–3 mg/kg PO q 24 hours |
* Antihistamines in bold are preferred by the author.
FIGURE 7-4 Canine Atopy.
Close-up of the dog in Figure 7-3. The periocular alopecia and hyperpigmentation caused by facial pruritus are typical of allergic disease.
FIGURE 7-5 Canine Atopy.
Periocular alopecia, erythema, hyperpigmentation, and lichenification caused by pruritus.
FIGURE 7-7 Canine Atopy.
Pododermatitis demonstrating the salivary staining caused by chronic licking.
FIGURE 7-12 Canine Atopy.
Sterile otitis caused by allergy often presents with erythema of the ear pinna and external canal.
FIGURE 7-14 Canine Atopy.
Secondary bacterial pyoderma (erythematous papular rash) on the inguinal area of an allergic dog.
FIGURE 7-17 Canine Atopy.
Severe erythema of the feet caused by intense pruritus and by the patient self-mutilating his feet.
FIGURE 7-18 Canine Atopy.
Same dog as in Figure 7-17 demonstrating the severe pododermatitis typical of atopy. Note the erythema on the abdomen, which is common with allergies.
FIGURE 7-20 Canine Atopy.
Same patient as in Figure 7-19. This intradermal allergy test (IDAT) demonstrates positive reactions with classic erythematous, well-demarcated, raised reactions. Note the difference between negative and positive reactions.
FIGURE 7-24 Canine Atopy.
Severe interdigital dermatitis with ulceration caused by secondary bacterial and yeast infections.
FIGURE 7-27 Canine Atopy.
Same dog as in Figure 7-26. Periocular dermatitis caused by the allergic reaction is obvious.
FIGURE 7-28 Canine Atopy.
Interdigital dermatitis with salivary staining is extremely common in atopic patients.
Canine Food Hypersensitivity
Diagnosis
1 Perianal dermatitis with or without recurrent otitis is the most common and unique feature of food allergy. However, food allergy can manifest in many patterns and should be suspected for atypical pruritic patients, including those with recurrent infections without pruritus.
2 Dermatohistopathology (nondiagnostic): varying degrees of superficial perivascular dermatitis. Mononuclear cells or neutrophils may predominate. Eosinophils may be more numerous than in atopy.
3 Food allergy testing (intradermal, serologic) (nondiagnostic): not recommended because test results are unreliable. Some dogs will have positive reactions to storage mite antigens, which may be clinically relevant, or that may be caused by cross-reactivity with other insects. Storage mites are ubiquitous, and their clinical significance is currently unknown.
4 Response to hypoallergenic diet trial: symptoms improve within 10 to 12 weeks of initiation of a strict home-cooked or commercially prepared restricted diet (one protein and one carbohydrate source). The hypoallergenic diet should not contain food ingredients previously administered in dog food, treats, or table scraps, nor should flavored heartworm preventive, flavored medications, nutritional supplements, or chewable treats (i.e., pig ears, cow hooves, rawhide, dog biscuits, table food such as cheese) be administered during the hypoallergenic diet trial. Beef and dairy are the most common food allergens in dogs, and avoiding these alone may result in clinical improvement. Other common food allergies include chicken, eggs, soy, corn, and wheat.
Treatment and Prognosis
1 Infection prevention: any secondary pyoderma, otitis externa, and Malassezia dermatitis should be treated with appropriate therapies. Controlling and preventing secondary infection is an essential component of managing atopic dogs. Bathing every 3 to 7 days and treating the ears after every bath helps wash off pollens and disinfect the skin and ear canals, preventing secondary infections from recurring.
2 Symptomatic therapy (itch control) is variably effective for food allergy:
a An integrated flea control program should be instituted to prevent flea bites from aggravating the pruritus.
b Topical therapy with antimicrobial shampoos and anti-itch conditioners, and sprays (i.e., those containing oatmeal, pramoxine, antihistamines, or glucocorticoids) applied every 2 to 7 days or as needed may help reduce clinical symptoms.
c Systemic antihistamine therapy reduces clinical symptoms in many cases (see Table 7-1). One- to 2-week-long therapeutic trials with different antihistamines may be required to determine which is most effective.
d Oral essential fatty acid supplements (180 mg ecosopentanoic acid [EPA]/10 lb) help control pruritus in 20% to 50% of cases, but 8 to 12 weeks of therapy may be needed before beneficial effects are seen. Also, a synergistic effect is often noted when essential fatty acid supplements are administered in combination with glucocorticoids or antihistamines.
e Dextromethorphan, an opioid antagonist, may be a useful adjunct in managing the licking, chewing, and biting behaviors associated with allergic dermatitis in dogs. Dextromethorphan 2 mg/kg PO should be administered every 12 hours. A beneficial effect should be seen within 2 weeks.
f Systemic glucocorticoid therapy is only variably effective (unpredictable minimal to good response) in controlling pruritus caused by food allergy but almost always results in adverse effects ranging from mild (PU/PD) to severe (immune dysfunction, demodicosis, and calcinosis cutis) (see “Atopy” section).
Potent, long-acting injectable steroids are contraindicated for the treatment of allergies because of their comparatively short anti-inflammatory benefits (3 weeks) relative to their prolonged metabolic and immunodepressive effects (6–10 weeks).
