Reaching a diagnosis in equine dermatology – It’s easy if you try...or is it ?

In equine practice, the discipline of dermatology is very badly overlooked as being a significant diagnostic challenge. This is largely because it is easy to see the signs and even easier to “attempt to treat” the symptoms. In some cases, this might even work but it relies more on luck and less on clinical acumen and evidence-based medicine. A good example of the dangerous approach is the common attitude to “pastern dermatitis” ! Here the use of the term “mud fever” as a diagnosis simply magnifies the concept that it is easy to make a diagnosis in equine dermatology. If a vet wants to add credibility to the diagnosis and add some “fancy names to impress or confuse” the owner, then the term dermatophilosis can often be heard ! The fact is that pastern dermatitis has up to 9 different potential causes and several others that are rare or very rare. It is however true that with a modicum of logic and clinical acumen combined with determination and a compliant and honest owner, a diagnosis can usually be reached ; the outcome often depends on the extent to which each of these can be undertaken in a proper professional manner. Dermatological disease in equine practice is however, always a challenge. A definitive diagnosis can provide immense professional satisfaction – even when the condition has little or no treatment available and even when no clinical evidence is published. At least the owner knows the reasons for this rather than just throwing every known medication at the animal without thought or justification. Polypharmacy obscures the basic and uncomfortable truth that the clinician does not really know what is going on ! It has been said that the number of different drugs administered is inversely proportional to the clinical acumen of the physician.

The skin is by far the most easily examined organ– it is directly visible easily and conveniently palpable and is very easily sampled if necessary. Veterinarians will be a called to see chronic skin cases more that those of acute onset; most owners will either have ignored it (in the hope that it would “go away” or will have applied some medications (usually ill-advised and inappropriate. This may confuse both the clinical presentation and the diagnostic possibilities. Failure to declare previous “treatments” also does not help.

The good thing about equine dermatology is that many of the conditions can be treated or at least they have a reasonably well-defined prognosis. However, extrapolation from other species may lead to problems when the treatment is also extrapolated from other species. Rare diseases are only rare because they are not common ! In the clinical investigation process, every potential differential diagnosis should always be included regardless of the rarity and even “impossibility. The clinical process in dealing with a difficult case still requires the basics of a clinical investigation ; there is nothing that is not within the remit of an ordinary practicing veterinarian. Specialists only look good when the first opinion vet has done a poor clinical job ! When the specialist is called upon for genuine clinical expertise the referring vet can be satisfied that the case is a real challenge.

A logical clinical approach is also essential – many of the conditions can be easily and definitively diagnosed with clinical and historical information alone. Intuitive supposition can be useful but there are significant dangers because many conditions have a common presenting clinical appearance. Only once a diagnosis has been established can treatment be expected to succeed.

The skin is also an effective window on the health or otherwise of many other organs and systems and so all the available possibilities should be considered. For example, an acute onset of an apparent photosensitisation affecting the blaze and muzzle could be associated with advanced liver disease, or ingestion of a photoactive plant such as St Johns Wort (Hypericum perforatum) or could indeed simply be a matter of sun exposure (actinic burn). In the former case management of the skin alone would be pointless while in the latter simple reduction in exposure to sunlight by application of sunblock or stabling the horse would be curative. If the problem was being caused by ingestion of a photoactive plant then of course the grazing and management must be investigated. This simply illustrates the importance of an encompassing /holistic approach to the investigation – there should be no assumptions and certainly no speedy conclusions. In spite of this, intuitive supposition is a common approach to skin diseases but serious errors can be made because of the clinical similarity of many of the skin conditions. [...]