Plus-Hex CLINICAL to identify which specific protein or group of proteins causes a flare. If one group causes a flare of clinical signs, the group can be further split to identify which allergens are safe and which are to be avoided.
Table 2. Groups of proteins with cross-reactivity.
Group
Common pitfalls
Owner factors
Protein sources 1 Beef, lamb, pork, venison, cow’ s milk 2 Chicken, turkey, other poultry, white fish
The patient ' s owners will be responsible for ensuring that the diet trial is adhered to; however, without being educated on how to perform a diet trial, they can also be responsible for invalidating it. Often, owners are unaware of the differences between a hydrolysed diet and a‘ hypoallergenic’ diet that can be bought in a pet shop. Hypoallergenic is a personal term: a food is hypoallergenic only if it does not contain the allergens to which the specific patient is allergic, which at the point of starting the trial are unknown. Often, these diets are advertised as having a single protein source. Unfortunately, due to multiple proteins being handled in the same factories, cross-contamination cannot be avoided. Studies of dry dog foods have shown that they contain DNA from animal sources that are not declared in the list of ingredients [ 7, 8 ].
Another issue that can arise is when owners are giving their pets flavoured supplements that they forget to mention when asked about the patient ' s current medications; this also applies to flavoured toys or natural toys made of antler or bone. Another confounding factor can be the owner ' s home circumstances. Having elderly relatives, young children or other pets in the home can make being strict with the patient ' s food very challenging, as can encounters with dog walkers, friends in the park or other carers, such as at doggy day care [ 3 ].
Veterinary factors
Even though VSs advise diet trials and educate the owners in the process, they can also inadvertently invalidate the trials. Common ways in which this occurs are by the VS prescribing flavoured medications, such as antibiotics, analgesics or prophylactic ecto- / endoparasiticides, or recommending toothpastes or pre- / probiotics that contain flavourings [ 3 ]. Even some antipruritic medications, such as oclacitinib( Apoquel, Zoetis), are available in flavoured and non-flavoured versions, which can be switched by mistake if both are available at the practice. Giving patients a treat during or after a veterinary consultation can also be second nature for veterinary nurses( VNs) and receptionists, who may not be aware that an individual patient is undergoing a diet trial.
Patient factors
Sometimes it is the patients themselves that make diet trials challenging. Canine patients that steal or scavenge food sometimes require muzzling on walks. However, owners may be less willing to use a muzzle to restrict access to sources of dietary indiscretion in the home, such as bins or cat litter trays.
Muzzling is not possible for cats, and cats that have outdoor access and are either hunters or visitors to other households are not suitable candidates for a diet trial. Keeping these cats indoors for the duration of a diet trial can be a source of stress and is not always in their best interests. Switching cats to a new food can also be a problem; some cats will avoid eating novel diets to their detriment.
The majority of dogs will usually eat any diet when they get hungry enough; however, owners can be unhappy to wait for this to happen. Having multiple pets in the household with different dietary requirements or restrictions can also cause issues. It is easier to feed all pets of the same species the same food, but this is not always possible, for example, if a pet needs a renal or hepatic diet. Pets that require medication in their food can also have poor diet trial compliance because the infrequency of effective dosing can lead to a flare.
Any time a patient is known to have had a dietary indiscretion during a diet trial, the diet trial must be restarted from day 1, regardless of how many days or weeks have passed since the trial originally started. This can be frustrating for owners and the VS, and can mean the diet trial is abandoned if compliance is not possible.
Interpretation
There are two ideal outcomes from a well-performed diet trial:
1. A patient that is doing better on antipruritic medications remains in remission when these are tapered and stopped, but reliably flares when re-challenged
2. A patient that is doing better on antipruritic medications, but is unable to be tapered off their medications without flaring.
Patient 1 would be given a diagnosis of CAFR. Patient 2 would most likely be diagnosed with atopic dermatitis. In reality, though, the results from diet trials can be much harder to interpret.
Volume 41( 5) • October 2026
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