Dogs and cats may display adverse reactions to food that can be broadly divided into two groups – immunological (food allergy) and non-immunological. A food allergy is an inappropriate immune reaction to a normal food or ingredient (e.g. a protein in the food) which can result in dermatological (e.g. red, itchy skin) and/or gastrointestinal (e.g. diarrhoea, vomiting) signs in dogs and cats (Verlinden et al., 2006). While ingredients such as chicken, beef and soya are readily recognisable as protein sources, it is also important to consider ingredients that are primarily providing carbohydrates to the recipe such as rice, wheat, potato, and corn as these also contain a small amount of protein. Non-immunological adverse food reactions can result from a number of causes such as food intolerance, food poisoning, food toxicity and dietary indiscretion (Verlinden et al., 2006).
Clinical signs of food allergy, food intolerance, food poisoning and dietary indiscretion can overlap, making it difficult to determine the cause and may lead to an adverse food reaction to being indiscriminately labelled by an owner as a ‘food allergy’. In some respects, it is not necessarily important to an owner whether their pet has, for example, an allergy or an intolerance to wheat gluten since the symptoms in both cases can be resolved in the same way – by removing the ‘offending’ ingredient from the diet of the pet.
Food allergies can develop in response to any dietary protein and those most frequently associated with food allergies are shown in the table below. It is not that these protein sources are intrinsically more ‘allergenic’ than other proteins but probably reflect the widespread use of these ingredients in pet food.
If a food allergy is suspected, the gold standard approach to determine the cause is by exclusion diets and re-challenge trials, although this would not discriminate between a food allergy and a non-immune mediated food reaction.
The key to exclusion diet trials is to exclude the possible ingredients the pet is allergic to by selecting a novel protein (and carbohydrate) source to which the dog or cat has not previously been exposed. The diet chosen must be fed exclusively (meaning no other food or treats should be fed alongside) for the trial period. The duration the diet is fed for will depend on the initial severity of the symptoms and on how quickly the clinical signs recede, for example, 2-4 weeks for gastrointestinal symptoms and 4-8 weeks or potentially longer for dermatological symptoms (Verlinden et al., 2006).
Once symptoms have improved sufficiently, re-occurrence of symptoms when re-challenged with the original food or individual ingredient supports the diagnosis of food allergy and identification of the specific protein to which the pet is sensitive.
Although elimination diets help owners to single out particular food types, it can be quite daunting for the owner. The process can be slow and time-consuming, to be effective it takes dedication and owners may not see immediate results.
Historically, hypoallergenic pet food diets avoided the use of common food allergens (e.g. beef, dairy, wheat, soy) and instead were based on novel proteins (e.g. lamb or salmon), which were not typically used to formulate pet foods, on the basis of a reduced likelihood of being allergic to a protein to which a pet had not previously been exposed. This approach can be very effective for many dogs and cats, although it may be necessary to try more than one novel protein diet until a suitable diet is found. It is also possible that the pet may eventually develop an allergy to the novel protein.
A number of pet food companies have marketed their products as good or suitable for pets with allergies or intolerances, due to being a ‘Limited Ingredient Diet’. The recipes usually use only one source of animal protein and one carbohydrate source. The ‘Limited Ingredient Diet’ trend seemingly links back to the idea of an elimination diet, as previously mentioned, excluding certain ingredients to try and limit the risk of an adverse food reaction.
More recently, hypoallergenic pet food diets have been introduced to the market that contains hydrolysed proteins. By using controlled enzymatic hydrolysis, proteins can be partially or extensively broken down into smaller peptides that can be too small to be detected by the immune system, making them hypoallergenic. One advantage of this approach is that they can be effective even in pets allergic to intact protein. For example, dogs that showed adverse gastrointestinal and/or dermatological signs following ingestion of soy protein did not show any clinical signs in response to ingestion of hydrolysed soy protein (Puigdemont et al., 2006). Similarly, in a study of 12 dogs with cutaneous manifestations after exposure to chicken meat, all but one showed a reduction in clinical scores when fed hydrolysed chicken (Ricci et al., 2010).