Stuart Fitzgerald
Dr Stuart Fitzgerald (MVB MANZCVS, University College Dublin)
Itchy dog

Just as hay fever and food allergies are common complaints in human medicine, veterinary surgeons are being presented with increasing numbers of dogs with allergies.

Unlike humans, in whom signs of rhinitis and conjunctivitis usually predominate, causing runny eyes and sneezing, dogs more commonly develop skin problems as a result of hypersensitivities. Around one in six allergic dogs will develop hay fever-like signs, and these symptoms often occur with, rather than instead of, skin irritation.

Development of Allergy

Allergy, or hypersensitivity, reflects over-activity and dysfunction of the immune system. The primary function of the immune system is to recognise and eliminate ‘foreign’ substances posing a risk to health. These foreign substances are often viruses, bacteria, or parasites, and are termed antigens. However, in allergy, the antigen being targeted by the immune system is actually benign, and is called an allergen. Symptoms of allergy are caused by the body’s defence mechanisms rather than by the allergen itself.

The mechanism underlying the development of allergy is often an impaired barrier, in the respiratory tree, the gut, or the skin, between the outside world and the blood stream. Allergens, which are tiny particles, pass through these impaired barriers and are carried by antigen presenting cells. These interact with other cells, called lymphocytes, which then produce antibodies to the allergen. Antibodies are proteins, custom-made to attach tightly to individual antigens/allergens. In genetically predisposed dogs, these antibodies are produced in great numbers, and are shipped around the body for the purpose of defending from future ‘attack’ by the allergen.

The antibodies, which in the case of allergy, are a particular type named immunoglobulin E (IgE), are carried on the surfaces of yet other immune cells that contain granules of caustic and inflammatory chemicals, including a protein called histamine. Attachment of the surface IgE to a recognised allergen causes an eruption of these granules, a process intended to degrade and destroy the allergen, but which in fact causes pain and irritation to the animal.


Atopy is the most frequently encountered form of allergy in veterinary practice. As discussed above, it is due to increased permeability of the skin or respiratory tract, combined with a genetic predisposition to developing hypersensitivity. This predisposition may or may not be inherited, as genetic disorders often arise through chance mutations of genes, but strong breed associations do exist. For example, West Highland White Terriers, Shih Tzus, Labrador Retrievers, and Shar Peis, to name but a few, are very prone to allergy, and pups born to parents with atopy are very likely to be affected themselves.

Signs of Atopic Dermatitis

The signs of atopic dermatitis usually first manifest between 6 months and 3 years of age. Itch is the predominant sign, and may be noticed by owners before other signs develop. Over time, the skin becomes more irritated and damaged, and reddening, scabbing, and flaking become noticeable. Pustules (spots) and blisters may appear, further irritating the dog, and the vicious cycle of itching and scratching leads to progressively worsening damage. As the skin becomes more damaged, its permeability further increases, and dogs with atopy tend to develop hypersensitivity to increasing numbers of allergens over time, providing another reason for early intervention and management of this debilitating problem.

Many atopic dogs are seasonally affected, with spring and summer being the times of year when most allergens abound. Although dust mite allergy is now the most prevalent cause of symptoms in atopy, pollens and plants are also common allergens, and so symptoms may only appear during the warmer months of the year.

The sites usually affected by atopy are the areas around the lips and eyes, the ear canals (with otitis being the only sign in many animals), the paws, and the perineum. Redness and itch may be the only signs in mild cases, but most dogs will eventually develop secondary infections with bacteria and yeast, self-excoriations, and hair loss. A strong ‘doggy’ smell is noticeable in many atopic patients, and this is largely due to the increased numbers of Malassezia yeast living on the skin. This yeast overgrowth also creates a greasy feel to the skin, which may be noticeable when stroking the dog.

Diagnosing Atopy

Correctly diagnosing atopy requires a careful a. I would stress the word ‘correctly’, as many dogs are erroneously diagnosed with the condition, when their skin disease is actually due to other conditions. The treatments used to treat atopy will alleviate symptoms in any itchy dog in the initial stages, but may in fact, worsen the underlying condition if used inappropriately. Atopy requires a diagnosis of exclusion, meaning that there is no definitive test that can be used. Instead, we must prove that a patient’s itchy skin is not caused by some other problem. Parasites, flea allergy, primary infections, and food allergy must all be ruled out before committing to the long term course of treatment required for an atopic dog. The procedures required to rule out these disorders is outlined below.

