Food Allergy: Definition and Diagnosis
Food allergy is a term that is widely used in today’s society, with more and more publicity and awareness of the human condition it seems to be more prevalent than ever before. But what about pets? What are the potential signs and how do we diagnose it? Should we be using serological testing, food trials, home prepared diets, commercial preparations? Do you believe the hype(ersentsitivity)?
There are several definitions associated with food allergy. An Adverse Food Reaction (AFR) is any abnormal response to a dietary component. A Food Hypersensitivity is a true allergy in that it is an immunologically driven adverse food reaction mediated by IgE, IgG or cell-mediated hypersensitivity. Food intolerance is an AFR that is not immunologically mediated and is driven for example by direct toxicity, idiosyncratic reactions or pharmacological / metabolic reactions.
Food Allergy in Man
Food allergy in man is widely researched and better defined than in animals and has revealed a number of key points including the difference between IgE and IgG mediated reactions. IgE-mediated reactions occur within a short timeframe (minutes – hours) following exposure whilst IgG reactions are delayed for many hours. Human studies have also revealed that food allergies in man contribute to atopic dermatitis. For IgE-mediated reactions, serology has excellent reliability and has replaced double-blind placebo controlled trials as the gold standard for selection of the most appropriate dietary trials.
The IgG Question
One question regarding food allergy is “are IgG antibodies relevant?” Studies on humans with IBS have shown that a significant majority of patients will respond positively to a diet which excludes foods to which a significant IgG response is shown (Atkinson et al, 2004, Shanahan and Whorwell, 2005). Although this does not necessarily mean that IBS is mediated by IgG, it does demonstrate that measurement of food antigen-specific IgG is of value in selecting an appropriate diet leading to improvement of symptoms in these patients. A study comparing serum antibody levels in normal dogs, dogs with Atopic Dermatitis and dogs with confirmed AFRs suggests that both IgE and IgG are helpful in the management of dogs with suspected AFRs (Halliwell et al 2004).
AFRs in the Dog
Canine AFRs are much less well defined than those in man. Dermatological signs can often be similar to canine atopic dermatitis (CAD). There are, however, some specifics such as perianal pruritus being more suggestive of AFRs, and interdigital pruritus more suggestive of CAD. Cases in which where gastrointestinal symptoms run alongside dermatological symptoms are more indicative of AFRs. More details on the contrasting clinical signs of AFR and CAD are displayed in table 1.
AFRs in dogs may occur in the form of gastrointestinal problems and/or dermatological problems. According to a recent review, approximately 10% of allergic dogs are suspected of having AFRs (Hillier and Griffen, 2001). Other studies have reported up to 20% of allergic dogs may have AFRs (Chesney, 2002. Loeffler et al, 2004).
5-10% of population
2-5% of population
Age of onset
Peak age 1-3 yrs
16% < 1yr
Less common < 6mo
Does not occur > 7yrs
Peak age < 1yr
48% < 1yr
Quite common < 6mo
Can occur at any time
Facial and ventral volar pedal and interdigital common
Can mimic CAD although volar pedal and interdigital less common.
Otitis Externa characteristics
Commences on ear flap and vertical canal
May commence in horizontal canal
Recurrnt staphylococcal folliculitis
Common, may be more briskly relapsing.
Concomitant gastrointestinal signs
Occur in the majority
Westie, Boxer, French Bulldog, Bull Terrier, Basset, Vizla
Same breeds as for CAD and also: German shepherd, Pug, Ridgeback.
Table 1: The contrasting clinical signs of CAD and AFR
Clinical signs in Cats?
Let’s not forget cats, who can also suffer from AFRs. Indeed AFRs in cats are considered to be relatively more common than in dogs (Scott et al, 2001) It is thought that AFRs in cats occur at about 20% of the frequency of atopic dermatitis. As with dogs dermatological symptoms of AFR can often mimic those of atopic dermatitis and the coexistence of gastrointestinal symptoms is an indicator of an AFR. One third of dermatologically presenting AFR cases in cats has dermatological symptoms around the head and neck. Interestingly, 20% of cats with AFR symptoms are symptomatic on any home prepared diet and symptomatic on any commercial diet indicating afood intolerance rather than a true allergy.
