Pruritus is a common canine presentation in veterinary practice.

Many skin diseases cause pruritus. Major differentials are summarised in Table 1.

History

A full dermatological history guides the clinician to the most likely underlying cause and focuses on the diagnostic tests indicated to rule out the differential diagnoses. Some skin conditions, such as canine atopic dermatitis (CAD), are familial, with some breeds over-represented (such as the West Highland white terrier, golden retriever and Labrador retriever, French bulldog, German shepherd dog and boxer).

Dietary history should be obtained and include information on all foods fed. Many owners are unaware their dog may have abnormal gastrointestinal function.

Questioning the quantity and consistency of faeces produced may lead to dietary or digestive problems being uncovered. The age of onset of the itch – and whether it is constant or waxing and waning – should be noted. Allergies frequently have an onset before three years of age and epitheliotropic lymphoma is generally a disease of aged dogs.

The location of lesions and distribution of the pruritus may be suggestive of a particular disease. CAD most commonly causes symptoms involving the face, paws, ears, axillae, groin and, sometimes, the perineum (Figures 1 to 3).

In contrast, flea infestation or allergy generally causes symptoms on the caudal dorsum, tail base and caudomedial thighs. Sarcoptes mites characteristically affect the pinnal margins, elbows, hocks and ventrum. Some diseases cause constant or perennial itch (such as Sarcoptes or house dust mite allergy in CAD), whereas others are seasonal (such as Neotrombicula mite infestation and pollen allergy in CAD).

The severity of pruritus may be graded 1 to 10 on a visual analogue scale (Rybnícek et al, 2009) or recorded for simplicity as mild, moderate or intense. This helps to monitor the response to prescribed treatments.

Sarcoptes and some cases of epitheliotropic lymphoma may present with extremely distressing pruritus that significantly reduces the quality of life of the owner and dog. CAD can present with mild to severe itch, depending on the patient.

Historical information of concurrent diseases should also be noted. Drugs required for other conditions may influence the choice of prescribed anti-itch medications (for example, oral glucocorticoids should be avoided in an arthritic patient on NSAIDs).

Responses to previous anti-itch medications prescribed should also be investigated (for example, CAD is generally responsive to use of lokivetmab, oclacitinib, ciclosporin or systemic glucocorticoids). The pruritus response may be partial rather than complete.

The author is commonly presented with severely pruritic dogs with presumed allergies refractory to a cocktail of concurrently used anti-itch therapies. In these cases, we should always step back and question whether CAD has been misdiagnosed in the case.

If the symptoms or lesion distribution are unusual, and not responsive to conventional anti-itch therapies, secondary infection or other significantly itchy disease must be considered.

Examples of cases of pruritic dermatoses misdiagnosed as allergy are presented in Figures 4 to 7.

History of contagion with the owner or in-contact pets may suggest ectoparasitic diseases or dermatophytes. Contact with wildlife or other infected pets may be the source. The travel history should be noted. Ectoparasite treatment history must be thorough, including the frequency of administration, type of product used on each household pet and any use of an environmental spray.

Physical examination

A thorough general clinical and dermatological examination should be completed.

The distribution of lesions and presence of primary lesions (such as erythema, papules or pustules), or secondary skin lesions (such as excoriation, alopecia secondary to self-trauma or inflammation from bacterial folliculitis) must be noted. The ear canals should be examined.

Favrot et al (2010) formulated criteria that have a diagnostic sensitivity and specificity of around 80% for CAD (Panel 1).

Once the lesions and their distribution have been determined, a differential diagnoses list may be formulated and diagnostic tests undertaken.

Diagnostic tests

Diagnostic tests should be focused according to the presenting symptoms and differential diagnosis list. This avoids need for unnecessary procedures for the pet, and cost implications for the owner.

Some laboratory tests are not required in every pruritic case. For example, skin biopsies for histopathology are often overused in suspected CAD cases, where they are not diagnostic. They are most useful if the differential diagnoses suggest a pruritic disease that can be firmly diagnosed using histopathology (such as epitheliotropic lymphoma).

Skin scrapes and hair plucks should be performed on all itchy dogs that are not treated with isoxazolines at licensed doses and frequencies to help exclude Sarcoptes and demodicosis.

Demodicosis is non-pruritic until the mites cause folliculitis and secondary infections. Hair plucks are useful to find Demodex mites in areas that are difficult to scrape (such as the paws); although, five deep scrapes (squeezing the skin to extrude the mites from hair follicles) are required to exclude these mites.