Food Allergy

Chronic Allergic Otitis

Chronic Allergic Otitis (self /

In around 10–20% of allergic dogs, their symptoms are caused by hypersensitivity to food rather than environmental allergens. The lining of the gut is rich in immune cells, and the intestinal contents are laden with foreign, potentially allergenic, proteins. As with atopy, a genetic predisposition in individual dogs allows them to develop inappropriate responses to some of these substances. Although the evidence for breed predispositions is far weaker in food allergy, Labrador Retrievers and West Highland White Terriers appear to be affected more frequently than most. In addition, some breeds are prone to developing a condition called protein-losing enteropathy, and have by definition increased gut permeability. These include the Old English Sheepdog and Wheaten Terrier, and so these too may be more likely to develop food allergy.

The distribution of the skin changes in food allergy are very variable, although it is more common to see patients suffering only with ear problems than it is in atopy. The symptoms can appear at any age, and may manifest even in elderly dogs with no prior history of skin disease. There is a common misconception among pet owners that allergies develop to new foods, whereas, in fact, most food allergies develop after a dog has been fed the offending protein source for two years or more.

Approach to the Itchy Dog

Flea Closeup

Closeup of a Flea

The first step for an owner faced with an itchy dog is to ensure he is up to date with anti-parasite treatments. Fleas are by far the most common reason for a dog to scratch, and are usually easy to deal with. In a light infestation, applying a good quality spot-on product or oral medication purchased from a veterinary clinic or pharmacy will resolve the problem quickly, as these products will kill all fleas within a matter of hours. In heavier infestations, it is also necessary to treat the household and the dog’s bedding, as a great number of fleas live for long periods in the environment. It is important that all in-contact animals are treated at the same time so as not to allow the fleas anywhere to hide.

A dog that is scratching because of fleas should show signs of relief within 48-hours of receiving one of these treatments. I would strongly advise any owner whose dog continues to scratch after this point to seek veterinary attention. While there are some at-home measures that may be taken to help allergic dogs in the long term, and which are discussed below, these do not provide immediate relief, and the itch-scratch cycle usually results in a progressive and worsening problem over time.

Ruling out Parasites

From a vet’s point of view, the first step in these cases is to rule out parasites as a cause of the symptoms. Mites and lice are other insects that commonly cause itching, are harder to identify, and often harder to kill than fleas. Finding these creatures, or ruling out their presence, requires the veterinarian to perform multiple skin scrapes. This involves using a bladed instrument to scrape a layer of cells from the affected skin. These cells and associated debris are then mounted on a slide and examined under a microscope. At least five, and preferably seven scrapes should be taken from different areas. If mites are present, their species should be identified, something the vet can do simply by referring to reference pictures, and appropriate measures should be taken to eliminate them, including treatment of in-contact pets if necessary.

Tape smears should be taken from itchy skin, using a special form of sticky-tape. These smears can then be stained and examined for signs of bacterial or yeast infection. Samples of hair or pus, particularly from ears, should be submitted to a laboratory to identify the species of microbes present, as well as their susceptibility to a range of medications. Treatments targeting these microbes, for example antibiotics, antifungals, or medicated shampoos, can provide great relief from itch, and these therapies should be pursued before specific anti-inflammatories are given, as the anti-inflammatory medication will mask symptoms, making it difficult to assess the effect of each treatment. It is not unusual for antimicrobial treatments to be continued for 8 weeks in cases of established infection.

Food Trials

Food Trials

In persistently itchy dogs, a food trial is now indicated. Unfortunately, the only way to either identify or rule out a food allergy is by eliminating all potentially offending foods from the dog’s diet. This may be accomplished by either feeding a novel protein source (i.e. a food which the dog has never eaten before) or using a hydrolysed diet.

These hydrolysed diets are prescription foods containing only proteins that have been chemically broken down into such small fragments that they are no longer capable of inducing an allergic response. A food trial is only effective if all other sources of protein are eliminated, and it must be continued for a minimum of 2 to 3 months. Other foods, milk, and even flavoured treats need to be withheld, something which can be very difficult to accomplish, especially in homes with children or other pets.

The Process of Elimination

A positive response to a food trial is indicated by a decrease in itching, which is usually evident by around 6 weeks into the trial. Should the dog appear symptom-free by the end of the trial, most vets will advise the staged introduction of ‘challenge’ foods. For example, the owner may introduce small amounts of chicken with the elimination diet over the course of 2 weeks. If the dog does not begin to scratch again, chicken may be considered a ‘safe’ food, and the next protein (e.g. beef) may be introduced in the same way. Food-allergic dogs will normally begin to scratch within 1–10 days of the introduction of an allergy-inducing protein, and in this way we can hope to establish a range of foods that can be fed longer term.

