Food allergies and food intolerances are different conditions, yet are often confused with each other due to their similar symptoms such as itchy skin or loose stools. The true prevalence of food allergies in domestic dogs and cats is unknown but is thought to sit approximately between 1-3% (1). Thus, if you have an itchy pet with gut issues, they’re more likely to have an intolerance over an allergy (Craig). Allergy reactions cause immediate inflammation, whereas an intolerance can develop gradually over time, causing recurring skin problems and chronic loose stools.
Adverse food reactions may be caused by an allergy, intolerance or toxin. A food allergy is an adverse reaction which does involve the immune system (mediated by antibody IgE), known as a food hypersensitivity. Whereas an intolerance does provoke a systemic immune response, it mostly affects function of the gastrointestinal tract (2).
Food Allergy
A food allergy is an exaggerated response from the immune system to a particular dietary component, most commonly caused by proteins (2). Allergen proteins kick off an immune cascade, leading to histamine release, and subsequent skin or gut symptoms.
The most common symptom of a food allergy is pruritis (itching), with common hotspots on dogs being the tummy, ears, and paws. Excessive licking and nibbling of the paws indicates discomfort, and may be the result of an allergy, be it food or environmental. Whilst pruritis is also the most common symptom in cats, it tends to be localised to the head and ears (1). Unfortunately, skin abnormalities caused by food allergies can mimic other skin conditions, such as canine atopic dermatitis (3), further making diagnosis of a true food allergy challenging. Gastrointestinal illness is the most common secondary issue associated with food allergies. As a result, it can be difficult to differentiate an allergy from an intolerance, of which the gastrointestinal symptoms are very similar.
Food Intolerance
Intolerances are more associated with gastrointestinal symptoms, such as diarrhoea, vomiting, flatulence, softer stools, or increased frequency of defecation. However, symptoms may also be present on the skin, due to poor gut health. Reactions may be metabolic, pharmacologic, or idiosyncratic in nature.
- Metabolic: E.g. Lactose intolerance, which is relatively common in dogs and cats. It occurs due to the high lactose content of cow’s milk, and the significant decrease in lactase (enzyme) production post-weaning, meaning it cannot be broken down as efficiently (4).
- Pharmacologic: E.g. Histamine in certain foods, include vasoactive amines, such as histidine in spoiled fish, which are converted to histamine by the intestinal flora, resulting in gut inflammation (3).
- Idiosyncratic: E.g. The reaction to certain food additives (3).
Food intolerances tend to be easier to resolve than allergies, and can be overcome with healing of the gut and an elimination diet to figure out which proteins work for your dog.
| Symptoms | |
| Food Allergy | Food Intolerance |
| Itching | Recurring gut and skin problems |
| Vomiting | Itching |
| Diarrhoea | Vomiting |
| Breathing abnormalities | Diarrhoea |
| Weepy eyes | Flatulence |
| Coughing | Loss of appetite |
| Anaphylactic shock (severe cases) | SIBO (Small Intestinal Bacterial Overgrowth) |
What Causes Adverse Food Reactions?
All food is antigenic – it’s a foreign material that is ingested. However, oral tolerance in healthy animals ensures immune ‘unresponsiveness’ to the food. Without oral tolerance, an allergic reaction (IgE response) develops towards a certain antigen. IgE then binds to gastrointestinal situated mast cells, and on contact with the food antigen, degranulation occurs, this in turn releases inflammatory mediators such as histamine (5). A healthy gut is a crucial requirement for oral tolerance. Inflammation of the gut lining increases its permeability to undigested food molecules, allowing absorption of allergenic antigens, thus a breakdown in oral tolerance (3).
The tissue surrounding the gut is known as the gut-associated lymphoid tissues (GALT), it monitors what permeates across the gut barrier. Food molecules which are poorly digested can antagonise the body’s immune cells, 80% of which reside in the intestinal tract. This results in inflammation, followed by leaky gut. This weakens the tight intestinal barrier junctions further allowing partially digestive food molecules to pass through. Whether this be chicken, wheat, beef etc… the immune system ‘tags’ these molecules as invaders (5).
| Common Allergens in Dogs | Common Allergens in Cats |
| Beef | Beef |
| Chicken | Chicken |
| Lamb | Fish |
| Wheat | Wheat |
| Corn | Corn |
| Soy | Soy |
Whilst an allergic response occurs quickly following consumption, reactions associated with intolerances are likely to be delayed (sometimes up to 48 hours), which can make it challenging to identify what food is causing them (4).
In the following blog, Elimination Diets For Dogs and Cats: Troubleshooting Allergies and Intolerances, we will discuss how you identify the food(s) causing the issues and why raw is recommended.
References
- Pucheu-Haston, C. M. (2020) ‘Cutaneous Food Allergy in Animals,’ MSD Veterinary Manual. Available at: https://www.msdvetmanual.com/integumentary-system/food-allergy/cutaneous-food-allergy-in-animals
- Sanderson, S. L. (2024) ‘Nutrition in Disease Management in Small Animals,’ MSD Veterinary Manual. Available at: https://www.msdvetmanual.com/management-and-nutrition/nutrition-small-animals/nutrition-in-disease-management-in-small-animals
- Gaschen, F. P. and Merchant, S. R. (2011) ‘Adverse food reactions in dogs and cats,’ Veterinary Clinics of North America: Small Animal Practice, 41(2), 361-379. Available at: https://doi.org/10.1016/j.cvsm.2011.02.005
- Ackerman, N. (2015) ‘Allergies and intolerances in cats and dogs: critical factors and role of diet,’ Vet Times. Available at: https://www.vettimes.co.uk/app/uploads/wp-post-to-pdf-enhanced-cache/1/allergies-and-intolerances-in-cats-and-dogs-critical-factors-and-role-of-diet.pdf
- Verlinden, A. et al. (2006) ‘Food Allergy in Dogs and Cats: A Review,’ Critical Reviews in Food Science and Nutrition, 46(3), 259-273. Available at: https://doi.org/10.1080/10408390591001117
Lucy James, BSc (Hons) Bioveterinary Science
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