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Allergic Skin Disease

The skin is the largest organ in the body, and has many complex functions. The skin can be affected by hundreds of different diseases, and allergic skin disease is one of the most common disease complexes we see in dogs and cats in south-east Queensland. How allergic skin disease occurs, the signs seen and how we manage it can vary a lot. Similarly, how our pets respond to treatment and management can also vary a lot, making it also one of the most frustrating chronic conditions for both our pets and their family.

What is Allergic Skin Disease?

Allergic skin disease is the reaction observed at the skin level in response to a stimulus. These stimuli can broadly be particles circulating in the air, particles on surfaces, particular chemicals or compounds that touch the skin surface, or compounds in foods or oral treatments. Typically, the skin can only react in a limited number of ways, which means that allergic skin disease can often appear the same regardless of its cause. Sensitivities to certain allergens trigger inflammation and the release of chemical signals in the skin, which leads to a cycle of clinical signs that, in many cases, only lead to more inflammation in the skin. Allergic skin disease can be a vicious cycle of inflammation that causes daily discomfort for our beloved companions. In many cases, allergic skin disease is more pronounced in some individuals due to abnormalities in their skin structure. Normal, healthy skin forms a strong, slightly waxy protective barrier with tight pores that protects the body from allergens and pathogens. Animals with allergic skin disease often have pores that are more open, and their skin may be prone to dryness. These factors can allow allergens and pathogens to penetrate the skin barrier more easily, triggering an immune response.

Put simply, allergic skin disease is the result of altered skin barrier function, and hypersensitivity to allergens.

Clinical Signs

The signs of allergic skin disease can be divided into two categories: the primary signs, and the secondary signs.


A thorough clinical history is key to diagnosing allergic skin disease. Important information is often revealed during discussion with your vet that may not seem like anything significant on its own. Your vet can start to understand whether allergic skin disease is likely to be affecting your pet with information such as:

  • What time of year the skin disease seems to occur
  • How frequently the skin disease occurs
  • Activities prior to the occurrence of the skin disease (e.g. been to the dog park, ran through the back paddock, stuck their head into the garden bushes, floors recently cleaned with a new cleaning product, etc.)
  • Types of foods fed and any recent changes
  • Types of topical treatments applied (e.g. shampoos or conditioners, lotions, powders, flea treatments, etc.)

Information like this can help your vet to start to piece together the picture why your pets skin is reacting the way it is. 

A thorough clinical exam is important in determining which parts of the body are affected and how. Is it just the area that comes in contact with the ground? Is it all over the body? Are there signs of this disease being a chronic, long-term reaction, or are there signs of it being a sudden, acute reaction? Are there other signs that might lead us to consider another disease process?

Some hormonal diseases can cause skin and hair coat changes, and your vet may recommend blood tests to try to determine if these processes are likely to be affecting your pet. Some external parasites can cause significant skin disease, and can be diagnosed by skin scrapings, microscope examinations of hair and/or skin, and other skin examinations during the consult. Sometimes skin disease can occur due to auto-immune disease (the body inappropriately reacting to itself, attacking its own cells), and tissue biopsies are recommended. 

Generally, allergic skin disease is diagnosed in general practice based on the clinical history and physical examination during the consultation. However, this diagnosis is often unable to determine what the animal is allergic to specifically. We can usually only determine that the skin disease seen is likely to be associated with allergies, but often not exactly what the animal is allergic to. Veterinary dermatology specialists can provide additional testing options such as skin-scratch tests to try to determine the exact allergen that causes the skin reaction, and referrals are available. 


This is a complex question to answer, and is dependent on understanding the likely cause of the skin disease in that individual. Treatment for allergic skin disease is often a combination of reducing exposure to the allergen (if possible), managing the itch and inflammation, managing secondary signs (such as infections), and provide support for the skin to try to maintain its natural barrier function. Many allergic skin conditions need management and treatment in some form for the whole of the animal’s lifetime.

Environmental Allergies

Environmental allergies encompass Contact Allergies and Atopy (or Atopic Dermatitis).

Contact allergies require direct contact of an allergen or substance with the skin to cause a reaction, which means that the areas affected are expected only to be the areas with little or no hair. Common causes of contact allergy are plants such as Wandering Jew (Tradescantia species), Moses In The Cradle (Rhoeo species), and ground succulents in the Commelinceae family. However, many substances can cause contact allergies, such as concrete, floor polishes, and other household chemicals. Like humans, contact allergies can be very specific and vary between individuals.

Management of contact allergies are focused around avoiding contact with known allergenic substances, and reducing skin exposure to allergens. Using hypoallergenic, oatmeal based shampoos and conditioners can help reduce the allergen load on the skin by physically washing away the substance from the skin. Some people fit bodysuits to their pets to reduce their exposure to allergens. Treatment for contact allergies may involve using medicated lotions and sprays, or oral steroids, antihistamines and other anti-itch medications. These may only be needed in the event of a sudden allergic reaction if the substance can be avoided most of the time. Avoidance is not possible for all cases though, and sometimes long-term medical management is needed.

