Diagnostic Tests: Tips from the Dermatologist #7

We are often presented with itchy dogs. They lick, nibble, bite, rub, scratch and roll. In referral practice I see more and more cases with a long history of pruritus in which several tests have been done, including skin biopsy and allergy tests.

Were they really necessary? Was our colleague just hoping to find some answers from these tests as he/she was desperate to help an unhappy patient and a frustrated owner?

Below are some practical steps that should direct you on when is the right time to do allergy tests or perhaps refer to a dermatologist for allergy tests (intradermal and/or blood tests).

In the vast majority of cases the pruritus has dermatological causes and is associated with ectoparasite infestations (fleas, Sarcoptes mites, Cheyletiella spp, harvest mites, and lice) and/or concurrent bacteria and/or yeast infections. It can also be associated with allergic dermatitis: environmental (atopic dermatitis), and/or flea, and/or food allergens.

Itch is “multifactorial” and the approach to the itchy dog is aimed at discovering, and possibly removing, all the identifiable pruritogenic factors.

Pruritus in dogs and cats is not commonly due to other dermatological problems, such as adverse drug reactions, irritant/allergic contact dermatitis, arthropod bite reactions and epitheliotropic cutaneous Tcell lymphoma. It is also rarely associated to neuropathies in dogs (e.g. Chiari-like malformation and syringomyelia and acral mutilation syndrome) or behavioural disorders (e.g. acral lick dermatitis). Sporadic cases of canine pruritus attributable to systemic causes (i.e. portosystemic shunt and pseudorabies) have also been reported. In these cases, signs suggestive of pruritus of non-cutaneous origin, lack cutaneous inflammatory changes (e.g. lichenification and hyperpigmentation) and the presence of well-demarcated excoriations, ulcers and scars with mild perilesional inflammation. In some cases skin inflammation is completely absent. Other useful clues to diagnose pruritic diseases of non-cutaneous origin include breed predilection (e.g. Chiari-like malformation in Cavalier King Charles Spaniels), typical distribution of lesions (e.g. distal extremities in acral mutilation syndrome) and presence of systemic signs (e.g. pseudorabies).

The diagnostic work up of a pruritic dog may not be simple. Sometimes it involves the collaboration of colleagues with different expertise. In addition to a dermatologist, a neurologist, a behaviourist or an internist may be consulted, depending on the dermatological and systemic clinical signs. If behavioural problems are suspected, a complete dermatologic workup is recommended.

In “standard” dermatologic cases the diagnostic approach to a pruritic dog is best dealt with step by step through progressive elimination of pruritogenic components associated with parasites, bacteria and yeast, and is commonly followed by exclusion of allergic components. The workup has to be sequentially planned and methodically performed, otherwise unreliable and inconsistent conclusions are made. There is no point rushing in with allergy tests if a patient has ectoparasites or if ectoparasites have not been thoroughly ruled out. In referral practice I have seen patients which fleas, Cheyletiella spp., Sarcoptes spp. or harvest mite infestations that have been blood sampled for allergy test when the only problem was the ectoparasite infestation. As you can imagine, this ends up with clients becoming even more frustrated with the money spent so far (visits, tests) and often disappointed by the approach followed by the colleagues in practice who may have wasted their clients money or what was left of the annual insurance premium for the skin condition !

Please think twice before deciding to run allergy tests and ask yourself:


  1. Have I ruled out ectoparasites? - Treat sarcoptic mange and other parasitic dermatoses. Skin scrapings and other diagnostic procedures that are useful in order to rule out parasitic mites have to be performed. If negative results are obtained but the presence of mites (except Demodex) is still suspected, an acaricidal therapeutic trial is recommended. Strict flea control has also to be instituted.
  2. Have I ruled out infections? - Do cytology to rule out the presence of Malassezia and bacterial overgrowth or the presence of an infection. Malassezia and bacterial overgrowth would benefit from topical shampoo therapy. If an infection is present, oral and/or topical antimicrobial treatments should be dispensed.

Only when parasites, bacteria and yeasts have been ruled out and/or effectively treated and the pruritus persists, does an underlying allergy has to be investigated, as long as clinical signs are compatible.

Rule out an adverse food reaction! While keeping the patient on strict flea control, begin a strict food trial (8-12 weeks).

  • If a reduction of pruritus is evident, a provocative test with the previous diet has to be done.
  • If no reduction of pruritus is evident, and there is signalment, history and clinical signs suggestive of an allergy, atopic dermatitis can be diagnosed and an allergy test may be useful to select the allergens to be included in the allergy vaccine. Serum based in-vitro allergy tests are now widely available and used, and the methodology for these tests varies by laboratory. Furthermore, there is no point at this stage to run the allergy test if the owner is not interested in the immunotherapy.
  • Medical treatments are available and more drugs will soon be available which are very effective in controlling the dog pruritus and the clinical signs caused by atopic dermatitis.

Commonly, dogs with a long history of pruritus are presented to the dermatologist with a variety of provisional diagnoses, often including a rarely demonstrated food allergy (or diagnosed based on a blood test which may not be very reliable!) and/or contact dermatitis. Their skin shows evidence of long-term scratching, and Malassezia and/or bacterial overgrowth or infection is common.

These dogs have changed diets numerous times and have already undergone extensive investigations, sometimes including blood work and skin biopsies. However, data from history are often not convincing for the specialist (e.g. incorrect diet trials, ineffective flea control, lack of ruling out ectoparasites), and the diagnostic workup must be frequently restarted when they are presented to the dermatologist. Perhaps deciding to refer these tough cases for an allergy work up earlier, may be better as owners will be less frustrated but grateful to the colleague in practice for having referred them. The earlier the better for the owner’s finances and the dog’s health!