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Allergic Skin Disease

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Martin C. James

All dogs scratch: it’s part of normal canine behaviour. However if an itch-scratch-itch cycle starts and itching gets excessive, the consequences for the dog can be horrendous.

As with asthma in children, itchy dogs are getting more and more common – probably for similar reasons. Also, there appears to be an inherited predisposition with certain breeds; Shar-Peis, Westies, Labradors, Staffies, German Shepherds and English Setters are all more commonly affected.

The condition starts when the dog is quite young: generally 1-3 years old, although it is occasionally seen in puppies of only 6 months. Dogs with allergic skin disease can be itchy all over, however, the feet, face and ears, tummy and anal area are often worst affected.

There are 3 common types of allergic skin disease in dogs - flea allergies, food allergies and atopy (environmental allergies). An animal can suffer from one, two or three of them. Studies have shown that up to 1 in 3 dogs may be affected with allergic skin disease.

Flea Allergic Dermatitis (FAD)

Most dogs will tolerate a few fleas on themselves with few signs. However some dogs can develop an allergic reaction to the fleas’ saliva, so any bites received produce an intense itch. As fleas live mainly in the thick fur along the back, around the tail base and on the tummy it is these areas that are worst affected, while the feet and head are okay. Complete eradication of fleas from the animal’s environment is necessary for effective treatment, as a single bite will be enough to start the problem. All in-contact animals and the animal’s environment must also be treated – see our article on flea control for more information. An itchy dog must be thoroughly treated for fleas before a diagnosis of FAD can be ruled out.

Food Allergy or Intolerance

This is the least common allergic skin disease. Some animals can develop reactions to particular proteins in the food – beef, lamb, dairy products, chicken, wheat, eggs, corn and soya are the usual culprits, although anything and everything can cause a reaction. To develop a food allergy, an animal must have had prior exposure to the protein so it may be a food the animal has been on for years. Some animals will also show signs relating to the gut, such as vomiting, diarrhoea or increased frequency of defecation.

Diagnosis can only be confirmed by using a food trial. The dog is either placed on a novel home cooked diet, (such as pork and potato, rabbit and rice, or even venison and maize!) or a manufactured hypo-allergenic diet. The best of these contain hydrolysed protein – the proteins can be broken up into pieces too small for the immune system to recognise. The diet needs to be fed for at least 4 weeks, and in some cases up to 3 months to see an improvement. If an improvement occurs then the dog should be fed or ‘challenged’ with the ingredients of the old food, one at a time, to see if the symptoms reoccur. This is the only way to demonstrate that the change of diet was responsible rather than other factors such as flea control, seasons or medication.


Atopy is an allergic reaction to a substance or substances (allergens) in the environment. Like hayfever in people, the allergens can be inhaled, although absorption through the skin is probably more important in dogs. The problem can be seasonal or non-seasonal. Seasonal allergies are most commonly due to the windborne pollens from grasses and trees – these pollens are smaller than those spread by insects, so are more likely to pass through the dog’s skin. Non-seasonal allergies are frequently due to house dust mites; while storage mites, fungal spores, bird feathers or even human dandruff can also play a part. With time, many seasonal allergies develop into all year round problems.

The causative allergens can be determined by either intra dermal skin testing, or a blood test. Skin testing is considered to be more accurate than the blood test, although it does involve sedating the dog, clipping up a large area of skin on the flank, and injecting 40-60 different allergens to see which provokes a reaction. The blood test is just a blood test.


The problem with itchy skin is that it looks like itchy skin whatever the cause. Therefore it is important to approach these cases methodically so no simple causes are overlooked.

  1. Rule out the common culprits:
    • Fleas - All itchy dogs, as well as other in-contact dogs and cats, and the house should be thoroughly treated.
    • Other Parasites - Coat examinations and skin scrapes should be taken to look for mites (mange) and lice.
    • Fungal Infections - Depending on the lesions and the type of dog it may be a good idea to culture the dog’s hair for ringworm, a fungal infection.