As skin scrape sensitivity is low in Sarcoptes cases, treatment trials using an isoxazoline, imidacloprid/moxidectin or selamectin should be used to exclude this parasite.

Pruritus secondary to Sarcoptes infestation can take a few months to resolve. Due to potential cross-reaction with house dust mites, any allergy testing should be delayed for around six months after a suspected or confirmed infestation. Blood testing for Sarcoptes IgG can be taken six weeks after the time of infection.

Some laboratory tests may produce false positive results in house dust mite allergic dogs, and if the patient does not improve with appropriate therapy, further workup for CAD is indicated.

Neotrombicula mites bite seasonally in the UK. They are visible with the naked eye as pinhead-sized orange dots in the skin folds (such as Henry’s pocket and the interdigital spaces).

Superficial skin scrapes are most useful to remove these mites from the skin for microscopic examination. Coat brushing is useful to detect fleas or lice, and a wet paper test of the collected material for flea faeces. When flea evidence is detected, thorough treatment of the dog, in-contact pets and the environment should be undertaken.

The clinical response should be monitored for at least around eight weeks before considering other causes for the itch.

Lice and Cheyletiella are investigated by coat brushing or acetate tests of the dorsal areas. Examination of unstained ear cerumen swabs are useful to detect Otodectes mites if they are not seen on otoscopy. Wood’s lamp and fungal culture should be considered in young animals that are at higher risk of dermatophytosis infection, and older animals that have a history of hunting rodents or contact with wildlife (suspected sylvatic ringworm cases).

Pruritus can occur or be significantly worsened in many dermatoses when skin lesions are complicated by overgrowth or secondary infections with bacteria (commonly Staphylococcus pseudintermedius) or Malassezia species yeast.

Secondary infections

Bacterial pyoderma and Malassezia dermatitis commonly cause or worsen canine pruritus. These should be identified by impression smear cytology and acetate strips of relevant skin lesions (such as pustules, epidermal collarettes, moist intertriginous regions and scaling areas). If diagnosed, they should be treated before reassessing the distribution and severity of any remaining pruritus and skin lesions.

Due to antimicrobial stewardship, topical antiseptic therapy – most commonly chlorhexidine-based shampoo formulations – should be initially used to treat the microbial overgrowth or infection.

Systemic antibiotics should only be considered if topical antiseptic therapy has been unsuccessful. A subset of dogs are hypersensitive to Malassezia yeast and benefit from trial systemic antifungal treatment to improve or resolve their pruritus. Ketoconazole is the only licensed canine systemic antifungal therapy (labelled for dermatophytosis) but has more potential for side effects than unlicensed itraconazole.

If ectoparasites and secondary infections are excluded, and the dog remains pruritic, food allergy and CAD should be considered if the clinical features remain characteristic.

Food allergy

Food allergy may affect around 20% of patients with CAD, with food flare factors being a major or minor pruritus trigger in these cases.

Many patients with food allergy also have concurrent environmental allergies. A full dietary history should include the main foods fed, table scraps, treats and those given by carers other than the owner (for example, dog walkers).

Flavoured medications, dental chews and foods used to administer medications also need to be disclosed. A list of ingredients fed should be examined. The best diet trial for the patient is one that is novel, palatable, fully balanced, suitable for life stage, easy to maintain good body condition, fulfils satiety, and supports good bowel and skin function. In practice, more than one diet trial may be needed to achieve this.

Communication and owner education is the key to achieving compliance, and the motivation required to food trial a dog correctly.

The author often spends a significant amount of time in the consultation, ensuring the process is fully discussed and understood.

The food trial is described as a diagnostic test requiring a diet change that lasts eight weeks. It is not a forever change in most cases.

If food allergy is diagnosed, this will reduce the overall itch burden. This subsequently reduces the requirement for anti-itch medications and their associated ongoing costs.

Once clients have grasped these concepts, they generally have more enthusiasm for the process.

Good evidence exists for the use of hydrolysed diets for food trials, as the hydrolysation process breaks up the potential allergen, rendering it truly hypoallergenic. These diets should be the first choice wherever possible for a diet trial. The commonly used hydrolysed diets available on the UK market are summarised in Table 2.

Hydrolysed diets have acceptable palatability for most dogs and can, therefore, be successfully used in most patients. Small, firm faeces are commonly seen and adding water to the diet can reduce mild constipation symptoms. Osmotic diarrhoea is rarely seen, and an alternative non-hydrolysed diet trial should be considered if this occurs. Some itching can remain with partial chicken hydrolysed diets in around 40% of chicken allergic dogs (Bizikova and Olivry, 2016). If this is suspected, use of more completely hydrolysed (lower molecular weight) diet – such as feather hydrolysate or soya hydrolysate – may be more successful.

