Dogs & Itching - By Dr Tom Shurlock
It’s the archetypal image, a dog scratching; a dog standing on three legs, a hind leg furiously paddling at the neck or behind the ear; in a cartoon there would be a few fleas flying out! And although it may be a jokey illustration, it is also very misleading!
An itchy skin, or pruritus, is the manifestation of a huge range of potential causes. It may be a single cause, such as an allergic response to an insect bite, or a build-up of causes increasing the stress threshold of the dog. Additionally, behavioural stress can also affect pruritus, as can environmental factors and underlying physiological disorders.
A recent review showed that 5% of all veterinary consultations were related to pruritus; additionally, canine atopic dermatitis which is an allergic response, adds to the incidence of skin related disorders. These cues include dietary and ectoparasitic allergens, and airborne and environmental irritants. Finally, stress factors, including mounting, chewing, hyperactivity, coprophagia (eating faeces), begging for and stealing food, attention-seeking, excitability, excessive grooming and reduced trainability are all associated with itching, although it is sometimes difficult to assess whether these behavioural traits are cause and effect. What is evident is that an itchy dog can disguise a whole range of physical, physiological and behavioural dysfunctions. So, what is happening to the skin?
The skin is a dynamic organ; although it is mainly regarded as a barrier of dead cells, its support levels cover a multitude of functions – temperature control, immunological control, nutrient storage, sensory control etc. – and is subject to any number of dysfunctional challenges, all of which involve the inflammation cycle. Unlike some responses, such as the inflammation associated with muscular activity, the dermal layers are packed with nerve endings. Some are motor neurones, whilst others are sensory – for sensation, irritation and pain – and involve a number of sensory metabolic systems such as GABA and endocannabinoid systems. Inflammatory cues can interact with these systems and these lead to the metabolic pathways that result in the sensation of itching. Itching demands a response, such as scratching and this leads to skin damage resulting in potential microbial or exotoxin invasion, as well as purely physical response; both result in inflammation.
It is worth repeating that inflammation is the body’s natural response to anything it regards as alien to its normal function. Any irritant can evoke an inflammatory response and the body responds by generating inflammatory cues that “ringfence” the potential damage. After the damage limitation comes the repair, followed by anti-inflammatory cues that allow the surrounding tissues to return to a normal state, and any debris is the flushed away. Under many chronic situations, or where the irritant is continuous the cycle becomes stuck in the inflammatory phase. Irritants include allergens, stresses, toxins and pathogens – all of which can give an itchy skin. These can be grouped into external and internal cues.
Although the outermost part of the dermis is several layers of dead cells, they are not totally impermeable, and the irritants can come into contact with living tissue. Examples of these can be anything from a microbial infection to an insect bite, from aerial spores to household chemicals. To help counteract these, and to maintain the suppleness of the skin, there are sebaceous glands – situated where hair follicles emerge – and these secrete sebum. Sebum is a waxy secretion that helps improve the impermeability of the skin and has antimicrobial, antifungal and anti-mite properties. Unfortunately, it can also act as an attractant to specialist parasites, such as ticks. These alongside other biting arthropods (flies, fleas, etc.) can bypass the sebum by penetrating the skin with their mouthparts, leaving allergens under the skin.
Fortunately, we can change the dog’s nutrition to support this natural defence. A component of sebum is a bioactive ingredient, called a terpenoid (plant versions of this are called essential oils) and there are some that have arthropod repellent properties. For example, turmeric terpenoids have been shown to affect the behaviour of ticks, confusing them.
There are a number of internal cues, such as dietary or airborne, inhaled, allergies as well as potential behavioural stress factors. Again, the body’s response is to initiate inflammation from whatever source. Where it impacts on the skin is releasing markers that stimulate neural responses. In some situations, but by no means all, internal inflammatory cues, and stresses can cause itching, invoking a scratching response. As with the external cues, itching can lead to sensory stimulation, invoking a response that can lead to skin damage.
Two major neural systems are involved in the physiology of itching. GABA is a natural neurotransmitter that calms and lessens neural transmission. It has been associated with reducing convulsions in dogs and the dampening of neural stimulation of the sensory nerves of the skin will help reduce overreaction to inflammatory cues. Research has shown that natural antioxidants, such as curcumin in turmeric can interact with inflammatory cues, in particular interleukin that have an effect on neural excitability.
The exotically named endocannabinoid is a regulator of cognitive and physiological function and is believed to be involved in pain function. Although named out of research on the psychoactive components of cannabis, it is now recognised as being an essential neural monitoring system. It also has been shown a number of nutritional parameters can help support pain suppression, such as pre- and pro- biotics, essential oils, flavonoids etc.
Dietary components can help maintain normal sensory parameters which in turn reduce over response to discomfort and itching. This is particularly relevant to the protective function of the skin. Reduced response to itching will reduce subsequent behavioural responses that can lead to dermal damage.
In summary, pruritus, canine atopic dermatitis, or just plain itching, can come from a variety of sources and, like many conditions, may be the sum of several parts. However, understanding the variety of components, how bioactives support antioxidation, their role in the interaction between oxidation and inflammation, and their role in dermal protection can lead to the development of a supplement that can help support the body’s defences.