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 Figure 1. The itch cycle begins with a cutaneous Langerhans cell presenting an antigen to a naive T lymphocyte, which it polarizes for the correct response. The lymphocyte may be “directed” to produce cytokines that classify it as a T-helper (Th) 1 cell. The cytokines primarily kill intracellular pathogens or assist white blood cells to kill extracellular pathogens. In allergy, the immune dysregulation is a shift toward a Th2 response, releasing cytokines that mediate neuronal itch.
Implications for Treatment
Pruritus is the primary reason that owners present their pet with allergic skin disease to the veterinarian. In the past, therapies, such as corticosteroids, were useful in the short term to get a rapid decrease in clinical signs; but the fact that corticosteroids have effects in every cell of the body causes multiple side effects that often outweigh the benefits. Now, based on the new science, there are therapies that are as effective and have fewer side effects.
Given the complexity of the pathogenesis of AD, the
goal of treatment is to find the precise combination of therapeutic approaches that effectively provides relief
for the individual patient. Multimodal treatment aimed at both the primary disease and at secondary complications can be required. Because many atopic patients require lifelong management, it is also important to consider long-term safety as well as convenience and the cost- effectiveness of treatment regimens.
Treatment strategies must address multiple elements:
• Manage itch and control scratching that can perpetuate skin damage.
• Avoid, prevent, or eliminate allergens (such as fleas or foods) when possible.
• Control secondary infections, commonly due to Staphylococcus pseudintermedius or Malassezia yeast, which contribute to discomfort and augment the allergic and inflammatory responses.
• Improve the skin barrier.
• Modify the immunologic response through allergen
immunotherapy in patients diagnosed with AD.
An Urban Experience
Controlling Itch and Inflammation
Fatty Acids (examples: alpha-linolenic acid, eicosapentaenoic acid, docosahexaenoic acid)
Fatty acids are specific kinds of polyunsaturated fats. The two main classes are omega-3 and omega-6, which are based on their molecular characteristics. Oral and topical supplementation with omega-3 fatty acids – alpha- linolenic acid (ALA), eicosapentaenoic acid (EPA), or docosahexaenoic acid (DHA) – may help in moderating inflammation and improving the skin barrier (Figure 2). EPA is the big performer of the omega-3 fatty acids and is incorporated into the cell membrane.
Allergies occur because the pet’s immune system reacts to normally innocuous proteins, such as house dust and mites. When a cell membrane is damaged, substances, such as arachidonic acid, are released and metabolized by enzymes (lipoxygenase and cyclooxygenase) to proinflammatory byproducts, which add to cutaneous inflammation and pruritus. When omega-3 fatty acids are incorporated into the cell membrane, they utilize
the same enzymes and produce less inflammatory substances after cellular damage.
Figure 2. In cases involving itch and inflammation, oral and topical supplementation with omega-3 fatty acids – alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), or docosahexaenoic acid (DHA) – may
help in moderating inflammation and improving the skin barrier.
Antihistamines (examples: diphenhydramine, hydroxyzine)
The complexity of the mediators produced by the immune system may explain why there is
a poor response to treatment with antihistamines (Figure 3). In fact, antihistamines aimed primarily at H1 and H2 receptors are a common treatment choice in veterinary medicine, but there is little evidence to support their efficacy in the treatment of allergic skin disease. New experimental evidence suggests that targeting H4 receptors may be worthwhile because these receptors are expressed on T lymphocytes, antigen-presenting
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