Dr Primrose Moss – Freelance Veterinary Writer

Have you ever had an itch you just can’t scratch? That’s life for a dog with allergic skin disease. Also known as atopy or atopic dermatitis, allergic skin disease is seen in 10-15% of dogs¹. Causing persistent itching, soreness and secondary infections it can seriously impact the lives of both dogs and their owners. 

In this first of two articles, we’ll focus on what causes allergies in dogs and how we can recognise them; understanding these allergies is key to helping these dogs live the best possible lives.

What are allergies?

We’re all familiar with allergies in humans – from hay fever to anaphylactic reactions – but this doesn’t necessarily translate well to dogs. Allergies can be defined as an exaggerated immune response to allergens, usually from either food or the environment. Unlike humans, anaphylaxis is not a typical presentation due to allergies; it can occur but is incredibly rare and usually a response to an insect bite or sting. They can experience allergic rhinitis (inflammation of the nasal passages) and conjunctivitis, like hay fever in humans, but this is usually relatively mild; they may experience some sneezing, ocular discharge or a runny nose.

Usually, when people refer to allergies in dogs they are referring to atopic dermatitis, also known as allergic skin disease. Skin changes are the main sign of allergies in dogs, in contrast to humans. This is true for both dietary and environmental allergens – you might think a food allergy would cause gastrointestinal signs like vomiting or diarrhoea, but this is relatively unusual. It’s important to note that dietary allergies should not be confused with diet-responsive Inflammatory Bowel Disease (IBD), which is an immune-mediated disease that certainly does cause gastrointestinal symptoms. 

Other types of allergy do exist – for example, contact allergies and flea allergies – but these are less common.

What causes the allergic reaction?

Dog allergies are an immune ‘overreaction’ to proteins; the immune system perceives the proteins in common substances like beef or pollen to be a threat. White blood cells that incorrectly recognise the proteins as a threat produce a specific type of antibody (IgE), which stimulates other white blood cells to release inflammatory molecules. This process results in itching, redness and other skin changes. 

For environmental allergens, it’s believed this is largely the result of allergens being detected on the skin itself – which makes sense. For food allergens, the link between gut and skin is a little less clear. While research has proved a strong link, the exact mechanism is still under investigation. 

Environmental allergens include a range of specific pollens, dust mites, mould spores and insect bites. Food allergens can be any protein, from those in chicken to fish. Proteins in different foods can also be ‘cross-reactive’ – for example, a dog that is allergic to chicken may well also be allergic to duck and turkey. Significantly, there is no evidence for allergic skin disease being triggered by things like pesticides or antibiotics and allergies to wheat (and gluten) are incredibly rare. Well intentioned owners may think an organic, grain free diet is the best option, but this isn’t necessarily the case (and it’s worth looking into the risks potentially associated with grain free diets).

Can we predict which dogs will be affected before they show symptoms?

Unfortunately, there’s no way to diagnose a dog with allergies before they show symptoms. There is evidence, however, that there is a genetic predisposition to allergic skin disease – breeds like West Highland White Terriers, Boxers, French Bulldogs and Staffordshire Bull Terriers are overrepresented², although any dog can be affected.

What changes do allergies cause in the skin?

The activation of white blood cells by allergens results in an ‘inflammatory cascade’ in the skin, in which molecules including cytokines, histamines and prostaglandins are released. These lead to increased blood flow, resulting in the classic pinkish red skin seen in allergic patients, and itching (pruritus). 

Atopic dermatitis is also associated with an abnormal skin barrier in dogs³ – although it’s not yet known if this contributes to the development of allergies or if it is the result of them. The skin barrier is essential for retaining moisture and preventing the environment – including allergens – from penetrating the body. 

The combination of an abnormal skin barrier and inflammation combine to predispose dogs with allergies to further skin problems. When an allergic dog’s skin ‘flares up’, we frequently see multiple conditions that have combined to cause intense itching. This commonly includes bacterial skin infections (pyoderma) as well as yeast infections with their characteristic ‘cheesy’ odour, commonly seen in the ears as well as the paws and skin folds. Ear infections (otitis externa) are also very common – the inflammation and mild swelling of the skin create a warm, moist environment in the ear canal that is perfect for bacteria. Parasites like fleas or mites can also contribute. 

Any dog with recurrent skin or ear infections should be evaluated for allergies – it’s essential to have a long-term plan to manage allergies in order to prevent recurrent secondary infections. Recurrent ear infections, for example, cause permanent changes to the ears that can make it almost impossible to prevent further infections and sometimes necessitate surgical removal of the ear canal. Managing allergies and reducing flare ups and secondary infections is crucial.

So, how is allergic skin disease diagnosed?

Any dog with a history of itching or recurrent skin or ear issues should be evaluated. It’s important to note that every dog is unique, and we often don’t see all the possible symptoms together. For some dogs, their main symptom will be recurrent ear infections. For others, it may be persistent licking and chewing at their paws. 

Vets base their diagnosis of allergic skin disease on a combination of factors described by Favrot’s (2009) diagnostic criteria⁴:

• Onset of clinical signs before 3 years of age
• Mainly indoor lifestyle (true for most dogs in the UK today)
• Itching that responds to steroids
• Itching that began with no skin changes (i.e. there was no rash or infection before they started to itch, though one may have developed later)
• Front feet are affected
• Inside the ears is affected
• The outside of the ears and ear margins are unaffected
• The back (lumbodorsal area) is unaffected

Not all of these criteria need to be true for a dog to have allergic skin disease. Around 85% of affected dogs will fit at least 5 criteria, though it’s important to note that some dogs with 5 of these criteria (approximately 30%) may not be suffering from allergic skin disease. There’s no perfect protocol to detect all cases of allergic skin disease, but your vet will evaluate your dog using these criteria as well as further testing (including treatment trials) and their own clinical experience. 