Anti-Allergy Medications

For those dogs that do not respond to the food trial, the next step is the use of specific anti-allergy medications. A range of such drugs are now available, though the long-established option of steroid drugs is usually the first to be tried. A dog that has undergone the staged investigation outlined above without significant improvement, and who responds well to the use of steroids, can now be diagnosed with atopy. Steroids are effective, and usually very affordable, and so many owners elect to continue their use long term. However, they are also well known to cause many adverse side effects, and they are best suited to short-term use in dogs with seasonal allergies.

Safer Alternatives to Steroids

In recent years, a growing range of safer alternatives to steroids has become available to vets. These mediations share a common mode of action, in that they suppress the immune system and its exaggerated response to allergens. However, they do so in different ways, with some having very specific actions on cellular pathways involved in hypersensitivity, and others simply subduing all immune functions. Specific antibody therapies are another recent development, which offer the exciting prospect of managing atopy without compromising the pet’s normal immune function. The downside to all of these other medications is their cost, which can be prohibitive for some owners, particularly when treating a large breed dog.


One diagnostic option omitted from the process above is that of specific allergen testing. This is an option that is often misunderstood and abused in the diagnosis of allergy. Two types of testing may be performed: intradermal testing, where a large number of potential allergens are injected into distinct sites over a patch of skin, and the ensuing reaction is observed; and IgE blood testing to measure antibody levels. Positive results to these tests do not prove the presence of allergy; they simply indicate exposure of the immune system to an allergen. This is where these tests are misused, as they are sometimes used as a ‘shortcut’ to obtaining a diagnosis. Unfortunately, this is a flawed approach.

However, should a dog test positive on these tests having gone through the full diagnostic work-up above, then we may have more confidence that the results are indeed indicative of at last some of the hypersensitivities in the patient. In most cases, the allergens identified are so prevalent in the environment that it is not possible to eliminate them. However, their identification allows the preparation of a type of immunologic vaccine, tailored to the individual dog.

By developing an individualised vaccine, we can administer tiny quantities of the offending allergens by injection, very frequently at first, and then both reducing the frequency and increasing the volume administered. The concept underlying this is that the immune system can develop a tolerance to the allergens contained in the preparation. While this does not happen in every case, around 50% of dogs show clear signs of improvement with the use of this immunotherapy. Though very few respond so well as to not need any further treatments, it is the only therapeutic option offering even the slightest hope of a permanent cure for atopy.

Supportive Measures

Specialised Shampoo to help with allergies

Although the appropriate use of prescription medications to control itching and secondary infections will produce a good response in most dogs, consideration must be given to the mechanisms underlying the development of allergy. The impaired barrier function discussed above can be improved in many atopic dogs by using prescription shampoos, designed to build up a protective layer of lipid within the skin’s surface. These shampoos can be used regularly – usually once a week – to reduce the burden of allergen reaching the dog’s immune system. Fish oils, especially of the omega-3 family, have a range of anti-inflammatory effects, and can offer significant benefit to some allergic dogs. In general, the more expensive products are far more effective, and although human fish oil supplements can be used, there are several veterinary products specifically formulated for atopy that tend to give better results.

During acute flare-ups of allergic skin disease, antibacterial and antifungal shampoos may be used in concert with medications to clear secondary infections and reduce the need for systemic drug use, and to speed up the resolution of the itch. However, these are not suitable for long-term use, as they tend to strip the skin of essential lipids and other protective elements. The number of different shampoos and topical treatments available is quite bewildering, and it is very useful to take the advice of a veterinary surgeon or nurse before using a particular product on an allergic dog.

Finally, parasite control is vital in any animal with sensitive skin. Although it may not be the primary issue, a light flea infestation on an atopic dog can lead to a dramatic and very distressing worsening of symptoms. Effective flea treatments should be routinely used on all animals in the household to prevent the introduction of these parasites.


In dogs, signs of allergy are usually confined to the skin, though a small percentage can show hay fever-like signs. While many are affected year-round, other allergic dogs may have seasonal symptoms, which are most commonly seen in the spring and summer months. Though achieving a cure is an unrealistic expectation in most cases, a methodical, staged approach to diagnosis and treatment should allow dogs with allergies to lead comfortable and happy lives.