Itching is the most common pet owner complaint in dermatology. Itch can look like licking, biting, nibbling, rubbing, and of course the good old scratch using a hind leg. Intensity can range from very mild to self-mutilation. There are three types of pruritus: psychogenic (altered brain chemistry), inflammatory (seen with atopic dermatitis, infectious etiologies and ectoparasites) and neurogenic (seen with adverse cutaneous food reactions).

Psychogenic itching is usually be discerned by taking a thorough history. This is a type of chemical addiction (licking will release endorphins in the brain, but prolonged licking will damage the skin in the area, causing inflammation and often secondary infection, which will further increase pruritus, inciting more licking, which becomes a vicious cycle of needing to lick to feel better). Treatment options can include stress-reducing pheromones, and “happy pills” like fluoxetine, clomipramine and amitriptyline to help regulate serotonin levels. Clomipramine and amitriptyline also have antihistaminic properties, and help with neuropathic pain, which can be very helpful when treating a patient that also has inflammatory or neurogenic pruritus. Some cases may even need mu-opioid receptor antagonists, like gabapentin.

Inflammatory itching is evoked directly by mechanical and thermal stimuli, and indirectly through chemical mediators, like histamine released from mast cells, neurotransmitters, and cytokines. This is the most common presentation of environmental allergies. The process of inflammation is a very complicated cascade, which involves the release of multiple chemical messengers (cytokines). These include interleukins: IL-2, IL-4, IL-5, IL-6, IL-10, IL-12, IL-13, IL-21, IL-22, IL-23, IL-31, IL-35.

Treatment for inflammatory itching includes anti-inflammatory medications like corticosteroids, antihistamines and cyclosporine. All of these treatments block, or decrease, different chemical interactions to suppress or prevent inflammation.

Let’s take a closer look at the inflammatory cascade: It starts with tissue injury (from scratching for instance) this causes the release of phospholipids. Phospholipids react outside of cells with phospholipase to become arachidonic acid. This is further processed into prostaglandins, which cause inflammation by recruiting white blood cells, sensitizing skin pain receptors, and stimulating hypothalamic fever response.

Corticosteroids inhibit phospholipase, which in turn prevents the cascade that results in inflammation.

Antihistamines work by stabilizing the cell walls of mast cells, so that they are not able to degranulate. When triggered by cytokines, mast cells will expel histamine granules, along with more cytokines, granulocyte macrophage colony-stimulating factor (GM-CSF), leukotrienes, heparin, many proteases, and metabolites of arachidonic acid, into the environment. These act on vasculature, smooth muscle, connective tissue, mucous gland and inflammatory cells, causing inflammation and fluid release as part of an allergic reaction.

Cyclosporine is an immunosuppressant that works by tricking the immune system into not paying attention. T cells, based on information received from other cells in the body, decide what type of a reaction to have when presented with an antigen.

Cyclosporine is a calcineurin inhibitor, which stops information being transferred in the T cell, which would direct the T cell to start an allergic or inflammatory cascade. If the message doesn’t get through, then there is no adverse response.

Neurogenic pruritus also results from a complicated cascade of chemical releases, including several of the interleukins (IL-2, -4, -6, -13 and -31) and neurotransmitters, called Janus kinase (JAK-1 and JAK-3). IL-31 is directly involved in the sensation of itch. JAK signaling occurs when a cytokine binds to a receptor on the surface of a cell. Oclacitinib works as a JAK inhibitor to prevent the “chemical messenger” from telling the nerves, to tell the brain that the body is itchy. JAK signaling is also an important messenger involved in other processes in the body, including bone marrow production.

Lokivetmab is another treatment option used for neurogenic itching. Lokivetmab is a monoclonal antibody which specifically targets the cytokine IL-31. If IL-31 is bound to an antibody, it is not able to deliver its “message,” which stops cascade leading to itch.

In allergic patients, we often see a combination of the different types of pruritus, which can make things complicated and is why a multimodal approach is necessary to achieve success.

Pruritus can be caused by many things, including parasites, infections and allergies, so a diagnosis of allergy is a diagnosis of the exclusion of all other possible causes for itching. Namely, ectoparasites and secondary infections.

By definition, an allergy is a “hypersensitive immune reaction to a substance that normally is harmless and would not cause an immune response”.¹ The most common hypersensitivities seen in private practice are: flea allergy dermatitis, food allergies and environmental allergies (atopic dermatitis). For food allergies, restricted diet trials are necessary with long term dietary management. For environmental allergies, allergy testing and subsequent treatment with immunotherapy offers excellent long-term management for many animals, but management of symptoms will need to addressed medically while working through induction. Immunotherapy can take up to 12 months before seeing a patient’s maximum response. Treating most dermatology patients requires multimodal therapies. This is often a combination of systemic as well as topical treatments.

When working topically, we can help the prevention of antibiotic resistance, as well as work to improve the integrity of the skin barrier. There is a vast array of topicals on the market today, all having a place in treatment protocols. When treating dermatology patients our goals are to make our patient more comfortable by addressing pruritus, inflammation and secondary infections. We also work to improve the integrity of the skin barrier so that our patient’s comfort is continued, and the chance of recurrent secondary infections is decreased.

Topicals

Topical therapy can be a very beneficial adjunctive treatment.

Clients need to know the rationale for the treatment and what to expect. It is important that clients understand this part of the treatment protocol and not an “extra” thing to do. Client education equals owner compliance. Make certain this is something the owner can actually do (consider a spray or mousse if bathing is not feasible) Because of the plethora of shampoos now available, it is important to understand their ingredients, concentrations and actions.

Consider spot treatment for focal problems.

Consider ear medications for focal lesions—most are combos for bacteria, fungus and inflammation.

Contact time, frequency and technique are paramount for success.

The aim of topical therapy is to promote a return to the “normal” microclimate of the skin surface. Products should be selected with the specific purpose in mind: cleansing, antiseborrheic, antimicrobial or antipruritic action. Product selection - depends on knowledge of the product attributes, product vehicle, response time as well as any risks or side effects.

Contact time—A contact time of 5–10 minutes is required to rehydrate the epidermis, 10–15 minutes contact time is preferable to allow penetration and achieve a treatment effect. Cool water will rehydrate, while warm water will dehydrate.

Frequency—During induction phase, this would be every 2–3 days. Maintenance may be moved to once a week, depending on the individual.

Technique—Owner compliance can be improved if clients are shown to how to bathe their pet using either handouts, videos, or by demonstration. Heavy-coated dogs may need to be clipped to allow the product to reach the skin surface. A shorter coat can significantly decrease the amount of shampoo used. Using a primer shampoo to remove initial dirt or grease, will allow a second stage bath with medicated shampoo to more easily reach the skin. Thorough rinsing is important as some products may have an irritant effect if left on the skin. A moisturizing rinse or humectant spray might be needed. Water temperature is also an important consideration, as stated above, warm water will dehydrate the skin and further irritate an already compromised skin barrier.

Ingredients: Understand the ingredients to best select the appropriate product. **Cheat sheets for veterinary shampoos and veterinary otic products currently available in Canada are available to members on the Canadian Academy of Veterinary Dermatology website: www.cavd.ca. This is a great resource!

For shampoo/mousse/spray selection

Ear medication selection for topical use on skin ear medication selection for topical use on skin

References

1.  National Library of Medicine (online April 2018) from www.ncbi.nlm.nih.gov/pubmedhealth/PMHT0030652.