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Diagnosing Food Allergies


Challenges and Pitfalls

Diagnosing food allergies seems like it should be a simple process, however, as you know, it can be very frustrating! It sounds straightforward enough: feed the pet a food that does not contain their suspected allergic triggers and monitor for changes in their clinical signs. However, many pet owners have already ‘self-diagnosed’ prior to seeing you, or, when you start to delve into the pet’s dietary history, there is a laundry list of proteins that the patient has been exposed to. 


The most common signs of food allergies in dogs are itching and chronic or recurrent skin and ear infections. It is estimated that up to 40% of food allergic dogs also suffer from loose stools and/or vomiting and that 20% of them have concurrent atopic dermatitis.  If a dog’s clinical signs disappear or improve during a food trial, you’ve got your diagnosis, right? 


Not so fast. 


Food trials can take a long time to complete. Six to eight weeks is typical, but some dermatologists  will go as long as 16 weeks before calling it quits. During that time, dogs have to eat ABSOLUTELY nothing but the recommended hypoallergenic food. No treats, no table scraps, no flavoured medications; nothing else but water.

What options are out there?

Most dogs and cats are allergic to one or more of the protein sources in their foods. Therefore, we need to find a food that contains only novel protein sources (i.e., ones they have never eaten before) or proteins that have been modified so they are no longer allergenic (hydrolysed). Determining what exactly is hypoallergenic for a particular pet is not always a straightforward process. Keep in mind that many niche brands are differentiating themselves by including more ‘novel’ proteins in their diets. This makes it increasingly problematic for choosing an appropriate novel protein.

Home prepared?

Home cooking is a popular option for the food trial phase. Unfortauntely, studies have found that the great majority (>90%) of home prepared diet recipes are nutritionally inadequate and not appropriate for long term use, especially in kittens and puppies. Even if the diet is formulated by a board certified veterinary nutritionist, it should be emphasised to the owner that recipe drift is a common cause of nutritional inadequacies.


Hydrolysed protein diets can be a great choice, particularly if the dietary history is unknown, incomplete, or when the pet has been exposed to a large variety of protein sources. Hill’s Prescription Diet z/d canine, for example, contains hydrolysed chicken and purified corn starch. The chicken protein in z/d is broken down enzymatically below the allergic threshold for the vast majority of pets. Protein contamination is reduced via meticulous cleaning between other product batches and the first portion of each z/d batch is discarded to ensure purity of ingredients. It’s a great option to consider for a dietary trial because it is also nutritionally complete and balanced.

Novel Protein?

When choosing a novel protein diet a thorough diet history is essential to ensure that the protein source chosen is indeed “novel”. A great diet history form can be found hereHill’s Prescription Diet d/d is a venison and potato diet (green pea in the cat formulation) which is a great option for a novel protein diet as long as the pet has not had venison in the past.


Carbohydrates are a less frequent, but not negligible, source of allergens. Hypoallergenic diets thus typically contain either rice or potato, to which most dogs and cats don’t react, or novel carbs such as sweet potato.

What about novel protein foods I can get from the pet store?

In an attempt to simplify and reduce the costs of a veterinary food trial, owners frequently ask, “Isn’t there an over-the-counter (OTC) food we could use?” This is a reasonable question given that a trip down to the local pet shop will turn up products with labels including fish, duck, rabbit, venison and other equally enticing sounding combinations. Unfortunately, there are two good reasons why the answer to this question should be “no.”


1. The ingredient list often reveals the presence of protein sources that aren’t advertised on the front of the bag. I looked at the label of two diets available at specialty pet stores. 


One, feline “grain free rabbit and chickpea” food, contained turkey meal. The other, a  “wild game and grain free” canine diet contained, in descending order, duck, lamb, turkey meal, wild boar, rabbit and dried egg.


2. Even if you find an OTC food containing only a single protein and carbohydrate source, the food might still be contaminated with other ingredients. This is because the manufacturing facility is unlikely to have undergone a comprehensive cleaning process between batches. A study published in 2017 revealed that

“In nine of 10 over-the-counter diets, DNA of one or more animal species other than declared on the label was identified. The DNA most frequently detected was derived from beef (n = 8) and pork (n = 6). Two hydrolysed diets(one of these being Hill’s z/d) only contained DNA of the declared animal source.”

Is a food trial really the only way of diagnosing food allergies?

We are occasionally advised on the Hill’s Helpline that the vet (or owner) has performed a saliva or serology test for diagnosing food allergies. The American College of Veterinary Dermatology does not recommend serum or saliva testing for food allergies. It recommends instead a diet trial as the gold standard.  This is largely because saliva and serological tests yield high numbers of false positives. Food allergies are not just an IgE mediated reaction. They can also include IgA and IgM hypersensitivities.

Tips and tricks

  • Ensure pet owners keep the food in the original packaging and use a clean cup each time. Cross contamination is surprisingly common. Many owners will simply dump the new food into the storage container without thoroughly cleaning.
  • Check that other family members are on-board with the ‘special diet’. Separate the pet from toddlers and young children during meal times. There is no use doing a diet trial if there is a well-meaning person in the house giving treats to the pet!
  • Get the owner to keep a food diary for “everything that passes the pets lips” for a week. This can help  if you feel there is a compliance issue.
  • Double check about treats and medications. Many owners do not realise that raw hides and heart worm medications are protein sources!
  • Suggest making a tag “my pet is on a special diet”. This is especially useful if the pet is a wanderer, or if there are pampering neighbours.
  • Send the pet home with a hand out on food allergies and diet trials. Remember that we only remember 30% of what we are told! It will also reinforce the importance of the diet trial
  • Set up follow up phone calls for 48 hours (any question?), 7 days (how did the transition go/remember no treats etc!) and 2 weeks (any response yet? Keep going!)
  • Recheck at 4 and 8 weeks

If in doubt, remember that Hill’s veterinarians are on hand to provide advice on nutritional management of your cases. Call our Veterinary Consultation Service on 1800 800 733. 


If you have enjoyed this blog be sure to check out ‘How nutrition transformed the lives of two atopic dogs’.

Dr Danielle Page BVSc, Bcomm, Professional Consulting Veterinarian, Hill’s Pet Nutrition New Zealand with Rosie

Danielle completed a Bachelor of Commerce from Sydney University in 2003. She then decided to pursue a veterinary degree and graduated from Massey University in 2008. She worked as a small animal veterinarian in Canberra, ACT and then Florida, USA for four years. In 2012 she became the Technical Services Veterinarian for Florida for a veterinary nutrition company and subsequently, Clinical
Trials Manager for the USA. In 2014 she moved back to NZ with her family and joined the Hill’s Professional Veterinary Affairs team and is the Hill’s Professional Consulting Veterinarian for NZ.

Dr Danielle Page


  1. Roudebush P, Guilford GW, Jackson HA, editors. Small Animal Clinic Nutrition, 5th Edition, Mark Morris Institute, 2010  pp. 610 – 611
  2. Loeffler A, et al. Vet Record 2004: 154; 519-522
  3. Roudebush P, Cowell CS. Vet Dermatol 1992; 3: 23-28
  4. Horvath-Ungerboeck, C 2017 Detection of DNA from undeclared animal species in commercial elimination diets for dogs using PCRVet Dermatol 2017; 28: 373–e86