Allergies in dogs: Clinical signs and diagnostic assessment

Introduction

The definition of the term “allergy” has undergone several changes since its first use in human medicine in the early 20th century.

In veterinary medicine, allergy has been defined as an antibody-mediated or cell-mediated hypersensitivity reaction to an allergen as the result of a specific immunological mechanism.¹ Although the term is often used to refer to dogs with atopic dermatitis (AD), other allergic diseases include food hypersensitivity (FH), flea allergy dermatitis (FAD), allergic contact dermatitis (ACD), venomous insect hypersensitivity (VIH), urticaria and angioedema. 

Assessing allergies in dogs: clinical signs

Pruritus is the most common clinical manifestation in allergic conditions, although its intensity, anatomical distribution and characteristics may vary depending on the underlying disease.

Canine atopic dermatitis (CAD)

In CAD, signs first appear in dogs aged between 6 months and 6 years, although 70% are usually 1–3 years old. Pruritus may or may not be seasonal, and if severe enough, it usually causes erythema, papules, pustules, crusts and abrasions, generally affecting the head (perioral and periocular regions and ears), flexor aspect of the elbows, carpi and tarsi, paw pads, digits and interdigital region, ventral abdomen, perineum and ventral aspect of the tail. These patients often suffer from chronic skin inflammation and recurrent infections.²

Food hypersensitivity (FH)

FH is characterised by generalised, non-seasonal pruritus, which is often clinically persistent and, somewhat counter-intuitively, is not usually related to food intake.  Primary lesions include erythema and papular eruptions; however, many cases are only diagnosed when secondary lesions such as alopecia, abrasions, scabs, lichenification and hyperpigmentation have already developed. The pattern of lesions may or may not resemble that of atopy. Chronic otitis externa is also a very frequent manifestation in dogs with FH, while gastrointestinal signs are observed in 6–50% of cases.³

Flea allergy dermatitis (FAD)

Dogs with FAD usually present a severe pruritic condition that tends to affect the lumbosacral region, base of the tail and hindlimbs, although in case of massive infestations it may be more generalised.³

Allergic contact dermatitis (ACD)

In the case of ACD, the lesions may be very pruritic and of a variable distribution that depends on the causal agent, ranging from highly localised to widespread involvement, e.g., the application of a shampoo or spray across the whole body.

Venomous insect hypersensitivity (VIH)

VIH typically courses with localised swelling and pain in the affected area, but can also provoke anaphylactoid reactions of varying severity, ranging from urticaria and angioedema to collapse, cardiorespiratory arrest and death.³

Urticaria and angioedema

• Urticaria refers to the presence of wheals or hives, which are raised, circular, well-circumscribed lesions caused by oedema of the dermis.
• Angioedema produces significant swelling in the area due to oedema extending into the deep dermis or subcutis that is often caused by hypersensitivity reactions to insects or arthropods, although there may be other causes. Unlike other allergic processes, these lesions are not always accompanied by pruritus.³

 Allergies in dogs: diagnosis

• The diagnosis of AD is based on the clinical picture and the exclusion of other diseases that course with pruritus, such as FH, parasitic infestations, pyoderma or yeast infections.² Signs of pruritus, initially without lesions, in a dog less than 3 years old which lives indoors and responsive to glucocorticoid therapy is highly suggestive of AD.^4,5 Once a clinical diagnosis of AD has been made, and on a case-by-case basis, the potential utility of intradermal or serological testing for allergens should be considered.⁵
• FH should always form part of the differential diagnosis in dogs with non-seasonal pruritus, either with or without gastrointestinal signs, especially if there is a history of an inconsistent response to glucocorticoids. In most cases, the diagnosis is confirmed after implementing an elimination diet.^3,4
• In FAD, the diagnosis is based on the clinical picture and the observation of fleas or their faeces. If FAD is suspected, but no fleas can be detected, intradermal or serological testing can be used to reach a diagnosis.³
• In allergy contact dermatitis and VIH, the diagnosis is established on the basis of the clinical picture and after eliminating the potential trigger from the dog’s environment or upon confirming exposure to the insect, respectively.
• The diagnosis of urticaria/angioedema is based on the clinical presentation, although initially it is often hard to find the underlying catalyst unless the owner reports recent exposure to a possible causal agent.

Conclusions

Dogs can suffer from a wide range of allergic diseases. In many of them, pruritus is the main clinical sign, but not the only one. Although diagnosis is sometimes straightforward, the absence of pathognomonic clinical signs and specific diagnostic tests for most allergic diseases can complicate the process in many cases. Therefore, it is important to obtain a thorough anamnesis and clinical history to guide the diagnosis. 

References

1. Halliwell R, and the International Task Force on Atopic Dermatitis. (2006). Revised nomenclature for veterinary allergy. Vet Immunol Immunopathol; 114: 207–208.

2. Santoro D. (2019). Therapies in Canine Atopic Dermatitis: An Update. Vet Clin North Am Small Anim Pract; 49: 9-26.

3. Noli C. (2014). Canine Allergy. In Noli C, Foster A, Rosenkrantz W (eds). Veterinary Allergy. Willey Blackwell: 5-195.

4. Nuttall TJ, Marsella R, Rosenbaum MR, et al. (2019). Update on pathogenesis, diagnosis, and treatment of atopic dermatitis in dogs. J Am Vet Med Assoc; 254: 1291-1300.

5. Hensel P, Santoro D, Favrot C, et al. (2015). Canine atopic dermatitis: detailed guidelines for diagnosis and allergen identification. BMC Vet Res; 11: 196.