Update on canine allergy (Proceedings)

Antihistamines

Antihistamines have variable efficacy in atopic dogs. If at least 3 different antihistamines are tried in an individual dog, you can reasonably expect to find one that works 25-40% of the time. My first-line choices of antihistamines are amitriptyline (2 mg/kg BID), clemastine (0.1 mg/kg BID), and hydroxyzine pamoate (2.2 mg/kg TID). They work synergistically with fatty acid supplements, and can have a steroid sparing effect even if they do not control the itching by themselves.

Fatty Acid Supplements

There have been a tremendous number of studies attempting to assess the efficacy and effective dose of essential fatty acids (EFAS) in atopic dogs. Many of these studies are poorly designed and/or contradict each other. The optimum dose or ratio of omega 3 to omega 6 fatty acids is still unclear. There is no doubt that EFAs can help to improve the appearance of the coat and skin. Realistically, I find them to be beneficial at decreasing pruritus in <25% of dogs. A recent study from Italy evaluated the efficacy of blackcurrant seed oil (BSO) in seasonal atopic dermatitis. Fourteen dogs with seasonal pruritus for more than 2 years were given BSO (100 mg/kg/day) for 2 months prior to their known allergy season, in 43% of them the pruritus did not appear at all, or was very mild, in 43% it was decreased, and in 14% it did not change.

Glucocorticoids

It is difficult to treat a chronic atopic dog without ever using corticosteroids even though our goal is to use them sparingly and in the safest method possible, if at all. Prednisone is the least expensive and most commonly used corticosteroid, however, it also tends to have the most side effects. It is estimated that at least 30% of dogs will have polyuria and polydipsia with prednisone. This side effect, while usually harmless, is bothersome both to the pet and the owner and can create compliance issues. It is more common in older dogs and in spayed female dogs, especially if there is already a problem with urinary incontinence. Therefore, I will often use oral methylprednisolone as there is only about a 10% incidence in PU/PD with this drug. Since this most commonly comes in 4 mg tabs, I use it more often in smaller dogs. Methylprednisolone is 20% more potent than prednisone, so a 4 mg tab of methylprednisolone is equal to a 5 mg tab of prednisone. Triamcinolone (has no mineralcorticoid) is another option that has an even lower incidence of PU/PD. Although due to its increased potency and longer activity, I prefer to use it only short term, and it should be tapered to every third day for maintenance. Dexamethasone also has a lower incidence of PU/PD, but it is the most potent of the four oral steroids and steroid side effects are common if used for extended periods of time.

All steroids are not created equally. If there are side effects with one, there may not be side effects with others. If one stops working (steroid tachyphylaxis) another may work fine. Unlike in our feline patients, dogs can almost always be dosed orally. Long-acting injectable steroids have no place in the treatment of atopic dogs due to the much greater incidence of side effects and inability to titrate to the lowest effective alternate day dose!

In difficult cases where immunotherapy or other treatments are not completely effective, it may be necessary to keep patients on a low, every other day dose of corticosteroids to keep them comfortable. To be safe, this must be <0.5 mg/kg of prednisone (or pred equivalents) every other day. I strive to get this dose as close to the physiologic dose (0.22 mg/kg SID) as possible. Occasionally, I will find that even large dogs can be maintained with physiologic doses, or even less, every other day (i.e. a 100 lb dog on 5 mg of pred EOD). If a patient is unable to be controlled with <0.5 mg/kg of prednisone EOD, Cushing's syndrome is a near certainty over time. It must be communicated to the owners that you are striving to reach a tolerable level of itching, not the complete absence of itching.

Topical therapy with a dilute triamcinolone spray (Genesis spray®, Virbac) can be very helpful. Especially when 90% of the itching is affecting only 10% of the body surface area.

Pentoxifyline (PTX)

PTX is a methylxanthine derivative with phosphodiesterase inhibiting activity. It also has rheologic effects by increasing the flexibility of red blood cells and reducing blood viscosity. PTX inhibits the complement cascade, neutrophil adherence, and cytokine production (especially tumor necrosis factor alpha). It can be effective in atopic canines at a dose of 15-25 mg/kg BID-TID. Side effects are uncommon, and mostly involve g.i. upset. It should be given with food, and can be used together with antihistamines to improve efficacy. PTX does not interfere with the results of intradermal allergy testing.

Immunotherapy

Allergen Specific immunotherapy (ASIT) is the most specific therapy available, is very safe, and is considered the treatment of choice for the atopic dog. Success rates vary depending on many factors including geographical location. In my practice, 80% of atopic canines have a good to excellent response to ASIT.

In my view, there are three equally important aspects of ASIT. I call these the three pillars of immunotherapy management.

Cyclosporine (Cyc A)

In my opinion, cyc A should primarily be used in patients that have been refractory to allergen specific immunotherapy. As a very potent immunosuppressive agent, compared to immunotherapy, its use is a philosophical step backwards. In refractory atopics, it is dosed at 5 mg/kg SID for the first 4 weeks, then tapered to an alternate day dose for 4 weeks, then, if possible to every 72 hours. Only 25% of patients can be successfully tapered to q72 hrs. A good thing about cyc A is that success should be seen during the first 4 weeks if it is to be effective in that patient. Small dogs tend to be the best candidates for cyc A use due to its high cost. In one recent study it did not interfere with results of intradermal allergy testing. For this reason, it can be very beneficial in order to keep a patient comfortable during appropriate drug withdrawal for skin testing.

New Treatments

Kiwi berry extracts, tepoxalin, masitinib, leukotriene inhibitors, tacrolimus, probiotics, and mycobacterium vaccae injections are all new therapies that have been evaluated in recent studies for atopic canines with some positive results.