Food allergy in dogs: symptoms and elimination diets

Overview

Adverse food reaction is an umbrella term for symptoms caused by food. Food allergy involves an immune response, whereas food intolerance can cause symptoms without one. A response to a diet change alone does not establish allergy. Food's contribution is confirmed when symptoms first improve on a restricted diet and then return during a controlled challenge.

Skin symptoms cannot reliably distinguish food allergy from atopic disease caused by environmental allergens. A dog can have both conditions.

A dietary trial investigates whether food contributes to symptoms, although it does not measure the mechanism of the reaction. Itching can also vary when a dog eats different foods on different days.

Causes

A particular food ingredient, most often a protein, triggers the allergy. One dog may react to more than one food. Allergens can occur outside the main diet, for example in flavoured medicines. Common allergens reflect what dogs are usually fed in different regions.

Among 297 dogs with at least one identified food trigger, beef caused symptoms in 102, or 34%. Dairy products were associated with symptoms in 51, or 17%. Chicken caused symptoms in 45, or 15%. Wheat was associated with symptoms in 38, or 13%.

Lamb was associated with symptoms in 14 dogs, or 5%, in the same group of 297. The ranking of foods in these cases does not show how often healthy dogs eating them develop allergy.

Risk factors

Food allergy symptoms often begin in young adulthood, at around one to four years old. Reported age at onset ranges from under six months to 13 years. Neither sex has been shown to be clearly more susceptible. Breeds identified as predisposed vary between regions according to local breed populations.

Labrador Retrievers and German Shepherd Dogs, for example, have been overrepresented in many Western countries. Cutaneous adverse food reactions have been identified in around 1–2% of dogs attending a vet for any reason. In groups of itchy dogs, the corresponding proportion has been 9–40%.

Among dogs presented for skin disease, the proportion with cutaneous adverse food reactions has ranged from zero to 24%. Some dogs develop food allergy after previously tolerating the same food.

Typical symptoms

The most typical skin symptom is itching, which may be localised or affect the whole body. Food-allergic itching is not usually confined to a particular season.

• A dog may lick the paws or rub the muzzle because of itching.
• Itching often affects the ear flaps, paws or abdominal skin.
• The skin may redden or develop small papules.
• Ear inflammation may return after treatment.
• Hair can thin where the dog scratches or licks.
• Digestive symptoms may include vomiting, soft stools or diarrhoea.

Vomiting, soft stools or diarrhoea occur alongside skin symptoms in around 19–27% of food-allergic dogs. Defaecating more than three times daily can also be a gastrointestinal sign. Environmental allergy may explain why year-round itching worsens in a particular season.

Digestive problems can also cause flatulence or unusually noticeable abdominal gurgling. A food reaction causing digestive symptoms may involve weight loss. Sometimes ear inflammation is an allergic dog's first or only noticeable sign.

Complications and consequences

Intense itching can make a dog scratch the skin open. Skin damage can in turn maintain inflammation and itching. Bacteria or Malassezia yeast can cause recurrent infections in irritated skin. Secondary infection can increase itching even after the triggering ingredient has been removed.

Prolonged scratching can thicken the skin. Allergic inflammation of the ear canal may involve secondary bacterial or yeast infection. Anaphylaxis is a severe reaction rarely reported with food allergy.

Skin infection can recur even when the dog is almost symptom-free between episodes. Microbial growth in the ears or skin can make it harder to assess how much itching is food-related.

Diagnosis at the vet

Other causes and choosing a trial diet

The vet reviews previous foods, treats and medicines before choosing a trial diet. Skin and ear samples help identify bacterial or yeast infection. External parasites are also investigated in an itchy dog. Blood, saliva or hair tests cannot reliably confirm food allergy.

A trial food may contain protein the dog has never eaten before. Another option is a hydrolysed diet, with proteins broken into smaller pieces to make a reaction less likely. Partially broken-down protein can still cause symptoms in a dog sensitised to its original source. Ordinary shop-bought limited-ingredient foods have been found to contain proteins not listed on the label.

If the dog's previous feeding history is unknown, a hydrolysed trial diet may be a practical choice.

Elimination diet duration and food challenge

Assessment during an elimination diet may take 4–12 weeks, according to symptoms and response.

During the trial, the dog receives only the selected food and water. Skin symptoms are usually assessed after around eight weeks on a strict dietary trial. A trial for skin symptoms may last 8–12 weeks if more time is needed to assess the response. Improvement in digestive symptoms alone can be assessed after two to four weeks.

