Desensitization in canine allergology.

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Atopic dermatitis can be controlled by symptomatic or etiological treatment. The latter includes the use of desensitization.

Author: William Bordeau
Cabinet VetDerm,
1 avenue Foch 94700 MAISONS-ALFORT

While symptomatic treatments conventionally used, such as antihistamines, corticosteroids, or cyclosporine, generally allow for rapid control, desensitization is much slower to reveal its effects. It is therefore much more of a long-term treatment that must be offered according to certain rules.
The principle of desensitization, also called hyposensitization, specific immunotherapy, or more recently anti-allergenic vaccination, consists of injecting increasing doses of the atopens to which the animal is allergic, up to a maintenance rhythm where this dose is then constant.
The mechanism of action of desensitization is largely unknown in veterinary medicine. In human medicine, it is assumed to act by modulating the production of certain IgG and IgE, by decreasing the number of effector cells such as mast cells, basophils, or eosinophils, by decreasing inflammatory mediators such as histamine, and by decreasing the production of interleukin 4 and 5, produced by CD4 lymphocytes.

desensibilisation-allergologie-canine1

Photo 1: Westie with highly developed atopic dermatitis.

This desensitization is classically used in human allergology, with extremely variable results, as in veterinary medicine. Some even go so far as to think that it is ineffective in atopic people. Three double-blind studies have been conducted in humans. In the first, carried out with various aeroallergens, a good response was obtained in 81% of people who received desensitization, compared to 40% in people treated with placebo. In the second, consisting of desensitization to Dermatophagoides pteronyssinus, which unlike in dogs, is the main aeroallergen responsible for atopic dermatitis in humans, no differences in efficacy were found between treated people and those receiving placebo. In the third, a significant improvement was also observed in people desensitized to Dermatophagoides pteronyssinus. More recently, trials have been carried out with the sublingual route. Desensitization proved effective in 64 to 74% of cases by this route.
In dogs, many trials have been carried out, but in the vast majority of cases, these were open studies. A response was obtained in 50 to 100% of cases in these studies. Only one double-blind study was conducted, nearly twenty years ago, by Ton Willemse. In this study, almost 60% of dogs showed a decrease of at least 50% in their clinical signs, with complete disappearance of clinical signs in 56% of these dogs, compared to a decrease of at least 50% in clinical signs in 21% of dogs treated with placebo, and complete remission in 80% of them.

Photo 2: In the same dog, significant erythema,
crusts, and lichenification are noted,
making treatment all the more difficult.

There seem to be many factors of variation in this response, even if many are still subject to discussion. This response therefore seems to vary with the type of allergens used, the laboratory manufacturing the product, the method of determining sensitization (in this context, it should be remembered that skin tests currently constitute the reference method in the determination of allergenic sensitization), the protocol used, the concentration used, and the duration of the treatment.
Although it is currently believed that these factors influence the response, no controlled study has yet demonstrated this. However, two factors seem important to take into consideration. The first consists of carrying out specific immunotherapy. Therefore, “soups” in which many aeroallergens are included should be banned, as many do not contribute to the onset and maintenance of clinical manifestations. It is better to target the aeroallergens to be injected, based on anamnesis and history, and in particular the seasonality of allergic dermatitis. The second factor is the injected dose. Indeed, it would seem that the immune response depends directly on the injected dose, without however reaching a dose that could cause side effects.
Different open studies have highlighted a number of factors that could predict the response to desensitization, such as the age at the onset of the first lesions, the age at the start of desensitization, the duration of allergic dermatitis, the severity of clinical manifestations, the breed, the importance of the positivity of skin tests, and finally the responsible aeroallergen(s).
Currently, in veterinary allergology, it is considered that the younger the animal, the more recent the onset of allergic dermatitis, the more moderate the manifestations, and the fewer aeroallergens the animal is sensitized to, the better the response to desensitization will be. However, these impressions must be supported by a double-blind study before definitively stating that these factors influence the response. In the meantime, it is currently considered that this would be the best candidate for desensitization.
The response to desensitization will be all the better if certain rules are followed.
Firstly, patients should be well selected, and in particular, atopic dermatitis due to aeroallergens should be confirmed by eliminating all other dermatoses included in its differential diagnosis such as sarcoptic mange. Secondly, aeroallergens should be well selected, and not include, without thought, all those that showed a positive response during intradermal reactions or serology. Thirdly, it is important to properly educate owners, by clearly explaining the objectives to be achieved, the long duration required before obtaining satisfaction, the risks of recurrence, the achievement of control and not a cure, and finally by explaining that it is a long-term treatment thus requiring regular monitoring to manage the animal as best as possible. These controls will notably be an opportunity to monitor the appearance of infectious complications, particularly Malassezia dermatitis, or an associated dermatosis, such as flea infestation. Finally, owners must be warned of the risk of rare but real side effects such as an exacerbation of clinical manifestations in the days following injections.
While all desensitization manufacturers agree on the need to inject increasing doses of aeroallergens until a constant dose is obtained, there is no optimal protocol. Although a general framework can be drawn, it is often necessary to adapt the classic protocol to the animal. Indeed, during the maintenance phase, some dogs will require an injection every 3 weeks, and others every 4 weeks. However, it is of course important not to switch to every 4 weeks if a result has not been obtained, eventually, every 3 weeks.
Desensitization is therefore a treatment of choice in the control of canine atopic dermatitis, bearing in mind that it is only a long-term treatment, which will often require the use of other molecules, at least at the beginning of treatment.