Updated: Sep 20
Many dog owners know the frustration of having a dog who is constantly itchy. Whether it’s being woken from a sound sleep by a shaking bed and jingling collar, or listening to never-ending paw licking during dinner, itching is a condition that affects the comfort of the pet, but also their human family.
Chronic itchiness, or pruritus, can lead to skin damage, painful sores and infections requiring multiple visits to the vet and the prescription of medication such as tablets, injections and frequent medicated baths. It’s no wonder that owners of itchy pets are eager to find a quick, effective solution!
Because caring pet owners are so motivated to give their pruritic pets and themselves some relief, an opportunity was created for a massive market of supplements, shampoos, and pet foods claiming to be the solution. But what is the evidence for these? Which ones really work and are worth their price? Many products are lacking evidence, and promising owners a quick resolution to a much deeper problem.
Many products marketed for itchy pets market themselves based on the assumption that the problem is an allergy or sensitivity to a food ingredient. Hundreds of over-the-counter pet food brands base their marketing on inclusion of an unusual protein source, or the exclusion of certain ingredients like cereals. This has led to the implication that food sensitivities, or “adverse reactions to food (AFRs)”, are the likely culprit for an itchy pet’s symptoms. However, allergic disease is a diagnosis of exclusion, meaning it is critical to rule out other potential causes of pruritus first. The vet will want to do a careful examination and a few tests to ensure that there aren’t any other causes for the itching. So what are actually the most common causes of itching and scratching in dogs and cats?
FLEAS and other ectoparasites
The very first thing it is important to rule out before jumping to a conclusion of allergic disease is parasites. Many of them, even fleas, can be hard to detect with the naked eye or a quick look. In one UK study, 21% of pet dogs and cats brought into vet practices for routine consults and selected at random were found to be infested with fleas. These were not pets with flea infestations known to their owners. Some dogs and cats have an allergy to fleas, which can result in severe itching and inflammation even if very few flea bites occur. Not only is it important to protect pets against flea infestations for their own comfort, fleas can transmit pathogenic microorganisms such as Bartonella and haemoplasma species, and intestinal parasites such as Dipylidium caninum (a type of tapeworm). These can cause illness in pets and some have zoonotic potential, meaning they can also be transmitted to people. In the study referenced, 14% of the flea samples were found to be infected with at least one of these species.
Bacterial and Fungal Infection
All animals (and people!) naturally have populations of bacterial and fungal species on our skin at all times. The vast majority of these are harmless or beneficial to us, or present in low enough numbers that they do not colonise or can be dealt with effectively by the individual’s immune system. Certain infections, like the fungal infection ringworm, caused by dermatophytes, can be quite infectious to pets and people. In other cases, there may be underlying allergic disease that has caused a break in the protective skin barrier, allowing normally harmless bacteria to colonise and cause clinical infection. It is important for vets to conduct tests for fungal and bacterial infections on the skin. Even if there is also allergic disease, not treating these infections simultaneously will prevent the pet’s skin from healing.
Systemic and Immune-mediated Disease
Less commonly, skin lesions can be caused by systemic diseases, or diseases other than allergies that affect the pet’s immune system, causing it to attack its own tissues. Diseases such as lupus, certain liver conditions, and even cancer, can cause skin lesions that need to be ruled out before diagnosing and treating allergic disease.
Diagnosing Allergic Skin Disease
Okay, say you’ve ruled out all of the above potential causes of pruritus in a dog or cat, and are suspicious of sensitivities or allergies. Is it food? Despite the explosion of marketing of limited ingredient pet foods that would lead anyone to believe that it is a very common problem among pets, the reality is that adverse food reactions are not terribly common. In one meta-analysis compiling the results of 28 published scientific papers from around the world, the median prevalence of cutaneous adverse food reaction was less than 1% of pets presented to the vet for any disease. When only factoring in dogs and cats presented for pruritus, the number is still only 15-20%.
Atopic Dermatitis (Atopy)
Most dogs and cats with allergic disease have atopic dermatitis, or sensitivities to something in their environment. The most common of these antigen triggers are pollen, moulds, and dust mites, all of which are difficult to protect the pet from encountering. In atopy, individuals have an abnormal skin barrier. This allows particles from the antigens to penetrate through the skin to trigger a response by the immune system. It also allows water loss through the skin. Both of these abnormalities can lead to the pet itching, scratching, licking and biting, and creating even more damage to the skin, which perpetuates a vicious cycle.