It is universally agreed that the only way of making a diagnosis of an AFR is the response of the symptoms to an elimination diet followed by relapse of the symptoms upon rechallange with components of the original diet.
There are several ways to approach dietary selection in the suspected AFR case:
Serological Testing as an aid to diagnosis
There are several commercial laboratories offering serological testing for food allergy. This involves running an Enzyme Linked Immunosorbent Assay (ELISA) to determine antibody levels in the blood to a range of common food allergens. Research has shown that there are significantly higher levels of IgE to the majority of relevant antigens in dogs with confirmed AFRs. Data shows that a positive response of 4 or more antigens has a positive predictive value for 82% of AFR, presumably IgE mediated. There are also significantly higher levels of food allergen-specific IgG in AFR cases, although it must be noted that normal dogs can have IgG antibodies to food allergens.
Serology provides a good aid to diagnosis in AFR cases. Specifically, it assists in the selection of an appropriate diet for trial. Serology enables identification of potential cross-reacting antigens which may be chosen if the diet trial is based purely on dietary history, it also identifies allergens that are being fed when components of the true diet are uncertain. Serology will also help to improve client compliance; an owner that is asked to feed their Great Dane on turkey and potato is much more likely to do so if the diet choice is supported with some test results. A recent study has shown that 61% of pruritic dogs responded positively (and then relapsed upon rechallenge) to a diet trial based on the results of serological testing compared to 35.6% of pruritic dogs responding to a home cooked diet trial not based on serology (Loeffler et al, 2006).
When recommending a home-prepared diet to clients select a single protein and single carbohydrate source based on a detailed dietary history and serological test results. No other foods are permitted (remind owners about treat, supplements and scraps). Whilst a home prepared diet is the Gold Standard for the initial dietary trial in circumstances when this is not practical a range of commercially available hypoallergenic and hydrolysed diets exist. Feed for a minimum of 6 weeks and if any improvement is shown continue for a further 4 weeks. If improvement is seen the diagnosis must be confirmed by rechallenge with the original diet and relapse of the symptoms. Studies show that 25% of AFR cases have a resolution of clinical signs in 1-3 weeks of starting a food trial and 74% show resolution by 6 weeks. Relapse upon rechallenge occurs within 2 hours in 18% of cases and within 3 days in 81% of cases (Rosser, 1993).
When working up a dermatological or gastrointestinal case always consider AFRs, especially if the 2 types of symptoms coexist. When planning a dietary trial it is recommended to utilise serological testing alongside a detailed dietary history. Ensure that the diet is of sufficient duration and that owners are aware of avoiding treat, snacks and supplements during the trial. Following a positive diagnosis, it may be possible to move the animal onto a commercially available diet for the purposes of long term management.
Case Study: Tess Heaton
Name: Tess Heaton
Age: 2 yrs
History: Long history of recurring staphylococcal pyoderma with pruritus. Lives in a pub, mainly in the bar area.
Serological testing results: High levels of IgE and IgG to potato.
Diet employed: Eukanuba lamb and rice, dog biscuits and chews. No potato crisps!!
Response: Normal within 1 month, no recurrence.
When the results of Tess’s serological test came back it highlighted questions as to what her actual diet consisted of. Her owners did not feed her potato as part of her regular meals. However, residing in the bar area of a pub Tess was regularly “hoovering” up all of the left-over potato crisps and chips left in the bar. When this was stopped her symptoms stopped! Potato would never of been considered as part of a dietary history so serology highlighted the offending allergen in this instance.
This article was provided by Avacta Animal Health, providers of SENSITEST®:
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