Potent, long-acting injectable steroids are contraindicated for the treatment of allergies because of their comparatively short anti-inflammatory benefits (3 weeks) relative to their prolonged metabolic and immunodepressive effects (6–10 weeks).
3 Food allergy treatment
a Offending dietary allergen(s) should be avoided. A balanced home-cooked diet or a commercial hypoallergenic diet should be provided.
b To identify offending substances to be avoided (challenge phase after food allergy has been confirmed with the dietary trial), one new food item should be added to the hypoallergenic diet every 2 to 4 weeks. If the item is allergenic, clinical symptoms will recur within 7 to 10 days. Note: Some dogs (approximately 20%) should be fed home-cooked diets to remain symptom-free. For these dogs, commercial hypoallergenic diets are ineffective, presumably because their hypersensitivity relates to a food preservative or dye.
4 The prognosis is good. In dogs that are poorly controlled, owner noncompliance should be ruled out, along with development of hypersensitivity to an ingredient in the hypoallergenic diet, secondary infection (caused by bacteria, Malassezia, dermatophyte), scabies, demodicosis, atopy, flea allergy dermatitis, and contact hypersensitivity.
FIGURE 7-32 Canine Food Hypersensitivity.
Close-up of the dog in Figure 7-31. Erythema, alopecia, and papular rash involving the ear pinnae. No infectious otitis is present—only external lesions associated with the underlying allergy.
FIGURE 7-33 Canine Food Hypersensitivity.
Close-up of the dog in Figure 7-31. Alopecia and erythema in the axillary area. Mild hyperpigmentation and lichenification are caused by a secondary yeast dermatitis. Note the similarity to lesions seen with atopy.
FIGURE 7-43 Canine Food Hypersensitivity.
Facial dermatitis (alopecia, erythema, and pruritus) is a common finding in allergic dogs.
FIGURE 7-44 Canine Food Hypersensitivity.
Perianal dermatitis is one of the most consistent and unique features of food allergy.
Acral Lick Dermatitis (lick granuloma)
Features
Acral lick dermatitis is first noted as excessive, compulsive licking at a focal area on a limb, resulting in a firm, proliferative, ulcerative, alopecic lesion. Causes of the licking are multifactorial, and, although environmental stress (e.g., boredom, confinement, loneliness, separation anxiety) may be a contributor, other factors are usually more important (Box 7-1). This dermatitis is common in dogs, with the highest incidence in middle-aged to older, large-breed dogs, especially Doberman pinschers, Great Danes, Golden retrievers, Labrador retrievers, German shepherds, and Boxers.
Diagnosis
2 Dermatohistopathology: ulcerative and hyperplastic epidermis, mild neutrophilic and mononuclear perivascular dermatitis, and varying degrees of dermal fibrosis.
Treatment and Prognosis
2 One should treat for secondary bacterial infection with long-term systemic antibiotics (minimum 6–8 weeks, and as long as 4–6 months in some dogs). Antibiotic therapy should be continued at least 3 to 4 weeks beyond regression of the lesion. The antibiotic should be selected according to bacterial culture and sensitivity results.
3 Anecdotal reports suggest good efficacy with combined antibiotic, amitriptyline (2 mg/kg q 12 hours), and hydrocodone (0.25 mg/kg q 8–12 hours) administered until lesions resolve. Then, one drug should be discontinued every 2 weeks until it can be determined which drug (if any) may be required for maintenance therapy.
4 Topical application of analgesic, steroidal, or bad-tasting medications every 8 to 12 hours may help stop the licking, but response is unpredictable and often disappointing.
5 When no underlying cause can be found, treatment with behavior-modifying drugs may be beneficial in some dogs (Table 7-2). Trial treatment periods of up to 5 weeks should be used until the most effective drug is identified. Lifelong treatment is often necessary.
6 Alternative medical treatments such as cold laser therapy or acupuncture have been beneficial in some patients.
7 Mechanical barriers such as wire muzzles and bandaging, Elizabethan collars, and side braces may be helpful.
8 Surgical excision or laser ablation is not recommended because postoperative complications, especially wound dehiscence, are common. Laser ablation may help sterilize the lesion and deaden nerve endings; however, response is highly variable.
9 The prognosis is variable. Chronic lesions that are unresponsive or extensively fibrotic and those for which no underlying cause can be found have a poor prognosis for resolution. Although this disease is rarely life threatening, its course may be intractable.
TABLE 7-2 Drugs for Psychogenic Dermatoses in Dogs
| Drug | Dose |
|---|---|
| Anxiolytics | |
| Phenobarbital | 2–6 mg/kg PO q 12 hours |
| Diazepam | 0.2 mg/kg PO q 12 hours |
| Hydroxyzine | 2.2 mg/kg PO q 8 hours |
| Tricyclic Antidepressants | |
| Fluoxetine | 1 mg/kg PO q 24 hours |
| Amitriptyline | 1–3 mg/kg PO q 12 hours |
| Imipramine | 2–4 mg/kg PO q 24 hours |
| Clomipramine | 1–3 mg/kg PO q 24 hours |
| Endorphin Blocker | |
| Naltrexone | 2 mg/kg PO q 24 hours |
| Endorphin Substitute | |
| Hydrocodone | 0.25 mg/kg PO q 8 hours |
| Topical Products | |
| Fluocinolone acetonide + Flunixin meglumine | |
| Deep Heet + Bitter Apple | |
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