Why Do Pets Itch?

Skin issues, particularly ones that cause a pet to be itchy, are one of the most frustrating and chronic diseases that pets and their owners experience. Studies have shown that 48% of owners with dogs that have skin disease believe that their pet’s illness affects their own quality of life. Owners with itchy pets report that scratching has interrupted sleep, their bond with the pet, normal daily activities, and family life. Understanding the itch-scratch cycle and the underlying diseases that can cause itching are important components in achieving a treatment plan that alleviates the chronic scratching. There are three main triggers of itch in dogs and cats: allergic inflammation, parasitic infestation, and skin infection.

What Exactly is an Itch?

Itch (aka pruritus) is defined as an unpleasant sensation of the skin that provokes the urge to scratch. When a human or a pet has chronic pruritus they often fall into a cycle (the itch-scratch cycle) in which the mechanical trauma of scratch leads to increased breakdown of the skin barrier and causes release of additional inflammatory mediators, leading to perpetuation of the itch sensation and thus the chronic scratching. Many therapies for itch focus on various pathways involved in this cycle to help reduce the desire to scratch.

Types of Allergic Inflammation

The three main categories of allergic inflammation in dogs and cats are environmental allergies (atopy), cutaneous adverse food reactions, and flea bite hypersensitivity.

• Environmental allergies are caused by transdermal (through the skin) absorption of pollens, molds, dust, and dander. Some pets will experience seasonal flares in itching, while others suffer from their allergies year-round. Pets will often begin showing signs at a young age and will generally grow into their allergies over time. This type of allergy is usually diagnosed by ruling out other triggers of pruritus.
• Cutaneous adverse food reactions (aka food allergies) can begin at any age. The main trigger of this allergy are protein sources in diet. Skin signs are identical to those seen with environmental allergies, but pets with food allergies may also have gastrointestinal signs including: flatulence, vomiting, diarrhea, and/or more frequent bowel movements. This type of allergic disease is diagnosed via strict prescription diet trials.
• Flea bite hypersensitivity is common in regions where fleas are endemic. Although they can have generalized itch, the most intense itch seems to be directed at the mid back and rump regions. Often, there will be evidence of fleas seen on examination. Diagnosis is based on the response to consistent flea prevention trials.

Types of Parasitic Infestation

Aside from fleas, the most common skin parasites to trigger primary itch are Sarcoptes spp. (Sarcoptic mange) or Cheyletiella spp. (Cheyletiellosis). Both mites are generally carried by other dogs, cats, or wildlife hosts (i.e. coyotes, foxes, rabbits, etc.). Pets with these types of mites are often extremely itchy and have less response to common therapies used to treat itch. These mites can affect multiple pets and occasionally people in the household. Another parasite that can cause varying levels of itch in pets is demodicosis, caused by demodex mites. Diagnosis of a parasitic trigger of itch is via demonstration of the mites from skin scrapings performed during the examination. It can be difficult to find these mites and trial treatment with antiparasitic medications may be recommended based on suspicion.

Types of Skin Infection

Finally, skin infections from yeast (Malassezia dermatitis) or bacteria (pyoderma) can cause a significant level of itch. Skin infections are rarely a primary disease and are generally related to underlying allergic inflammation or hormonal changes which affect the skin’s immune system. While they can cause itch on their own, infection often exacerbates the itch from the underlying conditions outlined above. It is important to have all infections under control to be able to effectively evaluate the efficacy of testing or therapies used to diagnose and treat the primary inflammatory conditions.

Ultimately, itching is a significant issue in many pets and a thorough workup to determine the exact cause is necessary. Once the primary cause is determined, a treatment plan can be developed to break the itch-scratch cycle and restore the health of your pet. It is important to remember that many of these diseases are manageable, but will require life long therapy.

References:

Linek, M, Favtot, C. Impact of canine atopic dermatitis on health-related quality of life of affected dogs and quality of life of their owners. Veterinary Dermatology. 2010;456-62.