Atopy is a generalised skin disease complex that is a result of abnormal skin barrier function, and hypersensitivity to allergens that do not require direct contact with the skin, and occur over any region of the body regardless of hair-coat. Atopy is often likened to eczema and hayfever, and may be triggered by things such as pollens, moulds, and dust mites. The changes in the skin barrier allows allergens and infections to more easily penetrate the skin, which worsens the immune reaction. Many atopic animals experience frequent secondary infections, which worsen their condition. 

Management of atopy can be approached in three ways: reducing the immune reaction, improving the skin barrier function, and treating the infections. The immune reaction can be treated with a vaccine following consultation and assessment with a veterinary dermatologist to desensitise the animal to the allergens. This immunotherapy aims to actually make the animal less sensitive long-term. Another type of vaccine is available with your regular vet that binds the allergens before they are able to trigger any immune response. We can also manage the immune reactions using anti-itch mediations and/or steroids. The skin barrier function can be improved by the addition of supplements containing essential fatty acids, and topical washes and conditioners to help keep the skin hydrated and provide some temporary relief from irritation. Infections can be managed and controlled by oral medications, topical creams, and medicated shampoos and rinses.

Management and treatment of environmental allergies can be complex, and must be tailored to the individual needs of each animal. The same treatment may not work exactly the same patient to patient. This is why in-depth discussion with your vet is key to finding the best management solution for you and your pet. 

Flea Allergy Dermatitis (FAD)

FAD results from a hypersensitivity to flea saliva that comes in contact with the skin when a flea bites and feeds from its host. FAD most commonly affects dogs, and cats are more likely to react to midges or mosquitoes. FAD in dogs often presents as intense itch over the rump and tail, and insect bite hypersensitivities in cats often affect the face and ears (areas with little or no hair). While the allergic reaction process is the same as other allergic skin diseases, management is more focused around avoiding exposure to the bites of these insects. Initial treatment may involve steroids, anti-itch medications, and antibiotics. Long-term treatment for FAD involves a strict flea control plan to minimise the flea burden on the individual dog. For cats, midge and mosquito hypersensitivities may require the cat to be restricted to the indoors, or the application of insect repellents to the affected areas. 

Dietary Allergies

Dietary allergies are a very commonly discussed topic with owners at the moment, with the emergence of many new diet options in stores. Grain-free diets are very popular at the moment, with many owners reporting that their pet’s skin appears to improve on theses grain-free diets. The concept of grain-free has been adapted from human nutrition, where grains are often a source for dietary allergies and intolerances. However, the same cannot be said for our four-legged friends. True dietary allergies are actually quite uncommon, seen in less than 1% of dogs. They are slightly more common in cats, affecting up to 6% of cats. The vast majority of dietary allergies in dogs and cats are based on proteins. The most common proteins associated with dietary allergies are beef and chicken, followed by lamb and fish.

That being said, we cannot discount the observations of many pet owners when it comes to their furry friend’s skin and coat. So why is it that grain-free diets seem to help? The theory is that grain-free diets contain higher quality proteins and higher essential fatty acid content. As we discussed earlier, when it comes to the natural skin barrier function, diet can make a big difference. A better quality diet on its own can improve the overall health of the skin and coat, which in turn will reduce the ability of allergens and infections to penetrate the skin barrier and cause an immune response.

True dietary allergies cause skin disease that appears similar to environmental allergies. Unfortunately, unlike people, blood and skin testing can’t diagnose dietary allergies in animals. Instead, we must rely on excluding the likely proteins causing the allergy from the animal’s diet. These are commonly referred to as “food elimination trials”. This can involve feeding a “novel protein”, which is a protein that your pet has never been exposed to before. This can be difficult for a lot of pet owners. The alternative is feeding a hydrolysed protein diet that is commercially produced and recommended by your vet. The proteins in these diets have been treated with enzymes to breakdown the protein into its building blocks that are considerably smaller fragments. These protein fragments are too small for the immune system to respond to, but will still provide the nutrition that the animal needs for a healthy lifestyle. If there is a good clinical response in the animal’s skin and overall condition, then we can consider “rechallenging” their immune system by carefully reintroducing regular proteins back into the diet and monitoring for a skin response. In some cases, the animals do best by simply continuing on the commercial hydrolysed protein diets, and these are designed to be complete and balanced diets for long-term use. 

If you have any further questions regarding allergic skin disease, your pet’s diagnosis or their treatment plan, please contact our friendly staff on 07 3297 0803, or come in and speak with us.
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