Derm Problems Again! What Should I do Next: Change the Diet?

This is the eighth in a series of short articles with the aim to inform colleagues in practice about diagnostic tests done on our patients with dermatological problems. This article will focus on food trials.

Food adverse reactions seem to be uncommon in veterinary general practice but it is frequently seen that cats and dogs with dermatitis and/or otitis had their diet changed as part of the diagnostic work up. The term ‘food allergy’ is commonly used with clients but the term “food adverse reaction” should be used unless an immunological basis of the disease has been demonstrated.

Most basic food ingredients have the potential to induce an intolerance or an allergic response, although most reactions are caused by animal proteins. In particular, dogs and cats can become sensitive to dairy products, meat, fish and less often, to carbohydrates, additives and preservatives. Cross-reactions between food allergens in the same group of food is well known and cross reactivity between certain types of food and pollen have also been reported.

Clinical Signs Are Variable!

In dogs and cats adverse reactions to ingested food components can affect many systems and can produce signs involving the skin, gastrointestinal tract, respiratory tract and central nervous system. Clinical signs are quite variable, depending on the individual response, although the most common are non-seasonal pruritus (paw licking, face rubbing), otitis externa or ventral dermatitis. The pattern of pruritus may mimic atopic dermatitis, ectoparasite infestation, and even flea allergy!

Recurrent skin infections (Staphylococcus pseudointermedius or Malassezia pachydermatis) may be the presenting complaint in dogs. These may or may not be associated with pruritus. Cats may have pruritus similar to canine patients, but they may also present with head & neck pruritus, miliary dermatitis, symmetrical alopecia, or eosinophilic plaques.


Diagnosis can be difficult, as there is no single quick test available to help the clinician to confirm or refute the presence of a food intolerance or sensitivity. The only accurate method of diagnosing adverse reactions to foods is to place the patient on a restricted diet.

Which diet to choose?

Start with a commercial prescription diet. Many families do not cook for the human members, and so are unlikely to cook for their pets! Commercially available ‘hypoallergenic’ diets commonly found in a pet shop or at the supermarket may not be the best ones to choose for the diet trial as they may still have ingredients which the pet has been previously exposed and sensitised to, as these diets are rarely made with a single type of protein or carbohydrate.

The diet typically recommended by dermatologists is either a home-cooked combination of a novel protein and a carbohydrate, a commercial prescription diet containing either a novel source of protein and carbohydrate, or a hydrolysed diet.

The new food should be slowly introduced over a period of 5-7 days so the pet can adjust to the new food. Left over old food can be stored in a dry, cold place until the time of the re-challenge.

A common request is what treats are allowed. It is rare to find truly hypoallergenic treats. Giving the clients choices undermines the principles of the trial. Giving them no options undermines compliance.

It is important also to switch to non-flavoured wormer tablets and drugs. A study at one of the USA veterinary schools showed that once-a-month heartworm pill (containing soy and beef) is sufficient to keep a soy-allergic dog symptomatic. Flavoured toys and dog toothpaste should also be avoided.

Are the clients ready & able to do a food trial?

Owner compliance is a major problem when performing an elimination diet. Food trials require a compliant client! This cannot be emphasized enough! The client should be well informed about the work-up for their pruritic pet; they should know what the options are for managing pruritus and what is the prognosis of allergy management.

If there is insufficient motivation to achieve this at-home diagnostic test, it will not get accomplished. A client information hand-out should be provided, as often there is not enough time during the consultation to explain to the clients the importance of a strict food trial and the items that need to be avoided.

Everyone in the family must be motivated and able to comply with the test diet trial. If there is a family member who is incapable of cooperation, then the pet must be crated during meal times and when snacks are at hand or the trial cannot be accomplished. Family motivation can be improved by asking the clients to keep a log and making this a family project. Daily records of itching level and honest recording of trial ‘accidents’ on the log can provide extra motivation and good information as well. Provide a hand-out with the log to be filled out.

Common Pitfalls

  • alternative food sources from small children or neighbours
  • accessible food bowls of other animals
  • pet is an escape artist and gets out of the house/garden
  • Getting into the rubbish bin and other bad habits (eating other dog or fox poo).
  • Dog or cat begging for food during social events!
  • Grandparents looking after the pet who think the food trial is ridiculous.

Patient compliance

Occasionally encountered problems are low palatability and refusal to eat the diet. You cannot do a food trial on a free-roaming pet. Don’t even bother to try! Food trials can only be accomplished when the clients have complete control over what their pet is eating. This is a particular challenge in trying to diagnose food-allergic cats. You may have to spend a couple of months or more converting a cat to an indoor only pet.

How long should a diet trial last?

The reported required duration of diet trials has varied over the years. Nowadays it is not uncommon to have dogs and cats on a diet trial for several weeks (usually 10-12 weeks). Recheck your food trial patients monthly! A food trial requires regular motivational meetings with the pet’s owner. Discuss pitfalls, accidents and progress. Monthly rechecks also allow you to check flea control, secondary infection and other things that can complicate interpretation.

Don’t Neglect to Re-Challenge!

It is important to prove that the previous diet was the cause of the problems. Otherwise it could have been the fleas that just finally came under control, or maybe it is seasonal allergy and the pollen picture changed. If there is a significant clinical improvement with the elimination diet, which lasts for a couple of weeks, the patient is then challenged with the previous food. Reported relapse times vary from a few hours to 14 days. If a subsequent change back to the elimination diet resolves the clinical signs it is likely that the patient has a food adverse reaction. This elimination-and-challenge principle is very time-consuming, but conclusively identifies the majority of patients with adverse reactions to food.

Remember to also challenge with old treats too, before assuming that the pet has not shown any problem after it has been re-challenged.

Have client cut out and save the ingredient list of the food/treats used for the challenge. That contains the list of all the suspect substances. Then find a commercial pet food that avoids all of the suspect ingredients for maintenance. Clients can re-challenge with individual ingredients if they wish so they can narrow down the list of pet foods and treats that are not safe to use!

Good luck!