How Vets Diagnose the Cause of Itching in Dogs

Article Summary

• Vets use a structured, step-by-step approach to investigate itching, ruling out causes in a logical order rather than guessing.
• Simple immediate tests (cytology, flea combing and skin scrapings) form the minimum dermatology database and are carried out at the first appointment.
• Canine atopic dermatitis is a diagnosis of exclusion: parasites and infections must be ruled out before it can be confirmed. [5,7]
• Food allergies are usually diagnosed through a strict elimination diet trial of at least 8 weeks, as blood and saliva tests for food allergy are not considered reliable. [8]
• Allergy tests identify specific environmental triggers for immunotherapy, they do not determine whether allergies are causing the itch in the first place. [5]

Why Accurate Diagnosis Matters

Treating symptoms without a diagnosis often provides temporary improvement followed by recurrence. Skin disease in dogs frequently involves more than one contributing factor at the same time, and a systematic approach is the most reliable way to identify all of them. [5,6]

Professor Tim Nuttall BVSc PhD FRCVS, Diplomate ECVD, a veterinary dermatologist and co-author of the BSAVA Manual of Canine and Feline Dermatology, emphasises that systematic investigation is essential because the signs of different skin conditions overlap considerably. Identifying the specific combination of causes in an individual dog is the foundation for a management plan that will be effective in the long term, rather than one that only partially addresses the problem. [3]

Step 1: Taking a Detailed History

The diagnostic process begins with a thorough discussion about your dog's signs. Your vet will ask questions that help narrow down possible causes before any tests are performed.

You may be asked about when the itching started, which body areas are affected, whether signs are seasonal or present all year, your dog's diet, where they live and spend time outdoors, any previous skin problems, and what treatments have been tried. Patterns in this information provide important diagnostic clues. Seasonal worsening often suggests environmental allergens; year-round signs may raise suspicion of food reactions, house dust mite allergy, or ongoing parasite exposure. [1,2]

Step 2: The Minimum Dermatology Database

After taking a history, your vet will usually carry out a set of simple tests that provide immediate information about the skin. These are referred to in dermatology guidelines as the minimum dermatology database. [1,3]

Skin cytology — involves collecting samples from the skin surface, ear canals or between skin folds using tape strips, swabs or impression smears. These are examined under a microscope to identify bacteria, yeast and inflammatory cells. Cytology helps determine whether infection is present and guides immediate treatment decisions. [3]

Flea combing — the coat is combed to check for fleas or flea dirt (small dark specks that dissolve red when placed on damp tissue). This remains important even if fleas have not been seen at home, as they can be extremely difficult to spot on a dog. [2]

Skin scrapings — performed if parasitic mites are suspected. Samples taken from the skin surface are examined microscopically to look for mites such as Sarcoptes (which causes intense itch at the ear edges and elbows) or Demodex (associated with hair loss and variable itch). [1]

As Dr Chiara Noli DVM Dip ECVD, European Specialist in Veterinary Dermatology and co-author of the international revised treatment algorithm for pruritic dogs, explains: "A structured diagnostic approach is essential in pruritic dogs, because multiple conditions often coexist and may mask one another." [4]

Step 3: Ruling Out Fleas and Parasites

Ruling out fleas is a priority even when none are visible, because they are one of the most common triggers of itching and because dogs with flea allergy dermatitis can react intensely to very small numbers of bites. [2,5]

Your vet may recommend strict flea control for all pets in the household as part of the investigation. As most of the flea life cycle takes place in the home environment rather than on the animal, absence of visible fleas does not exclude them as a cause. Flea control needs to cover the environment as well as the pets themselves to be effective. [2]

If mites are found on skin scrapings, specific treatment will be prescribed. Where your vet has strong clinical suspicion of mites — for example, based on the pattern and intensity of the itch — they may recommend a therapeutic trial even if mites were not found on the first scraping, as sensitivity of a single scraping is not 100%. [1]

Step 4: Checking for Bacterial or Yeast Infection

Skin cytology often reveals bacterial or yeast overgrowth as a contributing cause of itching. These infections are frequently secondary, and they develop because an underlying skin condition has disrupted the normal skin barrier, creating conditions in which microorganisms can multiply. [3,6]

Treating an identified infection will typically improve comfort, but if infections recur, this signals that an underlying driver, most often allergic skin disease, still needs to be identified and addressed. Recurrent infections that respond to treatment but return soon after stopping are one of the strongest clinical indicators that an underlying allergy is present. [6]

Step 5: Considering Allergy

Once parasites and infections have been treated or ruled out, allergic skin disease becomes a key diagnostic consideration. Vets group allergic skin disease into three main categories, each requiring a different approach to diagnose: flea allergy dermatitis, food-related skin reactions (cutaneous adverse food reaction), and environmental allergies (canine atopic dermatitis). [2,5]

Canine atopic dermatitis is defined as a diagnosis of exclusion, meaning other causes of itching must be ruled out and treated before this condition can be formally confirmed. The Favrot criteria, a validated set of clinical signs used to support a diagnosis of atopic dermatitis, have a reported sensitivity of approximately 85% and specificity of around 79% when the optimised five-criterion combination is used. [7] These criteria are one of several tools vets use alongside the history, examination findings and treatment responses to reach a diagnosis.

Your vet will use information gathered across all previous steps — history, examination findings, response to parasite and infection treatment — to determine which allergic category is most likely and what further investigation is needed. [5]

Step 6: Food Trial for Suspected Food Allergy

Diagnosing food-related skin reactions requires a strict elimination diet trial. This involves feeding your dog a specially selected diet exclusively for at least 8 weeks — during which they must not receive any other food, treats or flavoured medications. [8]

The diet contains either novel proteins (ingredients the dog has not previously been exposed to) or hydrolysed proteins (proteins broken down into fragments too small to trigger an immune reaction). If signs improve substantially during the trial and return when the original diet is reintroduced — the rechallenge phase — this supports a diagnosis of food allergy.

Step 7: The Role of Allergy Testing

Allergy tests — whether intradermal skin testing or blood-based serological tests — identify which specific environmental allergens cause a reaction in an individual dog. These tests are used to formulate allergen-specific immunotherapy (desensitisation) once a diagnosis of atopic dermatitis has already been confirmed. [5]

It is important to understand what allergy tests do and do not tell you. They do not determine whether allergies are the cause of itching — that conclusion must be reached through the earlier diagnostic steps. For food allergy, blood and saliva tests are not considered diagnostically reliable, and an elimination diet trial is still required regardless of test results. [8]

Why Step-by-Step Diagnosis Matters

The systematic approach to diagnosing itching exists because skin disease in dogs is rarely caused by a single factor. Allergic skin disease disrupts the skin barrier, making infection more likely. Infection worsens itch, which leads to more scratching and more skin damage. Parasites can trigger or worsen both. Working through the diagnostic steps in order means each contributing factor is identified and treated rather than treating only what is most visible. [5,6]

When to Contact a Vet Urgently

Most itching can be assessed during a routine appointment. However, seek prompt veterinary attention if you notice any of the following:

• Sudden, severe itching accompanied by obvious distress
• Extensive skin damage, bleeding or open sores
• Marked redness, swelling or discharge from the skin or ears
• Facial or muzzle swelling
• Sudden hives, welts or raised patches across the skin
• Scratching accompanied by any difficulty breathing — this may indicate a systemic allergic reaction requiring immediate treatment