The skin comprises two major layers, the epidermis and the dermis. In terms of disease processes, the epidermis is the most commonly affected, so this is the layer we will focus on here.
The epidermis
The epidermis consists mainly of cells called keratinocytes, which make up 90% of the cells in this layer. These cells start life at the innermost part of the epidermis, and as they age, they move towards the outermost layer, flattening and dying to form a hard surface layer. This whole process takes 3-6 weeks, depending on the part of the body and the thickness of the skin in that area. The main purpose of these keratinocytes is to prevent harmful substances from getting access to the body, so they are held together with tight connections. These connections prevent bacteria and water from getting in and also prevent too much water from escaping.
Other cell types that make up the skin include melanocytes and Langerhans Cells. Melanocytes produce pigment – they are the reason why some dogs have darker skin than others. Langerhans Cells, on the other hand, are spidery cells that form part of the immune system. They act as sentinels, watching for ‘invaders’, gathering information on them and signalling to other cells that the immune system needs to attack.
The dermis
The dermis contains the glands which produce sweat, skin oils and scent signals.
Symptoms of skin conditions
The skin has a relatively limited number of ways that it displays symptoms. After all, there aren’t many cells present, and each can only react in a certain way. Symptoms of skin issues include:
- Redness
- Spots, rashes and pimples
- Thickened skin
- Itchiness
- Hair loss
Redness
When skin goes red, it is usually because the blood vessels in the area have opened wide (called dilating) to let more blood flow through them. They do this in response to messenger signals from other cells, such as from the Langerhans Cells recognising bacteria or pain signals coming from the brain. These signals are picked up by the cells in the skin, and this starts a chain reaction, ultimately resulting in the dilation of the blood vessels and the rushing of fresh blood to the area ready to deal with the threat.
Spots, rashes and pimples
Most rashes are red for the above reasons – the body recognises an issue and sends more blood. This is the cause of inflammation around spots and pimples too. The classic pimple contains a central pore filled with white pus, surrounded by a raised, red area. The red area is the result of the body opening blood vessels and flooding the infected pore with blood, healing proteins and immune cells. The pus in the pimple is actually all of the dead white blood cells that have been busily fighting the infection.
Thickened skin
Skin becomes thicker for one of two reasons. Either the body is producing too many new skin cells (the body recognises a higher turnover of cells from inflammation and produces new cells at a faster rate to compensate), or the dead skin cells are not falling away from the body correctly (because they’re being ‘programmed’ abnormally and instead of dying at the correct time, they remain attached to the body.)
Itchiness
Itchiness is a prevalent symptom of skin disease. In the case of allergic skin disease, there are three major steps to ‘feeling’ an itch. Firstly, the immune cells searching for bacteria and foreign bodies recognise an allergen and release a messenger. A specific receptor picks up this messenger and starts a chain reaction, which triggers nerves to transfer the message to the brain. The brain then tells the dog to scratch the itch. The problem with this is that scratching damages the skin, making it more prone to feeling itchy, something usually referred to as the ‘itch-scratch cycle’.
Hair loss
Sometimes called alopecia, hair loss can occur for various reasons. Firstly, because the animal is itchy, they may break hairs early and lose more than they should due to scratching themselves. Secondly, the hair growth-loss cycle can get disrupted, usually because the hair follicle becomes inflamed, infected or invaded by parasites. This growth-loss cycle can also be disrupted by hormones or stress, and sometimes no cause for this disruption can be discovered.
Common diseases that affect the skin
Pyoderma
Pyoderma is an infection of the skin. It can occur in several layers of skin. The skin can fight infections pretty well thanks to the tight junctions between cells and the immune cells living in the upper layer, so an infection usually only occurs in immune-compromised individuals. This could be young pets, older animals, or pets with diseases of the skin that affect its functionality. Pyoderma is very itchy, and there are normally visible pimples or a rash.
Parasite infestations
Sadly, mites, lice and fleas are a significant part of pet ownership. These pests live in various parts of the skin, from simply hiding amongst the hairs like lice and fleas to burrowing down into the skin and follicles. These pests cause a severe itch and may allow infection in.
Allergies
Whilst 1 in 10 allergic dogs are reacting to something in their food, the vast majority of the rest are allergic to something in the environment. This is referred to as atopy or atopic dermatitis. For these dogs, tiny amounts of pollen or dander make it through the epidermis due to a defective skin barrier. Here, they activate the immune cells almost as if they were bacteria, causing an itch response.
Note: Wounds and injuries to the skin increase the risk of infection developing due to the skin’s barrier being damaged and the tight junctions between cells being compromised.
Tumours and growths of the skin
Tumours and growths of the skin develop due to unregulated growth of cells within the skin. There are multiple types of tumour, some benign and some malignant, such as warts, mast cell tumours, squamous cell carcinomas, melanomas and fibrosarcomas. Any new lump, bump, or sore should prompt a visit to your vet to rule out a tumour of the skin.
Take home message
Skin diseases can really affect the quality of life of our canine companions, so if you notice any symptoms of skin disease, chat with your vet promptly. As with most conditions, it is best to get them dealt with early to both improve your dog’s comfort levels and minimise the damage caused. Several drugs are available for treating these conditions, including surgical options for tumours, and your vet will be best placed to decide which will best help in your particular scenario.
