Allergic skin disease (atopic dermatitis)


The commonest signs of allergy in dogs occur in the skin.The condition is known as allergic or a topic dermatitis (AD). The affected animal will be itchy and can go on to develop inflamed skin, hair loss and secondary bacterial and yeast infections. The medical term for itchiness is pruritus. The most commonly affected areas are the ears, face, paws and along the abdomen including the groin and axilla (“armpits”). In some dogs, there can also be digestive upsets caused by allergy. These signs can rapidly become very distressing for both the pet and its owner.  

The following information is a guide to the various ways your pet’s condition can be investigated and then treated.  Unfortunately, there is no cure for allergy so it is possible that the treatment will be required for the pet’s lifetime.

First visit to vet - what to expect

Initially, the vet will perform a detailed examination of the pet and discuss relevant information e.g. its diet, if the signs occur at certain times of the year, the pet’s age when the signs were first seen, if the pet has been exposed to parasites (foxes in the garden or travel abroad for instance)and if any digestive upsets occur. Before a diagnosis of allergy can be made,other causes of itching and hair loss need to be ruled out.  Samples of hair and little scrapings of the skin are taken to look for bacteria, mites,yeasts and fungi such as ringworm.  In some cases, a blood test will also be performed.  If any parasites or infections are discovered in these initial tests, they will be treated before going on to investigate the possibility of allergy.  

Cutaneous adverse food reaction and exclusion diets

The next stage of the investigation is to rule out a condition known as cutaneous adverse food reaction (CAFR). Food allergy is thought to be the cause of about 5% of the cases of itchiness in dogs.  If this is found to be the cause the treatment is, simply, to identify the foods which trigger the signs and then avoid them. Unfortunately, at this time, there are no reliable blood tests to identify food allergies accurately.

The commonest ingredients, which are known to trigger allergic reactions in dogs are beef, dairy and chicken. Most standard commercial diets will contain at least one of these ingredients even some labeled as “hypoallergenic”or “sensitive”. The only reliable method to diagnose CAFR is by doing an elimination diet trial.  This can be in the form of a home cooked diet or a commercially manufactured one. With a home cooked diet, a source of protein has to be selected which the dog has never experienced before. This can be very difficult to manage as standard dog diets can contain a range of different proteins. They are also time consuming to prepare and it can be difficult to make sure the meals contain the correct range and balance of nutrients. The most reliable and convenient way to conduct a diet trial is to use a diet where the protein has been hydrolysed. This means that, in the manufacturing process, the protein molecules have been broken down into tiny pieces, which should not cause an allergic reaction. These products are made under very controlled and hygienic conditions to prevent any contamination in the factory. For the first week, it is recommended that the dog be transferred gradually to the hydrolysed diet by mixing it with their usual food.  Once on the diet exclusively, it is essential that the dog eats nothing apart from this food for 8 weeks. This includes treats and any other scraps.

Before starting this diet, the pet is treated for any infections and the itchiness is brought under control so that it is comfortable.  Once the pet has been on the diet for 7 weeks or so this treatment is withdrawn and the pet closely observed for any sign of relapse.  If the symptoms all recur immediately and while the pet is still eating this diet exclusively it is then possible to rule out CAFR.

On the other hand, if the pet remains well once the treatment is withdrawn it can be assumed that a food allergy is wholly or partly responsible for the symptoms.

Then, to identify the ingredients responsible for the pet’s clinical signs individual protein sources are added one by one to the hydrolysed diet.  One ingredient can be added for a period of 1-2 weeks and if no relapse is noted this can be changed for a second ingredient with this being repeated until all possible sources from the previous diet have been tested.  Any foods, which trigger a recurrence of the symptoms, can then be avoided by careful examination of ingredient lists on bags of food. Even some medications such as worm tablets contain protein-based flavourings so even these must be checked carefully in an allergic dog.

Once we have eliminated CAFR and parasitic and yeast infections we can then consider the diagnosis of allergic or atopic dermatitis.

The diagnosis is made after the ruling out of other causes of itchiness and by considering the breed of dog (some breeds are much more commonly affected than others), the age the symptoms started (most commonly in dogs of 1-4 years although there can be exceptions) and the parts of the dog’s body where the itchiness occurs.  

Causes of allergy

The underlying cause of this condition is quite complex, but a simple explanation is that the animal’s skin barrier is defective.  This barrier is a layer of cells, which resemble the bricks in a wall with grouting or cement filling in the gaps.  In these animals’ skin, it is thought that the cement or grouting is missing or reduced so that “allergens” or proteins from the environment can penetrate the skin and set off an allergic reaction.  In addition, the defective skin barrier allows moisture to escape from the skin resulting in a dry coat and scurfy and dry skin.  The allergic dog treats these “foreign” proteins in an excessive way and mounts a reaction,which in turn causes the skin to become inflamed and itchy.  Examples of these allergens are grass pollens and house dust mites, which are present in the environment of every dog.

Treatment of allergic dermatitis

Treatment can consist of one or several of the following:

  1. Corticosteroids (often known as steroids) have been used to treat AD for many years.  They are very useful as a short-term measure as they work very quickly and they are effective. They are not suitable for long-term cases due to the high risk of side effects.
  2. Cyclosporines are used in human medicine to prevent organ rejection after transplantation. In animals they are occasionally used to treat AD.  They can take a few weeks to take effect and are expensive so they are not used very often. Cyclosporines are the drug of choice in some itchy cats.
  3. Apoquel is a very useful drug, which has been available since 2013.  It interferes with the chain of chemical reactions, which occur in the allergic skin triggering the dog to feel itchy and start scratching. Apoquel works very quickly and makes most dogs comfortable in a day or two.  Once the dog has stopped scratching the skin becomes less inflamed and painful.  Dogs taking this drug long term need an annual blood test.  It is a safe treatment in most cases.
  4. Cytopoint is a new treatment.  It is an antibody and it is known as a biological therapy.  The drug binds to one of the chemicals produced in the allergic dog’s skin and stops the itchiness.It is quick to take effect and there are no reported side effects.

Immunotherapy requires a blood sample from the pet to be analysed for the presence of antibodies to a very wide range of allergens.  Once the results have been scrutinized a special “vaccine”,containing these allergens is manufactured specially for the patient. Then, the dog or cat is injected with the vaccine at gradually increasing doses until the final dose is arrived at.  The injections are finally done once monthly and can be administered by one of the vets at the practice or the owners can be taught how to do it at home themselves.  Immunotherapy is very safe but it is a commitment and if effective, needs to be continued for the dog’s whole life.  There are various reports on how effective it is. Some dermatologists say that only about 40% of animals benefit from the treatment and others report success rates of 60%.  In many cases, while not actually curing the patient, immunotherapy can reduce the frequency of other treatments being required.  Some animals, which are allergic to grass and other plants can have much worse symptoms in the summer months and might need additional help at this time.

In addition to the above treatments, most animals can have some partial relief of symptoms by using special shampoos to moisturize the coat and skin, dietary supplements to help improve the skin barrier, topical steroid sprays to treat small areas of inflammation and special diets with added oils.Most dogs need a combination of some or many of these treatments.  Regular checks at the surgery can be very useful to monitor the animal’s progress. If possible, once the animal is stable, any treatment should only be withdrawn after discussion with the vet.  Once relapsed, it can sometimes be difficult to attain the level of stability the animal had previously.