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Adverse food reactions in dogs and cats: What really matters in diagnosis and nutritional management – Part 1

  • Ecommercy
  • 2026-09-22
  • 15:45
  • 0 comments


Introduction and definitions

Adverse food reactions encompass a broad group of disorders with different underlying pathophysiological mechanisms, which are often confused or treated as synonymous in clinical practice. The most common distinction is between food allergy and food intolerance, although the boundary between the two can be unclear and their clinical presentations overlap considerably. The terms are often used interchangeably, and owners may regard any adverse reaction to food as an allergy.

Food allergy, however, is an abnormal immune-mediated response in which the immune system reacts to dietary components, most commonly proteins or protein fragments. The reaction may be immediate and IgE-mediated, or delayed, cell-mediated and independent of IgE. Regardless of the mechanism, a food antigen is recognised as a threat and an inflammatory cascade is triggered. In dogs and cats, food allergy most commonly manifests as chronic, non-seasonal pruritus and recurrent otitis externa. Gastrointestinal signs such as diarrhoea, vomiting or abdominal pain are much less commonly the only manifestation. Importantly, even a small amount of an allergen may trigger clear clinical signs.

Food intolerance, on the other hand, encompasses non-immunological reactions resulting from abnormalities in digestion, absorption or intestinal barrier function. These may be associated with enzyme deficiencies, bioactive substances naturally present in food or secondary intestinal hypersensitivity. Unlike allergies, the signs of food intolerance are usually dose-dependent and more commonly limited to the gastrointestinal tract, although in clinical practice this distinction is not always clear.

Diagnosis is further complicated by the frequent coexistence of other problems, including environmental allergies, chronic enteropathies and disturbances of the intestinal microbiota. In many patients, immunological and non-immunological mechanisms overlap, resulting in a complex and variable clinical presentation. For this reason, there is an increasing tendency to move away from a rigid distinction between food allergy and food intolerance in favour of the broader concept of adverse food reactions, with an elimination diet regarded as a fundamental diagnostic and therapeutic tool.

Epidemiology and clinical significance

The clinical significance of adverse food reactions in dogs and cats depends strongly on the population being assessed. Across the general population of patients presented to veterinary practices, these diagnoses are relatively uncommon. However, this does not reflect the true importance of the problem in veterinary dermatology, as most animals without allergic signs never undergo dietary investigation.

The prevalence is considerably higher among animals presenting with dermatological signs. In dogs with chronic, non-seasonal pruritus, recurrent otitis or allergic skin disease, a food-related component is considerably more common and, depending on the diagnostic criteria and rigour of the elimination trial, may affect a substantial proportion of patients. In practice, this means that an elimination diet should be regarded as a standard part of the diagnostic work-up in these animals rather than as an alternative diagnosis.

In cats, epidemiology is more difficult to assess because fewer studies are available, clinical signs are more variable and dermatological conditions frequently overlap with other causes of pruritus. Nevertheless, available evidence indicates that food-related reactions are clinically relevant in cats with chronic pruritus, eosinophilic lesions, miliary dermatitis or recurrent otitis. An elimination diet should therefore form part of the routine diagnostic approach in these patients.

Species differences are also evident in the clinical presentation. Dermatological signs predominate in dogs, while gastrointestinal signs may be absent, increasing the risk that a diet-related component will be overlooked. In cats, diagnosis is often based on non-specific dermatoses and pruritus, the interpretation of which can be complicated by numerous confounding factors. In both species, the absence of gastrointestinal signs does not exclude an adverse food reaction.

Available evidence does not indicate an age threshold beyond which an adverse food reaction can be ruled out. Clinical signs may develop in young, adult or senior animals, often after prolonged exposure to the same dietary ingredients. This is clinically important when interpreting the dietary history and planning an elimination diet.

There is also no clear pattern of breed predisposition that is specific to food allergy. In practice, assessing the clinical phenotype – such as year-round pruritus, chronic otitis or an incomplete response to antipruritic treatment – is more diagnostically useful than assigning risk on the basis of breed.

Previous dietary exposure plays an important role. The most frequently identified allergens are ingredients that have been commonly and repeatedly consumed over a long period, which can make it difficult to identify genuinely novel protein sources for an elimination trial. In patients with gastrointestinal disease, impaired intestinal barrier function or atopic dermatitis, a food-related component may exacerbate clinical signs rather than represent the sole cause of disease.

