I must confess to not having much enthusiasm for dermatology in the early days of my career but over the years I have spent considerable time dealing with skin conditions and I think the horse’s skin is regularly overlooked and perhaps the most forgotten organ of the body.
The skin provides a first line physical barrier against infection, protects the underlying structures and plays a role in heat regulation and electrolyte balance. When hot, the blood circulation to the skin increases -think of a thoroughbred after running a race- nearly all its surface veins are visible. The skin then produces sweat which evaporates to cool the underlying blood vessels. In the cold the coat will fluff up by a process known as pilo-erection- the hairs stand on end and create an insulating layer suited to the ambient conditions. When the surrounding air warms up, the hairs lie down again, thus allowing the horse to maintain the same body temperature. Moreover, the coat changes twice a year to fit in with weather conditions. I once wintered a thoroughbred foal outside with a field shelter. In the coldest mornings she would actually have frost on her back but a hand placed thorough her winter woollies revealed a warm skin. All this and waterproof too! In my opinion native types which are not being worked in the winter are better off without any type of rug. We have all seen native ponies which look drenched but the oil in the coat serves to repel the water. Thick manes, tails and feathers grow out of specialised areas of skin to provide protection not only from the elements but from insects too.
In addition to all this the skin has the ability to self-repair when damaged and on a much quicker time scale than most other parts of the body, small surface wounds are plugged quickly by a blood clot which helps prevent infection setting in. When proud flesh appears on a horse’s leg (never ponies as they are literally made of different stuff), this too presents a barrier, being rich in local antibodies.
So horse skin evolved perfectly over the last 3000 years or so to cope with all nature had to offer. Then we started to ride them and perhaps that is where the problems started to occur! Our horses no longer roam free, browsing for food, eating a variety of sparse grasses, supplemented by herbs and the odd tree leaf that might seem tasty, as well as having natural anti –inflammatory and anti -toxic properties, nor are they left to maintain their coats naturally by rolling in the mud and dust. (Most do, however exhibit this natural behaviour, usually before an important competition!) Our horses are kept on limited acreages, usually on pasture that is not as diverse as ideal- most grassland having been put down for farm animals. We want to ride or drive our horses- no-one wants to place their very expensive tack onto a filthy dirty animal and working animals with thick coats causes sweating and loss of condition, therefore we groom, clip and rug them up. Grooming will remove the natural oil content of the hairs and clipping more drastically removes them completely so they no longer provided a protective layer.
The general health of our horses is reflected in their skin. A healthy horse will have a glossy coat which shines without much elbow grease! Good nutrition will provide the necessary fat soluble vitamins and proteins to nurture the skin and provide a waterproof barrier. Together with normal skin bacteria an effective defensive layer is formed. A horse that is generally unwell will have a dull staring coat as vital nutrients are utilised elsewhere in the body in attempt to combat the stresses of disease.
The skin can only react to an insult, whether traumatic or infectious by reddening, swelling, (be it diffuse or in lumps), thickening and flaking, thus it can sometimes be difficult to determine the cause. This is particularly so when the horse has been rubbing or nibbling at the lesion and it gets secondarily infected.
For instance what initially looks like a few fly bites could turn out to be ringworm (fungal infection) or an allergic rash. Ringworm in horses nowadays is, thankfully, fairly easily treated but it is none the less very infectious and has the potential to spread to humans. Personal experience with my own point to pointer many years ago taught me to always suspect ringworm in skin conditions where there are any spots! In another case a small area of grey skin on the ear tip of a chestnut show pony turned out to be ringworm (fungal) infection. Ringworm in horses does not always look like the text book picture! If you have any concerns, I would advise strict hygiene until ringworm is ruled out due to rapid spreading and it is easy to be caught out. This means no sharing of tack, rugs or grooming kit which can be problematic on a busy competition yard.
Mud fever looks very like leukocytoclastic vasculitis and this can often get muddled with localised cellulitis due to the reactive swelling. I would always advise early veterinary intervention if anything unusual is seen, but do not always expect an instant diagnosis from your vet due to these difficulties. Swabs and scrapes or even biopsies may need to be taken.
Recent research into mud fever has shown that it is the cold, as well as the wet, which allows the causative organism to flourish. I have never been an advocate of washing legs off routinely and have always advised drying with a hair dryer (where a horse will allow!) which will not only dry the skin but blast the organisms as well. My clients thought me slightly batty but time has proved me to be right! It is always best to use wicking leg wraps which keep the legs warm and brush the mud off when dry than it is to wash. Horse owners are always surprised that their horse has mud fever when it has not been in a muddy field! It is the wet that harbours the bug and this can be wet grass as well as the arena surface, as was the case in an outbreak I dealt with.
The skin, where not pigmented, is liable to sun burn so a sun block should always be used on pink noses during the summer. Sunburn may be easily confused with photosensitisation where the pink and white areas of skin become crusty, hard and peel as a result of the horse eating something containing free radicles such as the weed St. John’s wort, or worse still as a sign of liver disease from ragwort poisoning for example. Should your horse exhibit signs of photosensitization, get him out of the sun in a barn where possible and avoid all green feeds until your vet can attend.
We do see allergic reactions which can vary from a few spots, to full blown urticarial reaction which can present with colic like symptoms due to the intense irritation before bumps are seen. I once injected a horse with penicillin and within 5 minutes he was throwing himself around the box rolling and twitching. His skin rapidly erupted in hundreds of marble sized lumps. Fortunately, quick intervention with steroids helped limit the condition. That horse had developed a penicillin allergy which was duly noted in large red letters on his passport.
Often owners will simply find their horse with itchy bumps and it is difficult to find the cause. It is safe to say that all animals have to be sensitised to an allergen before an allergic reaction can occur. It is virtually impossible to see a true allergy in a young animal, most taking about 2 years to become established. Some reactions are seen almost immediately the horse encounters the allergen and others may take several days to manifest. The allergen could be anything in the environment, from food stuff, including grasses, to tree pollens or insect saliva. As with human allergies, the best strategy is to eliminate the cause. Equine allergies also respond well to steroid treatment (less to anti histamines) but this has obvious implications in a competition animal.
Sweet itch is the most easily recognised and managed allergy: By preventing the midges from biting the horse in the first place. Unfortunately, this is a type of reaction known as delayed hypersensitivity which means that it takes a few days to show, so the damage is done by the time the horse is seen rubbing and an itch-scratch -itch cycle is set up. Some horses react very badly to horse fly bites, wheals the size of eggs or larger appearing within a few hours. There is no predicting which horse is going to react, it is due to the individual’s immune system, but there is possibly some genetic predisposition. In both cases, I would advocate a good rug and the use of a permethrin fly repellent.
Where lumps appear under the numnah or rug it is usually due to irritants in washing powder; horses being even more sensitive than humans to detergents as their skin is protected by hair. Riding the horse warms the skin up and rubs the irritant. Such cases are easily avoided by using only skin kind products in the future. Similarly, only specially prepared animal shampoos should be used for washing horses; human preparations are too strong and could well be irritating. Washing up liquid, even if it is kind to hands, will strip the coat of all its natural oils at the very least and at most could cause a reaction.
In conclusion,the skin reflects the general health of the horse. Poor skin can be an indicator of many systemic diseases and a healthy horse will always look well in its coat. Skin can suffer many insults but will react in one of a few specific ways which can present a diagnostic challenge for veterinary surgeons.
Dr Annie Ashman MA Vet MB MRCVS and veterinary advisor to KBIS