All About Dog Allergies

A veterinarian explains what having an allergy entails, how canine allergies differ from ours, and which allergens bother dogs most frequently. The most common? Flea allergies.


There are few things that make dogs (and their veterinarians) tear their hair out like skin allergies. Skin conditions are the top cause for veterinary visits in the United States, and most of those skin problems are symptoms of allergies. Dog allergy symptoms include itching – called pruritus – redness, hair loss, and damaged and infected skin. Vomiting, diarrhea, and weight loss are other but less common effects and are generally associated with food allergies. 


In the simplest terms, allergy is the result of an immune system gone awry. When it’s functioning as it should, immune system agents patrol the body, checking the identification (as it were) of every molecule in the body. The immune-system patrollers allow the body’s own molecules and harmless foreign substances to go about their business, but when they are operating properly, they detect, recognize, and attack potentially harmful agents such as viruses and pathogenic bacteria.

When a dog develops an allergy, the immune system becomes hypersensitive and malfunctions. It may mistake benign agents (such as pollen or nutritious food) for harmful ones and sound the alarm, calling in all the body’s defenses in a misguided, one-sided battle that ultimately harms the body’s tissues or disrupts the body’s usual tasks. Or the immune system may fail to recognize normal agents of the body itself and start a biochemical war against those agents (an “autoimmune” response).

Allergies can be very frustrating to diagnose, manage, and treat. Just as in humans, allergies are usually a lifelong condition without a cure. Management is focused on identifying the causative allergen(s) (often through extensive testing), eliminating the allergen(s) from the dog’s environment when possible – or, if not possible, reducing the dog’s exposure to the allergen(s) as much as possible, and treating secondary effects of allergies, such as infections and discomfort. 

In theory, this is very simple; in reality, it can be difficult and frustrating. Any breakthrough in controlling the dog’s exposure to the allergens (assuming they even can be determined!) can lead to itching and infections. Whenever I am counseling an owner on managing a dog’s allergies, I start with the caveat that we will never cure the allergy. We will only keep it under control.

Choose Wisely; Be An Informed Owner

I wish that more dog owners were aware of their preferred breeds’ genetic predisposition to allergy and planned accordingly when buying or adopting. 

Breeds to consider as particularly prone to allergies include West Highland White Terriers, Golden Retrievers, Boxers, Shar-Peis, Lhasa Apsos, Shih-Tzus, Boston Terriers, Staffordshire Terriers, and Bulldogs. There are many others as well, so thoroughly research any breed that you are interested in owning. If possible, ask about the parents and other offspring that have resulted from the same matings; if the parents or siblings have severe allergies, chances are better than even that the puppies will develop them, too.


All About Dog Allergies
Many dogs with allergies lick and chew their feet, causing raw swollen irritated skin and secondary infections.
Photo Credit: Raquelsfranca/

So how do you know if your dog has allergies? Allergies generally manifest as dermatitis – meaning skin inflammation. In particular, skin infections are called pyoderma, meaning “pus skin.” A one-time skin infection that responds to treatment is not a reason to suspect allergies. That can happen to any dog! It is when infections recur again and again that makes allergies the prime suspect as the underlying cause. 

Allergies and pyoderma are a vicious cycle. When a dog is exposed to whatever substance(s) to which he is allergic (whether it be fleas, food, or an environmental allergen like dust mites), his first and most intense symptom will be itchy skin. (In contrast, the most common symptoms of allergy in humans are sneezing and an itchy, runny nose and eyes.) As a result, the allergic dog will scratch and chew at his skin, damaging the skin’s protective barrier. 

Once the skin is broken, opportunistic bacteria, particularly Staphylococcus (which commonly lives harmlessly on the skin in some amount), enter the damaged area and colonize it. This stimulates the immune system to release white blood cells to fight off the invaders. The white blood cells release inflammatory substances to kill the bacteria – but they also irritate the surrounding tissue and cause more pruritus. The itching continues, the dog continues to scratch, and more bacteria enter the broken skin. The cycle continues.

Intense itching and discomfort, redness, and skin infections that recur despite appropriate treatment are the hallmarks of allergies. In the case of severe allergies, it may be hard to distract a dog from the maddening itch. 

Recurrent ear infections can also be associated with allergies. Or your allergic dog may scratch behind his ears, along his belly, or chew on his feet. The skin is often moist and red and may have a yeast-like odor. Epidermal collarettes – small, flaky circles – may be seen; these are most often noted on the belly, where there is less hair. 


When you take your dog into the veterinarian for itching and red skin, an initial skin work-up will be done. This includes three important steps: a skin scrape, tape cytology, and skin impression. This will determine if there is an easily treatable cause for the discomfort and if pyoderma is present. Mite infections can lead to these symptoms and are easy to treat. 

For a skin smear, a glass slide is pressed against the infected area, dried, stained, and examined under a microscope. 