Among 209 studied dogs with cutaneous adverse food reactions, symptoms had settled in over 85% by five weeks. In the same follow-up studies, the proportion rose to over 95% at eight weeks. Fewer than 5% needed a trial lasting up to 13 weeks before skin symptoms resolved completely.

The trial's implementation can be reviewed at two to three weeks to identify feeding problems early. If itching does not improve, assessment includes whether the dog had access to anything outside the trial diet.

If the first trial diet does not improve symptoms, the vet may consider another.

The vet needs to assess the effects of anti-itch medication before a challenge, because it can mask remaining skin symptoms.

Once symptoms improve, the veterinary-planned challenge usually reintroduces the previous food. Returning symptoms help confirm that improvement resulted from the diet. If a dog reacts to a mixed commercial food, this does not yet identify the responsible ingredient. When symptoms return, the dog goes back to the suitable trial diet.

Skin symptoms do not always return on the first day after a challenge. A small study of 11 dogs with confirmed adverse food reactions involved 71 controlled food challenges. Symptoms occurred in 35 challenges. In those challenges, symptoms returned after an average of 4.1 days. Reactions in the study occurred within one to seven days.

When investigating individual ingredients, only one is added at a time so a reaction can be linked to the correct food. In over 90% of dogs with cutaneous food reactions, symptoms returned within 14 days of a challenge. They often return within one to three days, but in some cases can take up to four weeks to be noticed.

Treatment

A suitable long-term diet

The vet prescribes treatment and any medication. Confirmed food allergy is managed by avoiding the triggering ingredient. A well-tolerated trial food can continue if it is nutritionally balanced for long-term use. A growing puppy needs food meeting growth-stage nutritional requirements. A simple homemade trial diet can cause deficiencies if continued long-term without balancing.

Secondary skin or ear infection needs its own treatment. Itch relief may also be needed early in a dietary trial to prevent self-inflicted skin damage. Concurrent atopic disease may still need separate treatment after the diet changes.

For long-term home-prepared feeding, a professional with expertise in animal nutrition can help balance the diet. Diet can keep symptoms absent or nearly absent if other causes of itching are not maintaining them.

Prevention

Further episodes are prevented by avoiding the specific ingredients shown to trigger symptoms. The same ingredient may occur in a treat, supplement or dog toothpaste. If symptoms return after starting a new bag of food, a recipe change or unlisted protein is a possible explanation. Separating other animals' food reduces accidental access to unsuitable food.

Recurrent symptoms can also relate to fleas or environmental allergy even if the food remains unchanged. A triggering protein may also occur in flavoured chewable medicines, even when it is not listed on the packaging.

Important for dog owners

Agree that everyone caring for the dog gives the same trial food during the trial. Tell the vet about flavoured medicines too, as they may contain food proteins. Keep a diary of changes in itching and digestive symptoms. Also record accidentally eaten foods or unusual events that may explain temporary worsening. Do not give flavoured chews or treats during the trial.

Keep other pets' food out of reach during the trial. Mention if your dog eats another animal's faeces, as this can interfere with the dietary trial. A diary helps reveal gradual changes when symptoms improve only partly.

Frequently asked questions

Can a dog become allergic to food eaten for a long time?

Yes: a dog can become sensitised to previously well-tolerated food. A familiar ingredient can also cause symptoms, so a trial diet may use a protein the dog has never eaten. A controlled challenge investigates an individual ingredient's contribution.

Is a blood test enough to diagnose food allergy?

No: blood tests do not reliably identify which foods cause a dog's symptoms. A dietary trial investigates whether symptoms improve when possible triggers are removed. Diagnosis requires symptoms first to improve on the trial food and then return during a controlled food challenge.

Can my dog have treats during the trial?

Treats outside the trial diet are not given because they may contain triggering proteins. Avoid chews and table scraps too. The veterinary-selected diet is the only food during the trial, with water provided as usual.

Why is the previous food given again?

A veterinary-planned food challenge checks whether symptoms return on the previous food. Itching may improve during a trial for another reason, such as the pollen season ending. Skin symptoms can return after a delay, so monitoring for up to 14 days may be needed.

What if itching improves only partly?

Partial improvement may mean food contributes to symptoms but another cause of itching is also present. Atopic disease triggered by environmental allergens, for example, can continue despite dietary changes. The food challenge monitors whether remaining symptoms worsen.