  2. Control secondary infections – As dogs scratch they break the skin’s protective barriers and allow bacteria and yeast infections in, adding to the inflammation and itchiness. If we can control these secondary infections adequately, the underlying allergy may be mild enough for the dog to live with. Unfortunately the skin takes a long time to heal so antibiotics may need to be used for extensive courses – typically 4-12 weeks. Special shampoos are available to control yeast infections.
  3. Manage or diagnose – Diagnosis of food allergies and atopy takes time and money. If the dog is older, the condition is mild or there are financial restraints then it may be better just to manage the allergy. However if we can diagnose the allergen and remove it from the dog’s environment, or desensitize the dog to it, the dog may need no drugs at all.

The first stage of diagnosis involves a food trial, particularly if the allergy is all year round. If there is no improvement after feeding a novel diet for 1-3 months then a food allergy can be ruled out. If an improvement is only partial then atopy together with a food allergy may be present.

The next stage is either a blood test or an Intra-Dermal Skin Test, to look for environmental allergens. If the allergy turns out to be something simple like bird feathers and there is a budgie in the house, then re-homing the budgie may be the answer. If there is an allergy to multiple widespread allergens like grasses, house dust mites and human dandruff then desensitization will be a better option. This involves having a vaccine made, containing the offending allergens, which is given to the dog regularly over several months. In 70-80% of dogs the results are very good, although the vaccine may need to be continued monthly for life. Young dogs and those with the least allergens will respond best.

If we decide to manage the problem, or the dog does not respond well to desensitizing, then there are several other treatment options – some or all of which may be required;

  1. Treat for fleas – Can not be emphasised enough. An allergic dog that gets fleas will react ten times worse.
  2. Control secondary infections – If the dog has scabs or sores, antibiotics for bacterial infection; If the coat is greasy or smelly, shampoos for yeast infection.
  3. Reduce itchiness – Essential fatty acids – These significantly reduce itchiness in 30-40% of dogs; when used with antihistamines the two work together to improve results. These must be used long term. Veterinary products like Viacutan work best, although evening primrose oil can also have some effect.


Antihistamines – These work very well with some dogs, although it is necessary to trial different ones to see which is most effective. We normally suggest treatment for 1 week each of 4-5 different types to see which is best. The only side effect possible is drowsiness – otherwise they are very safe, just don’t let your dog operate heavy machinery!

Steroids – for years steroids have been the main treatment for allergic dogs. They reduce itchiness very well and make the dog much happier. However side effects can be severe leading to weight gain, excess thirst and urination, lethargy and even liver or kidney damage. If they must be used, it should ideally be for only short periods, and the dose should be kept as low as possible.

Cyclosporins – These medications were originally used in human transplant patients to prevent rejection. In dogs they can be as effective as steroids at reducing the itch without the long term side effects, although occasionally they initially may cause vomiting. They are now considered the treatment of choice for allergic skin disease, however they are very expensive.

Shampoos – There are many types of veterinary shampoos available to help with itchy dogs. Fundamentally all will help to wash allergens off the dog’s skin, so are particularly useful after walks on long wet grass - you can just do the feet! Some shampoos contain agents to combat bacterial or yeast infections, reduce itchiness, hydrate the skin or remove excess scurf. They are often a useful adjunct to the other treatments so I’m afraid you may have to get used to bathing your dog.

A Cure?

Unfortunately, at present there is no cure for allergic skin disease, so if your dog has it at 2 years old, he will have it when he’s ten. Although having to manage your dog for life seems a depressing prospect, if fleas, bacteria and yeasts are kept under control, most dogs will be comfortable with minimal medication. However, failure to resolve the itchiness and secondary infection leads to chronic skin thickening and hair loss, leaving the dog looking a sight and feeling worse. If your dog is itchy don’t ignore it: we may not be able to cure him, but we can do lots to help.



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