One of the most important skills that any veterinary dermatologist needs, is the ability to recognise the typical distribution patterns of skin lesions seen in the different species. Although there are skin diseases that do not obey the rules, many skin problems present in a very typical way. In equine skin disease for example “Sweet itch” an allergic reaction to midge bites leads to rubbing and hair loss of the mane and tail. Where cats are allergic to flea bites, they often develop a crusty rash on their backs called miliary dermatitis. The same types of pattern analysis can be applied to skin problems that affect the dog especially allergy.
What types of skin allergy do dogs get?
Canine skin allergy can be divided up into four distinct categories. The most common canine allergy is flea allergy which, as its name suggests, is a reaction to flea bites, more specifically the saliva of the flea, which is injected into the dog’s skin when the flea feeds. Food allergy is also common, caused by a reaction to the different components of a dog’s diet. This can be a major constituent such as a protein like beef or chicken, a carbohydrate like rice, or to an additive or flavouring. The Westie along with the Labrador Retriever are recognised as two breeds that are overrepresented when it comes to food allergy. Atopic dermatitis which is the equivalent of atopic eczema in people is defined as an allergic reaction to environmental allergens such as pollens or house dust mites. The Westie is recognised as being predisposed to atopic dermatitis and across all breeds this is usually listed as the third most common canine allergy. The rarest of all of the canine skin allergies is a contact allergy. Contact allergy is common in people because their skin is exposed. Humans can react to a wide range of things including cosmetics, clothes or almost any household object. Most dogs are covered in a thick furry coat which tends to make them less susceptible to contact allergy.
What signs are typical of each type of canine skin allergy?
Dogs with flea allergy tend to have itchy sores and hair loss on their backs. There is an area which is usually referred to as the “flea triangle” that extends from the base of the tail to the middle of the back which is the area where signs are most commonly observed. Flea allergy never affects the ears and rarely affects the feet, face or underside. Therefore, as a dermatologist if I see involvement of the back of a dog, I always want to make sure that treatment is in place to protect an animal from fleas, in order to eliminate that as a cause for their itching.
Contact allergy in the dog is usually caused by something they lie on or walk on. This may be in their bedding e.g. the fabric it is made from, or a detergent, fabric conditioner etc. applied to it. Alternatively, it may be a chemical in their immediate environment such as oil or cement dust. The worst contact allergy seen by the author was on a Jack Russell, who reacted to soap powder on the floor of the owner’s laundrette. Contact allergy only affects the hairless areas of the body which means the itchy lesions tend to be seen on the undersides of the feet and on the hairless abdominal skin and especially in an entire male dog on the testicles. Where a contact allergy is caused by something applied directly to the skin such as a shampoo, cream, ear drop etc. then it will only affect the specific areas where it has been applied. Therefore, unless the contact allergen is a shampoo that can penetrate through a hair coat, contact allergy is always seen on relatively hairless sites so will not affect the back, legs or face.
Atopic dermatitis in its typical presentation affects the feet, creases of the elbows and hocks (ankles on the back legs), the armpits, groin and the face. In addition, it almost always affects the ears. Atopic dermatitis is probably the most common cause of ear disease in the dog and the Westie is no exception. Whilst all breeds can have signs in all of the predisposed areas, it has been shown that different breeds have different weighting of the different sites. A multi-centre international study looking at more than 1,000 itchy dogs studied by 35 veterinary dermatologists in 15 different countries showed marked breed patterns (Wilhem, 2010). A breed such as the French bull dog showed involvement of all of the areas listed but tended to be most severely affected on the front feet and in the armpits. German Shepherd dogs most marked signs were on the front feet and in the groin. A heat map of the areas affected in the Westie is reproduced below and shows that both front and back feet are severely affected, followed by the armpits, groin and ears.
The signs of food allergy in the dog can overlap exactly with those of atopic dermatitis which can make it challenging to differentiate between the two diseases by clinical signs alone. The institution of a low allergy diet is usually recommended to assess if improvement can be seen with a switch of diet suggesting that the food is a trigger for the irritation. Food allergy can however present with non-dermatological signs which can be present at the same time as skin disease. Food allergy commonly causes gastro-intestinal signs. Some dogs present with sickness and diarrhoea whilst other can have more subtle signs such as rumbling, called borborygmus, or increased frequency of defaecation. Where dogs pass motions more than 3- 4 times a day the author recommends some form of dietary trial. Mood swings, behavioural problems and even convulsions (White, 1988; Rosser, 1993) have been attributed to diet in some cases, where a resolution of signs has been recorded after the diet has been changed.
Are signs alone enough to make a diagnosis?
Whilst the distribution of signs is really helpful in the start of the investigation of any skin disease, by themselves the signs should only help direct further investigation and should not in the author’s opinion be used as a basis for treatment. Having said that, Westies are prone to both food allergy and atopic dermatitis and both conditions commonly affect their front feet, but it should be noted that there are other skin conditions that can also affect this area and look identical. Demodectic mange for example is caused by a mite that lives within the skin and causes irritation and infection of the feet. The therapy for this problem is very different to that for allergy and in fact some of the allergy drugs can make it worse. So, whilst a knowledge of clinical signs can help direct further tests, they can never replace them.
References:
Breed-associated phenotypes in canine atopic dermatitis. Wilhem, S et al Vet Dermatol 2010
Food hypersensitivity in 30 dogs. White, S.D JAVMA 1986
Diagnosis of food allergy in dogs. Rosser, E JAVMA 1993
