The most widely studied and well-established adverse food reactions in cats and dogs are mediated by IgE-associated type I immediate hypersensitivity reactions (Muller et al., 1989). Although the onset is “immediate”, this condition may evolve chronically.
What is type I hypersensitivity?
Type I hypersensitivity is caused by the production of IgE immunoglobulins (also known as antibodies) by B lymphocytes in response to a specific exogenous allergen. These immunoglobulins bind to mast cells and stimulate the release of chemical mediators (a phenomenon called degranulation) such as histamine, which causes different signs in various body systems depending on the route of exposure to the allergen: aerosol inhalation, direct contact, oral intake, injection, etc. This is classically referred to as an allergic reaction.
Type I hypersensitivity is caused by the production of IgE immunoglobulins (also known as antibodies) by B lymphocytes in response to a specific exogenous allergen
Signs can be local, affecting specifically the system whereby the body has been exposed to the allergen, such as: eyes (conjunctivitis), skin and mucous membranes (rash, hives, itching, oedema), gastrointestinal tract (abdominal pain, bloating, vomiting, diarrhoea), upper respiratory tract (sneezing, rhinitis, sinusitis, laryngeal oedema) or lower respiratory tract (bronchoconstriction and dyspnoea). In more severe acute cases, more generalised signs may be seen, with specific consequences on the cardiovascular system such as hypotension and, in the most severe form of anaphylactic shock, coma or even death (Muller et al., 1989; Plant, 2011).
What constitutes an allergen?
An allergen is defined as any innocuous molecule that can trigger an immune response. This seems to be a very broad definition, but in fact only complex molecules are considered as able to stimulate the immune system and trigger an allergic reaction (Verlinden et al., 2006). Food allergens are generally large water-soluble proteins or glycoproteins ranging from 10,000 to 70,000 Daltons (10 to 70KDa; Muller et al., 1989; Plant, 2011; Verlinden et al., 2006). Although size is not the only characteristic that impacts the protein’s ability to trigger an immune reaction, it is crucial for the necessary binding.
Food allergens are generally large water-soluble proteins or glycoproteins ranging from 10,000 to 70,000 Daltons
Substances in the intestinal lumen are cleared before they cross the mucosal barrier, composed of different interrelated immunological and non-immunological components. The absorption rate of intact protein depends on the integrity of this mucosal barrier. Different factors contribute to the function of this barrier, such as: morphology and functionality of enterocytes, presence of IgA, good digestion, food quality and composition, and presence of inflammation (Guilford, 1996).
How can a hydrolysed protein diet help?
Hydrolysed forms of intact proteins are used to ensure the diets are hypoallergenic. The hydrolysis process involves the cleavage of the peptide bonds of different amino acids that make up a protein (Figure 1; Knaul et al., 2017).The protein hydrolysis used in hypoallergenic diets aims to disrupt the protein structure to remove as many existing allergenic epitopes as possible, to alter its immunogenicity and the ability to generate an immune reaction, thus contributing to reduce the immune recognition by animals who were already sensitised to the intact protein (Cave, 2006).
Another option is to use an extensively hydrolysed protein that consists mainly of oligopeptides and free amino acids derived from protein breakdown. These are oligopeptides: peptides formed by few amino acids. Extensive protein hydrolysis is indispensable to prevent IgE-mediated food allergen recognition, especially in more complex cases of food hypersensitivity (Olivry et al., 2017).
Hydrolysed protein can contribute to the nutritional management of adverse food reactions. Digestibility expressed in percentage and utilisation of crude protein depend on the degree of hydrolysis and may also depend upon the source ingredient involved.
Research has shown that beef and chicken are among the top allergenic ingredients (Mueller et al., 2016) for cats and dogs. Considering this, the chosen hydrolysed protein source should be safe and effective, in which extensive hydrolysis may contribute to reducing the possibility of triggering/inducing allergic skin conditions in pets with adverse food reactions (Knaul et al., 2017; Olivry et al., 2017; Lesponne et al., 2018).
The chosen hydrolysed protein source should be safe and effective, in which extensive hydrolysis may contribute to reducing the possibility of triggering/inducing allergic skin conditions in pets with adverse food reactions
However, all proteins contained in a formula do not just come from the primary protein source itself. In fact, sources of carbohydrates and other raw materials may contain a small portion of protein in their composition which could contribute to allergic issues.
Pet foods formulated with a purified starch source instead of whole grains can be great options for those animals with adverse food reactions due to severe food sensitivities. Purified starch is obtained by breaking down cereal grains and removing proteins, fibres and oils, which leaves only the purified carbohydrate fraction. Since corn is not a common food allergen source in cats and dogs (Mueller et al., 2016), purified corn starch can be considered a preferred carbohydrate source for pets with suspected food hypersensitivity (Mueller et al., 2016; Olivry and Bexley, 2018).
The basics of the nutritional management of food allergies are very straightforward: prevent exposure to the potential allergen through the use of a diet that does not contain a protein source to which the animal has been previously exposed and sensitised, or that does not contain intact protein. Hydrolysed protein-based diets are advantageous because they are easy to use, readily available and cost-effective for pet owners.
Hydrolysed and extensively hydrolysed proteins provide the additional benefit of being highly digestible, which increases their utilisation and reduces waste. This helps cope with digestive issues caused by food sensitivities and gives us the opportunity to switch the pet to these diets without the dietary transition that takes seven days in most cases, if required.