A skin tape cytology is similar. A piece of acetate tape is pressed to a place on the dog’s skin in an infected area or where a pustule is seen; then the tape is removed and examined microscopically. The veterinarian is looking for the presence of mites, yeast, or bacteria to determine the cause and treatment for the infection. 

If mites are not suspected to be the cause and none are found on the scrape, the veterinarian will treat the infection based on the skin cytology and impression.

If bacteria are found, a topical shampoo with antibacterial effects may be used. Chlorhexidine shampoos are inexpensive and readily available in most veterinary clinics. Alternatively, if the infection is widespread and/or severe, an oral antibiotic like a cephalosporin may be more appropriate. 

If yeast is also present, a topical shampoo or mousse treatment with an antifungal agent will be needed. In some cases, oral antifungals such as itraconazole are also used. 

A one-time skin infection does not indicate allergies, but when infections recur – especially in susceptible breeds – allergies are the most likely diagnosis. At that point, it becomes necessary to try to identify what substances the dog is allergic to. 

We’ll discuss food and substances that are inhaled or that come into contact with the skin (such as dust mites, pollen, grass, mold, and other common environmental substances) that commonly cause canine allergies in upcoming issues. Below, we’ll discuss the most common canine allergy: Flea allergy.


All About Dog Allergies
Flea combs have fine teeth that are very close together. If you comb through the coat of a dog who has fleas, you may or may not capture a flea between the fine teeth; flea actually spend only a small amount of time on the dog, and hop off shortly after feeding. But if you see dark specks like those seen in this photo of a few comb, you are looking at flea “dirt” – the feces of the flea, which contains dried blood.

As vexing as they may be, fleas are the simplest cause of canine allergy to diagnose and treat. The offending agent is the flea’s saliva: When they bite into a dog, they inject a small amount of saliva into the dog’s skin, and the dog’s immune system responds to this material. 

Flea allergy typically develops when a dog is between the ages of 2 and 5 years, though dogs who are as young as 1 year or are senior dogs can also develop flea allergies. 

Some dogs are so intensely allergic to flea saliva that one flea bite can lead to widespread, severe itching and self-trauma, setting up the pyoderma cycle. (These dogs in particular need to be kept on flea prevention year round in most parts of the United States. Fleas can be a year long problem in many parts of the United States that stay warm or have warm days during the winter.)

All About Dog Allergies
If you are in doubt, drop those specks in a saucer of water. If the water turns a rusty red, it’s flea poop – and you need to address the flea infestation STAT.

Generally, diagnosing flea allergy is fairly simple. It presents with symptoms in the “flea triangle”– the base of the tail, rear legs, and inner thighs. There is hair loss in those areas, where fleas tend to congregate. Sometimes, live fleas can be seen crawling around in those areas, but not always.

Your veterinarian may use a fine-toothed comb (called a flea comb) to detect and expose living fleas. Whether or not fleas are found with a comb, she will also conduct a skin scrape, tape, and cytology as outlined above to determine what infectious agents may be present. The pruritus caused by fleas leads to the vicious cycle of skin trauma and pyoderma, so that must be treated in conjunction with eliminating fleas. 

It’s important to remember that just because you don’t see fleas doesn’t mean that they are not present. Fleas remain on a dog for only a few minutes to an hour while they are feeding. Once they feed, they return to their environment. By the time you begin seeing live fleas crawling on your dog, the infestation is severe. It is common for owners to be certain that their dog cannot have fleas, just for us to use a comb and find several live ones. (We veterinarians call this “fleanial.”) 


Treatment obviously involves killing the fleas. (See “Quick, Make Fleas Flee,” page 14 of this issue.) 

In addition to recommending products that kill the fleas, your vet may also prescribe a medication to help with the itching. Older medications like oral steroids are very effective but do come with significant side effects such as increased drinking, urinating, panting, restlessness, and the risk of inducing diabetes with long-term use. Some dogs also experience temporary behavioral changes. Steroids are very inexpensive, and generics are available. 

Newer medications such as Apoquel (oral tablet) and Cytopoint (injectable) specifically target the factors that cause itching, are safer, and are extremely effective without the side effects of steroids. They are also more costly, as no generics are yet available. 

But the pyoderma caused by self-trauma must also be treated. This is called flea allergy dermatitis (FAD). Treatment involves either topical shampoos or mousses and/or oral medications, depending on what the veterinarian finds on microscopic examination. 


If your dog has flea allergies, consider yourself (somewhat) lucky! Of the three main causes of allergies in dogs, flea allergy is the easiest to treat and manage. With careful attention to preventives, maintaining a flea-free house and yard through regular treatments, dogs with FAD can live a happy and itch-free existence. 

It’s far more difficult to identify food and environmental allergens that may plague your dog and, especially, to limit your dog’s exposure to them. We’ll discuss those challenges in upcoming issues. 

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