From a clinical perspective, the conclusion is clear: although adverse food reactions are relatively uncommon in the general population, they represent an important and modifiable factor in patients with pruritus and chronic dermatological disease. A properly planned elimination diet can be crucial for successful control of clinical signs and may reduce the need for long-term anti-inflammatory and antipruritic medication.

Pathophysiological mechanisms of adverse food reactions

Adverse food reactions in dogs and cats encompass a broad spectrum of pathophysiological mechanisms, only some of which are strictly allergic. In clinical practice, it is essential to understand that very similar clinical presentations can result from different biological processes and that several mechanisms may coexist in the same patient.

Classical food allergy is an immune-mediated response directed against antigens present in the diet, most commonly proteins or protein fragments. The best-understood mechanism is the immediate IgE-mediated reaction, in which re-exposure to an allergen following sensitisation leads to mast cell activation and the release of inflammatory mediators. Clinically, this may result in a rapid increase in pruritus, erythema, oedema or gastrointestinal signs shortly after eating.

Delayed, non-IgE-mediated and cell-mediated reactions are considerably more difficult to diagnose and are probably more common among veterinary patients. In these cases, T lymphocytes and local inflammatory responses within the skin or intestinal mucosa play a key role. Clinical signs develop more slowly, often over many hours or days, making them difficult to associate directly with diet. This mechanism helps explain the limited value of serological testing and why an elimination diet remains the primary diagnostic tool. Clinically, cell-mediated reactions most commonly manifest as chronic, persistent pruritus, recurrent otitis or chronic enteropathies.

Non-immunological reactions, referred to as food intolerances, form another important group. They may result from enzyme deficiencies, impaired digestion and absorption, or reactions to naturally occurring bioactive substances in food, such as biogenic amines or salicylates. These mechanisms predominantly cause gastrointestinal signs without direct involvement of the immune system.

Increasing attention is being paid to the role of the intestinal barrier and microbiota in the pathogenesis of adverse food reactions. Damage to the intestinal mucosal barrier increases permeability to food antigens and promotes excessive activation of the immune system. Microbiome disturbances may further impair mechanisms of immune tolerance, helping to explain why adverse food reactions may develop secondarily, for example following gastrointestinal infections, antibiotic treatment or in the course of chronic enteropathies.

It is also important to recognise that food allergy and food intolerance are not mutually exclusive. Immune-mediated reactions, digestive abnormalities and intestinal dysbiosis may coexist in the same patient. This multifactorial background supports a holistic approach in which an elimination diet serves not only as a diagnostic test but also as a means of modifying the intestinal environment and the body’s immune response.

Clinical presentation and differential diagnosis

The clinical presentation of adverse food reactions in dogs and cats is variable and largely non-specific, making diagnosis particularly challenging. Clinical signs may involve the skin, gastrointestinal tract or both systems simultaneously. The course is most commonly chronic or recurrent, with periods of partial improvement and flare-ups that cannot always be clearly associated with diet.

In dermatological practice, persistent pruritus is the predominant sign. In dogs, it most commonly affects the ears, paws, groin, perianal area and abdomen. Recurrent bilateral otitis externa that responds poorly to standard treatment is also common. These signs may be accompanied by erythema, secondary bacterial and yeast infections, and lesions resulting from intensive scratching and licking.

In cats, the clinical presentation is often more variable and may include miliary dermatitis, eosinophilic lesions, pruritus of the head and neck, symmetrical alopecia caused by excessive grooming and recurrent otitis. In both species, the absence of a sustained response to antipruritic treatment should prompt consideration of a dietary component.

Gastrointestinal signs are common, although they do not always occur alongside dermatological lesions. They most commonly include chronic or recurrent diarrhoea, vomiting, loose stools of variable consistency, mucus or fresh blood in the faeces, increased frequency of defecation and signs of discomfort after eating. In some patients, these signs are mild and intermittent and may therefore be overlooked. At the same time, the absence of gastrointestinal signs does not exclude an adverse food reaction.

Systemic signs are less common but may include weight loss, deterioration in body condition, poor coat quality and reduced exercise tolerance. In young animals, chronic adverse food reactions may affect growth and development, while in adults they may contribute to persistent low-grade inflammation.

Because the clinical signs are non-specific, differential diagnosis is essential. Parasitic diseases, bacterial and fungal skin infections, and primary gastrointestinal diseases that may mimic or mask food-related reactions must be excluded. In adult and senior patients, endocrine and systemic diseases affecting the condition of the skin and gastrointestinal tract should also be considered.