If a hydrolysed diet trial is not accepted by the patient, the author often considers a novel insect protein kibble diet or vegetable based diets for food trial.

Unfortunately, many non-hydrolysed diets are labelled as hypoallergenic in pet shops. These often contain a single meat protein source, but are not hypoallergenic if the pet is allergic to this protein. Owners often misinterpret these diets as suitable for food trials. Many dogs are fed a wide variety of meat proteins and, therefore, truly novel protein diets are often difficult to achieve in practice. Increasing evidence points out that many meat proteins cross-react, so changing the protein alone may fail to diagnose a food allergy (Olivry et al, 2022).

For this reason, the author only uses commercial novel meat protein or home-cooked diets if other suggested food trials have failed. Commercial diets may also be contaminated during the manufacturing process with non-labelled proteins or contain hidden allergens when “meat derivatives” are disclosed as ingredients. Most food allergic dogs appear to be reactive to one or more meat proteins they are fed rather than the carbohydrate in the diet.

Further research into the potential for canine food allergy to carbohydrate sources is needed, but it appears clinically uncommon.

At the time of writing, no reliable food allergy serology, hair or saliva tests are available to diagnose canine food allergy. Many positive serology results are clinically irrelevant false positives.

At best, food allergy serology results can be used for their negative predictive value. A negative result is around 70% likely to be truly negative and safe to feed.

Despite being informed by veterinarians that positive food serology results may be false positives, clients often unnecessarily restrict their pet’s diets based on these results, as they find the concept of a false positive test very difficult to understand.

How to conduct the food trial

The food trial diet may be mixed with the normal diet for the first few days to aid acceptance.

During the food trial, the selected diet is fed exclusively for around eight weeks. Use of a handout instructing avoidance of table scraps, treats, dental chews, and coprophagia during this time aids compliance. Flavoured medications should be changed for non-flavoured where possible. If hiding tablets or motivating training treats are requirement the author advises using moistened diet trial kibble or diet trial tinned food, small amounts of fruit or vegetables or pet peanut butter.

High-value motivational treats are often not possible during the trial.

Secondary pyoderma, Malassezia dermatitis and otitis must be controlled and settled before the diet trial is completed, and challenges undertaken.

Trial duration may need to be extended in chronic otitis, where unresolved secondary infections may mask any benefit from the diet or worsening with food challenges.

Owners cannot efficiently examine the ear canals at home. Therefore, in cases where otitis is the sole presenting symptom, the author recommends in-clinic otoscopic examination once the ears seem settled at the end of the food trial, and weekly for three weeks after challenge to monitor for worsening.

Food challenges are important to confirm any improvement is due to the food trial food and diagnose what proteins cause reactions, if a food allergy is truly present. The author challenges the dog for five days with a single previously fed protein source to identify any food-induced pruritus flares. Proteins to which the dog is not reacting can safely be fed.

During the food trial, anti-inflammatories such as oclacitinib or glucocorticoids (such as prednisolone) can be used for comfort, but must be stopped at the end of week six (two weeks before the end of the diet trial) to assess the response to the food trial alone.

Lokivetmab can be used at the start of the diet trial, but the author switches to oclacitinib for week four to six if required for welfare reasons, so anti-itch treatment is not present systemically during the food challenges.

A recent study has shown that genuinely food-allergic dogs can often be diagnosed by a shorter four-week diet trial (Fischer et al, 2021). Dogs that are environmental allergen allergic still require an eight-week diet trial. Using this method, oclacitinib or prednisolone are given daily for two weeks and then stopped. If the itch remains settled at the end of week four, food challenges can be performed early. If the itch relapses, the medication is restarted for comfort and stopped at the end of week six to assess the diet alone for the last two weeks. As this is more complicated to undertake than a traditional diet trial, the author provides the owner with written instructions (Table 3).

Confirming a diagnosis of CAD in canine pruritus

Where relevant, differential diagnoses are excluded – and the history and clinical presentation are suggestive, a diagnosis of CAD can be confirmed.  After CAD diagnosis, allergy testing using serology or intradermal allergy testing can be performed to select allergens relevant to the patient’s history for inclusion in immunotherapy.

These are not diagnostic tests for CAD, and if the owner does not want to pursue allergen-specific immunotherapy, they are not required, and topical and systemic anti-itch treatments should be considered.