The International Task Force on canine atopic dermatitis definition is as follows: “A genetically predisposed inflammatory and pruritic allergic skin disease with characteristic clinical features associated with IgE antibodies most commonly directed against environmental allergens” (Halliwell 2006). It is currently accepted, based on clinical experience, that dogs with atopic dermatitis may be hypersensitive to environmental allergens, food allergens (food induced atopic dermatitis) or both.
Certain breeds are recognised as being at increased risk of developing atopic dermatitis; these include the terriers (West Highland White, Cairn, Fox, Yorkshire), Chinese Shar Pei, Cocker Spaniel, Dalmatian, Bulldog, Boxer, Labrador and Golden Retriever.
The currently favoured route for allergen exposure is via percutaneous penetration, facilitated by a defective epidermal barrier and irregularities in cell mediated immunity.
The classical sign of atopic dermatitis is pruritus. The incidence of canine atopic dermatitis in the general population is unclear – studies quote between 3.3% – 30% – but this appears to be increasing, possibly due to the changing lifestyle of dogs; proposed risk factors include spending more time indoors, increased uptake of routine vaccination and ecto/endoparasite control (Hillier, Griffin 2001).
Clinical signs are rarely seen in dogs less than 6 months old, except perhaps in the Shar Pei. Most become symptomatic between the age of 6 months and 3 years. Late onset disease is less common and is rare in dogs over seven years of age.
Dogs may present with the “triad” of facial rubbing, axillary pruritus and paw-licking but lesions can be found anywhere on the body. Initially pruritus may be seasonal, depending on the inciting cause, but may become perennial as an increasingly wide range of allergens become problematic. Secondary (Staphylococcal) pyoderma and Malassezia dermatitis are common complications.
Otitis externa may accompany generalised pruritus but can be the major, or only, presenting sign.
Feline atopic dermatitis is less well characterised than its canine counterpart. The pathogenesis of disease is unclear and there are no recognised breed associations.
Disease usually manifests between 6 months – 3 years of age as a chronic pruritic, usually corticosteroid responsive dermatosis.
Lesions include miliary dermatitis, over-grooming/barbering, nonlesional alopecia or eosinophilic granuloma complex. These are not pathognomonic for atopy; Flea allergic dermatitis, cutaneous adverse food reactions, neurological and behavioural problems may have a similar presentation.
Age of onset of equine atopic dermatitis is variable, typically between 18 months and 6 years. Clinical signs commonly involve chronic relapsing pruritus and urticaria with scaling, hyperpigmentation, alopecia and secondary trauma. Lesions may develop anywhere on the body but frequently involve the head, mane and tail.
Diagnosing atopic dermatitis
Over the years various workers have developed and published criteria for diagnosing canine atopic dermatitis (Willemse 1986, Prelaud et al 1988).
These criteria were further refined by Claude Favrot and his colleagues following a large study involving over 1000 dogs, published in 2009 and clarified by Olivry in 2010. They are recommended for use in general practice to aid in the diagnosis of atopic dermatitis.
The 2009 Favrot diagnostic criteria for canine atopic dermatitis include:
- Onset of signs under 3 years of age
- Dog living mostly indoors
- Glucocorticoid-responsive pruritus
- Pruritus without lesions at onset
- Affected front feet
- Affected ear pinnae
- Non-affected ear margins
- Non-affected dorso-lumbar area
If 5 criteria are met the sensitivity is 85% with a specificity of 79%. If 6 criteria are met the sensitivity falls to 58% and specificity increases to 89%. Whilst these criteria could lead to a misdiagnosis in 20% of cases, by ruling out ectoparasitic disease and pyoderma, the specificity can be increased considerably. All animals should be evaluated for ectoparasites by skin scraping.
Therapeutic trials for fleas and sarcoptic mange (dogs) should be undertaken.
A combination of systematic and topical treatment for pyoderma and Malassezia should be employed if micro-organisms are contributing to the level of pruritus. Specific culture for dermatophytes should be performed when indicated.
In 2011 Doctor Favrot and his colleagues produced a set of criteria to help identify cats with non-flea induced allergic dermatitis. These criteria include:
- Presence of at least two body sites affected.
- Presence of at least two of the four clinical patterns:
- Symmetrical alopecia
- Miliary dermatitis
- Eosinophilic dermatitis
- Head and neck erosions/ulcerations
- Presence of symmetrical alopecia
- Presence of any lesions on the lips
- Presence of erosions or ulcerations on the chin or neck
- Absence of lesions on the rump
- Absence of non-symmetrical alopecia on the rump or tail
- Absence of nodules or tumours
If 5 of the 8 criteria are met a diagnosis of allergic dermatitis is likely although similarly presenting dermatoses (flea allergy dermatitis, dermatophytosis, adverse food reaction, neurological and behavioural factors) must be ruled out.
As for dogs, these feline criteria may be useful and practical, but it remains essential to eliminate other cause of pruritus to reduce the likelihood of a misdiagnosis.
Similar diagnostic criteria have not, as yet, been established for horses.
The information has been put together by NationWide Laboratories – an exclusive distributor of allervet® in the UK. allervet® offers a comprehensive package to assist in the diagnosis and treatment of allergic disease in dogs, cats and horses. For more details, please visit our website www.allervet.com or call us to talk to the team 01253 899215.