Differentiating adverse food reactions from environmental allergies is particularly important. In many patients, both mechanisms coexist, with the dietary component exacerbating the course of atopic disease. Non-seasonal clinical signs, an incomplete response to treatment and persistent lesions despite control of environmental allergens should prompt the introduction of an elimination diet as an integral part of the diagnostic work-up rather than an option reserved only for refractory cases.

Proteins most commonly associated with adverse food reactions in dogs and cats

In clinical practice, adverse food reactions most commonly involve proteins that have been a long-term or repeatedly consumed part of the diet rather than ingredients that are inherently “more allergenic”. Nevertheless, retrospective studies and clinical observations allow us to identify groups of proteins most frequently recognised as clinically relevant food allergens.

In dogs, adverse food reactions are most commonly associated with animal proteins, particularly beef and poultry. The high frequency of reactions to these ingredients is linked to their widespread use and frequent introduction early in life. Other commonly identified allergens include dairy products, chicken eggs and fish proteins. Reactions to lamb, pork and soy are reported less frequently, although they remain clinically relevant, particularly in dogs with a long history of dietary exposure.

In cats, the range of proteins associated with adverse reactions is similar, although their relative frequency may differ. Beef, fish and poultry, including chicken and turkey, are most commonly reported. Reactions to dairy products are also relatively common, although some clinical signs in cats may result from intolerance rather than a classical immune-mediated response. Importantly, an adverse food reaction may develop even after a very long period during which a particular protein was previously well tolerated.

It should be emphasised that the frequency with which a particular protein is identified as an allergen does not necessarily indicate greater allergenic potential. Rather, it reflects how frequently animals are exposed to that protein within the population. The choice of an elimination diet should therefore be based primarily on the individual patient’s dietary history rather than general lists of “common allergens”. A protein to which the animal has not previously been exposed has the greatest diagnostic value, regardless of its position in population statistics.

Limitations of laboratory testing

In everyday clinical practice, there is increasing pressure to provide a “quick diagnosis” of food allergy using laboratory methods such as blood tests, saliva tests or commercial allergen panels. The problem is that these methods have not been reliably validated as diagnostic tools for adverse food reactions in dogs and cats, and their agreement with the actual clinical response to an elimination diet is poor.

As a result, they may lead to incorrect conclusions, unnecessary dietary restrictions and delays in implementing the only procedure with established diagnostic value: elimination followed by controlled food challenge.

A fundamental limitation of serological testing is that the detection of antibodies does not necessarily indicate clinically relevant disease. The presence of specific IgE or other immunoglobulin classes may indicate exposure or an immune response, but it does not establish whether a particular ingredient causes reproducible clinical signs.

This is particularly important in adverse food reactions, where many mechanisms are independent of IgE and involve delayed, cell-mediated responses. Consequently, even correctly performed IgE tests do not cover the full spectrum of adverse food reactions.

Available systematic reviews indicate that serological tests have limited sensitivity and specificity for clinically confirmed adverse food reactions and cannot replace an elimination diet. Another problem is the variability of results between laboratories and testing methods, which significantly complicates interpretation.

IgG panels have even less diagnostic value because increased concentrations of these antibodies most commonly reflect regular exposure to a particular ingredient rather than disease. In practice, this can lead to overdiagnosis and the creation of long lists of “forbidden” ingredients without clinical confirmation.

Studies assessing the accuracy of commercial tests, including serum and saliva assays, demonstrate poor agreement between test results and diagnoses established through elimination diets and food challenges. This also applies to saliva tests and other panels marketed as rapid and non-invasive allergy diagnostics. Samples such as saliva are highly variable and susceptible to confounding factors, while the proposed methods lack clearly defined and reliable clinical thresholds. Their usefulness in everyday practice is therefore limited.

For these reasons, an elimination diet remains the diagnostic gold standard. It is the only method that allows the actual relationship between consumption of a particular food and the development of clinical signs to be assessed. It encompasses the entire spectrum of adverse food reaction mechanisms, regardless of whether they are immunological or non-immunological.

At the same time, an elimination diet also serves a therapeutic purpose by helping establish a safe and effective long-term feeding strategy, which in practice is the primary goal of management.

CONTINUE READING

Watch the webinar recording: “Food allergies in dogs and cats – how to recognise the problem and choose the right diet”


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