Atopy is a diagnosis of exclusion, meaning there are no reliable tests that can prove a pet has the condition. Once other conditions have been ruled out, and atopy is suspected, blood or skin tests can be performed to give some indicator of what the environmental antigens might be. This can be helpful in management strategies like avoiding heavy contact with certain types of pollen in peak season, or minimising dust and mould in the home as much as possible.
Adverse Food Reaction
While it is far less common, there are still of course some dogs and cats that react to food with dermatologic signs like itching and chronic ear infections, and sometimes chronic or intermittent gastrointestinal signs like diarrhoea. Approximately 20-30% of dogs with AFR also have concurrent environmental allergies (atopy or flea allergy dermatitis). Unfortunately, there are no simple laboratory tests that can be relied on to diagnose AFR in dogs or cats. While companies do exist that claim to be able to diagnose food allergies in dogs and cats based on blood or saliva samples, these tests are unreliable and their accuracy is variable at best. The only way to accurately diagnose an adverse reaction to food in a dog or cat is to feed a restricted standardised elimination diet for a period of time (usually 8-12 weeks) and watch for improvement in symptoms. As AFRs are generally to protein molecules, a proper elimination diet needs to exclude any possible protein molecules that the pet has previously been exposed to and could have a sensitivity to. Based on a meta-analysis of scientific literature reports, the most commonly reported causes of AFR in dogs are currently beef, dairy products, chicken and wheat. In cats, they are beef, fish, and chicken. Despite marketing to the contrary, cereals are not the cause of AFRs in the vast majority of dogs and cats. Elimination diets need to avoid intact protein molecules from these ingredients. This can be done using alternative protein sources such as venison, duck or egg, or with proteins that have been hydrolysed, or broken down into particles too small for the immune system to detect and cause a reaction. These are often combined with carbohydrate sources that have had their protein component removed, leaving only the energy-providing starch, or with intact carbohydrate ingredients not on the common allergen list. Which food is best will depend on the individual pet and its dietary history. These types of dietetic foods are available on the recommendation of a veterinary surgeon, and have the advantage of being complete and balanced, safe, and having careful quality control to avoid cross-contamination with antigenic ingredients, which can happen during manufacturing.
A positive response to an elimination diet will confirm that the pet suffers from AFR, but to determine exactly which component of the previous diet the pet was reacting to, challenges with those individual ingredients would be necessary. Some owners want to do this so they can know which specific ingredients to avoid in the future. Others are happy to maintain their pet’s newfound relief and continue on with the elimination diet for the long-term.
What To Feed My Allergic Pet
If a pet has been diagnosed with an adverse food reaction, the best diet is a complete and balanced pet food that avoids that pet’s individual triggers. There is no evidence that different preparation types (dry, wet, fresh-cooked, raw, etc.) have any effect. If a pet is allergic to chicken, it is generally going to be allergic to intact chicken protein in all of its forms. A hydrolysed protein food is a very common choice for pets with AFR.
If a pet has been diagnosed with an environmental allergy, it may seem counterintuitive to consider nutritional management, but it can actually be very helpful as part of multimodal management with medications. Certain ingredients, like omega-3 fatty acids from fish oil, can help to calm inflammation and restore the protective skin barrier. Some veterinary-recommended dietetic foods also contain other ingredients to help manage inflammation and dampen the immune system’s reaction to the environmental allergens, like phytonutrients. Because a significant proportion of dogs have both AFR and atopic dermatitis, these diets may also avoid the most common food reaction triggers.
Overall, it is best to follow a vet’s recommendation for stepwise diagnosis of itchy pets. It may take some time to rule out the various potential causes of itching, but this patience leads to a proper diagnosis and more effective treatment. There are many misconceptions around the prevalence and common causes of adverse food reactions in pets. Discovering the true cause of pruritus, and managing allergic causes with veterinary-recommended targeted nutrition, can help to save vets bills and much discomfort for pets and their owners over time.
Hillary Pearce, DVM, MRCVS
Hill's Pet Nutrition, Ltd