But what would these criteria actually look like in your dog? 

It’s common to think that environmental allergies should be seasonal, as opposed to dietary allergies which tend to be present all year round. This is frequently the case – but common allergens like dust mites are present all year round and can cause persistent symptoms. Any dog can occasionally be a little itchy or chew at a paw, but if you notice your dog persistently licking and chewing their paws or frequently scratching their ears, allergies could be to blame. Brown saliva staining around the paws is a classic sign. The skin of the belly and between the pads on the bottom of their feet are also frequently a brighter pinkish-red colour than usual. Some allergic dogs will have dry skin but others can have a greasy coat. 

Looking inside your dog’s pinna (the outer part of their ear), the skin inside their ears may also be pinker than in a ‘normal’ dog, or even red. If you notice a small amount of dark wax this can be normal but excessive wax and discharge are suggestive of an ear infection. Dogs with yeast infections secondary to allergic skin disease will also have an unpleasant, cheesy odour. Allergic skin disease can also affect the anal glands, predisposing dogs to recurrent anal gland impaction and/or infections and contributing to ‘scooting’ behaviour.

It should be noted that the vast majority of allergic dogs begin to show symptoms within the first few years of life; however, these can initially be mild and may not be picked up until they’re older. Food allergies may also develop when dogs are older; even if your dog has been fed the same food their whole life without issues, a dietary allergy should still be ruled out if they begin showing symptoms. Symptoms of dietary allergies are often indistinguishable from the symptoms caused by environmental allergens, although in some dogs occasional gastrointestinal signs may be seen. The best way to diagnose a food allergy is to trial treatment with a hypoallergenic diet (discussed in Part 2), rechallenging the patient with other foods at the end of the trial to see if their skin worsens in response. Dogs can also have both food and environmental allergies, in which case a partial response may be seen.

Because secondary infections are so common in dogs with allergic skin disease, it’s worth noting that your vet may also recommend diagnostics like skin scrapes to rule out other conditions prior to diagnosis. It’s essential to rule out or treat other possible causes like parasites when treating for allergies.

How does this affect treatment of allergies?

Allergic skin disease is a complex, lifelong condition. It cannot be cured, only managed – and the symptoms and response to treatment can vary over time, so it’s essential to work closely with your vet to keep your dog’s symptoms controlled and reduce the frequency of flareups. Some allergies require constant treatment while others can be treated when they flare up – for example, some dogs will have a regular flareup lasting a few weeks when a specific pollen appears in spring but are otherwise unaffected. Every dog requires a tailored treatment plan that takes into account their individual symptoms and the possible causes of their allergies. Some dogs can be managed partially or entirely by diet changes, while others require a combination of medication, supplements and home care to keep them as healthy and itch free as possible. 

Diagnosis of allergies can be tricky, especially if we only see a dog sporadically when they have an ear or skin infection, prescribe them treatment and don’t have a chance to discuss any longer-term concerns. If you suspect your dog may be affected then speak to your vet next time you see them. It’s often difficult to discuss allergic skin disease with owners due to the time limits in practice – and it’s difficult to explain that even though your dog’s ear infection will clear up with antibiotics, he may still need longer term treatment. Opening up the conversation with your vet can definitely help.

We all – quite understandably – want a perfect treatment that will ‘fix’ our furry friend and allow us to avoid the vets until they’re next needed. I hope this article has shed some light on why treating the cause of these skin problems – rather than fire-fighting the flareups – is the best way to keep our beloved pets happy and healthy.

Next month we’ll take a closer look at the different methods we have for treating allergies.

References

1 – Hillier, A., & Griffin, C. E. (2001). The ACVD task force on canine atopic dermatitis (I): incidence and prevalence. Veterinary immunology and immunopathology, 81(3-4), 147–151. https://doi.org/10.1016/s0165-2427(01)00296-3
2 – Picco, F., Zini, E., Nett, C., Naegeli, C., Bigler, B., Rüfenacht, S., Roosje, P., Gutzwiller, M. E., Wilhelm, S., Pfister, J., Meng, E., & Favrot, C. (2008). A prospective study on canine atopic dermatitis and food-induced allergic dermatitis in Switzerland. Veterinary dermatology, 19(3), 150–155. https://doi.org/10.1111/j.1365-3164.2008.00669.x
3 – Olivry T. (2011). Is the skin barrier abnormal in dogs with atopic dermatitis?. Veterinary immunology and immunopathology, 144(1-2), 11–16. https://doi.org/10.1016/j.vetimm.2011.07.014
4 – Olivry, T. and (2010), New diagnostic criteria for canine atopic dermatitis. Veterinary Dermatology, 21: 124-127. https://doi.org/10.1111/j.1365-3164.2009.00776.x
Jackson, H. A., & Mueller, R. S. (2012). Atopic dermatitis and adverse food reactions. In H. Jackson & R. Marsella (Eds.), BSAVA Manual of Canine and Feline Dermatology (3rd ed., pp. 130–151). British Small Animal Veterinary Association.