Definition
Adverse food reactions can be caused by food allergy (triggered by the dog’s immune system) or food intolerances. Food allergy (or canine food hypersensitivity) may cause non-seasonal pruritus due to a hypersensitivity reaction to one or more ingredients in the dog’s diet.
It is now thought that food allergy may manifest as atopic dermatitis in some patients (food-induced atopic dermatitis). It is estimated that 10 to 20% of dogs with atopic dermatitis have a food-induced component to their disease. Prognosis is good with dietary management.
Cause
Food allergy is caused by a hypersensitivity reaction to allergens (usually large protein or carbohydrate molecules) within the food.
The intestine has many protective mechanisms to prevent food antigens entering the circulation. A breakdown in either the physical barriers or the immune response is thought to enable a sensitisation reaction to occur.
Signalment and history
The clinical signs of food allergy may be identical to those of atopic dermatitis. Food allergies may also manifest as non-atopic dermatitis syndromes such as urticaria (hives) or gastrointestinal signs (for example vomiting or diarrhoea). Food allergy may occur concurrently with other allergic skin disease.
The disease can occur in dogs of any age, even at less than 6 months of age.
Some breeds (for example Labrador retrievers) have been anecdotally reported as being at higher risk of food allergy, but opinion on this varies.
The most common presenting sign is non-seasonal pruritus. Some seasonality may occur if the diet changes during the year, due to the presence of flare factors, or if the dog has additional conditions, such as flea allergy or environmental allergen-driven atopic dermatitis. Food allergy should be considered in any dog with non-seasonal pruritus.
Clinical signs
Signs of food allergy vary depending on the clinical syndrome but typically they can be identical to that of atopic dermatitis (i.e. food-induced atopic dermatitis).
Otitis externa and/or secondary infections, especially Malassezia, are common. Other lesions such as urticaria (hives) are less common.
In chronic cases, just like in atopic dermatitis, the skin develops alopecia, excoriations, lichenification and hyperpigmentation.
Dogs may also have a history of gastrointestinal signs, such as intermittent vomiting or diarrhoea, mucoid or bloody stools, increased frequency or volume of stools, borborygmi or colitis.
Diagnosis
There is no specific diagnostic test for food allergy. It is a clinical diagnosis based on exclusion of other conditions that could cause similar signs and a positive food elimination trial.
Differentials
Any other cause of non-seasonal pruritus including:
- Hypersensitivity reactions (for example atopic dermatitis, flea allergy dermatitis)
- Parasites (for example mange)
- Bacterial or yeast infections of the skin
To diagnose, a logical stepwise approach, just as for other causes of pruritus such as atopic dermatitis, is required. (Owners should be made aware that it may take months to reach a confirmed diagnosis.)
Diagnostic approach
- Compatible history and clinical signs.
- Rule out/treat parasites
- Comprehensive flea control
- Skin scrapes and coat brushings +/- trichograms to identify other parasites (for example Sarcoptes, Demodex, Cheyletiella)
- To rule out Sarcoptes, serology and/or trial therapy may also be necessary
- Rule out/treat infections
- Cytology (tape strips/impression smears) to identify yeast (Malassezia) and bacteria
- Food elimination trial – the gold standard to diagnosis of food allergy.
- Rechallenge – an essential step to confirm that any improvements are the result of food elimination rather than due to parasite control or seasonal changes in allergen exposure.
- Consider the possibility of additional hypersensitivity (for example atopic dermatitis) particularly if the dog partially improves on a food trial and relapses on rechallenge.
Treatment and management
If complete resolution of signs on elimination trial
- Complete treatment and introduce management strategies for any parasitic disease or secondary infections.
- Maintain on elimination diet or select a maintenance diet based on the tested novel protein and carbohydrate.
- Compile a list of ingredients from the original diet and consider rechallenging with individual ingredients over time.
- If the owners wish to increase dietary options, foods may be re-introduced into the diet one at a time, monitoring for relapse (usually seen within 14 days).
- Follow up: regular updates on progress, dietary changes and skin flare-ups can help maintain control of food allergy and provide the opportunity to address any questions or concerns the owners may have.
If partial resolution during dietary trial
- Check to ensure parasitic and secondary infections are resolved.
- Partial resolution may indicate accompanying hypersensitivities such as atopic dermatitis. A partial resolution may be seen as a change in lesions or in the pattern of pruritus and may not be noticed by the client as an obvious decrease in pruritus.
If relapse occurs
- Check for history of recent dietary indiscretion.
- Rule out flare factors such as secondary infections and parasites.
- Dogs may develop new allergies leading to relapse.
- Changing to a new protein and carbohydrate should stabilise the condition if the relapse is due to a new food allergy.
- Non-resolution may indicate the presence of a concurrent skin disease.
