The approach to the equine dermatological case

2 The approach to the equine dermatological case

There are many important aspects involved in the clinical investigation of a dermatological case. Often, the main clinical evidence of skin disease and the reason for the owner seeking a veterinary consultation is visually obvious in some form or another but the skin is easily overlooked as an important mirror for the health of the horse. Also it is often the most abused organ as a result of neglect, repeated washing, over-energetic grooming, poor harness and blanket hygiene, and of course the horse’s own propensity for trauma. This means that often the primary clinical signs have been either obliterated or so altered and confused that it is hard to know whether any particular sign is the primary one.

It is remarkable how many times a diagnosis can be achieved even in long-standing and difficult cases when extra time and care are applied in the process of clinical investigation. The primary or first clinical evidence and information on the progression (improvement/deterioration) of skin disease can usually be defined with the help of a really thorough history. In cases where the historical progression of signs cannot be established, repeated clinical assessments will often provide useful information.

Intuitive supposition is often the mainstay of clinical practice – sometimes the signs are pathognomonic but even very experienced clinicians can make diagnostic errors when important aspects of the clinical process are not followed. A solid black spherical mass in the perineal skin of an aged grey mare may be assumed to be a ‘typical’ melanoma but it could be one of several different other options as well as one of several different types of melanoma (Fig. 2.1). Also a superficial glance at the skin may be grossly misleading. A close and detailed examination is an essential part of the diagnostic process even when the condition is apparently quite obvious. Often there are occasions when there are two distinct conditions present in the same field of view and then the clinician could easily overlook one of them (Figs 2.2 and 2.3).

Figure 2.1 This large black nodule complex developed from long-standing smaller nodules similar to the ones in the tail base, over a period of around 2 months in this 16-year-old grey horse. Although the lesion was initially diagnosed as ‘melanoma’ it was in fact a highly malignant aggressive undifferentiated melanosarcoma with a high mitotic rate. Prompt removal of all the nodules was undertaken.

Figure 2.2 (A) This 3-year-old grey horse was presented for investigation of alopecic areas on the lateral neck. Close examination of this area showed a noticeably different black-coloured nodule. Further inspection confirmed that there were in fact many more of these small nodules. Biopsy confirmed both occult and verrucose sarcoid and melanocytoma. The treatment options for the two conditions are significantly different. (B) A diagnosis of melanoma was made for the lesions marked with arrows but the wider alopecic area was sarcoid.

Figure 2.3 This horse was presented for investigation of this pale wart-like area affecting the inner skin of both pinnae. A diagnosis of pinnal acanthosis (see p. 136) was made clinically. During the close examination a verrucose sarcoid was identified at the ear margin (arrow). Again the clinical approach is very different for the two conditions – the sarcoid has a much more threatening nature, and management options for the two conditions are very different.

Without a logical and systematic approach, there are few more frustrating situations than the diagnosis and treatment of skin disease in horses; treatment commonly fails when it is not focused on the cause of disease or recurrences develop when the pathogenesis and epidemiology are not considered. It is therefore essential that before any diagnosis is made, all the available facts are considered as a whole. Owners of horses commonly become distressed when skin problems fail to resolve, or even to follow a predicted course of treatment. Sometimes this leads to further frustration and often desperate measures such as seeking advice from non-professional sources or resorting to homeopathy. This may be a result of misinformation, misdiagnosis or in some circumstances improper treatment. There are some conditions that are extremely difficult to treat/manage and if the owner is not aware of these problems, there may be an unrealistic expectation.

A realistic and helpful prognosis and effective treatment usually require an accurate diagnosis. Without this diagnosis, any treatment becomes little better than an educated guess. For example, the clinical similarities between some forms of the occult sarcoid, linear keratosis and dermatophytosis (ringworm) may lead to an error of treatment choice. The simple expedient of a thorough, exhaustive history and a careful clinical examination involving the whole horse will invariably lead to a correct diagnosis, correct treatment and a satisfied owner. Even if the condition is not resolvable, or carries a poor prognosis, the owner will be satisfied that a proper diagnosis has been achieved. In the days of information availability it is even more important to develop clinical acumen. Symptomatic treatment is often palliative rather than curative and does little to improve our understanding of equine dermatological disease; often this approach relies on natural ‘healing’ mechanisms and when this does not take place there is opportunity for complaint and recrimination.

It is tempting to overlook apparently trivial skin changes, but where two or more conditions coexist, each may have very different clinical demands, significance and prognoses (Fig. CD2 • 1A–H).

A problem-orientated approach is an effective way of exploring a dermatological case; the diagnostic process is directed towards the identification of the recognizable problems which the horse has. The problems that are recognized do not necessarily have be dermatological – indeed, a comprehensive problem list will help enormously in both the investigative process and the selection of further tests. It is, for example, possible to overlook the more generalized clinical signs of liver disease when faced with a horse suffering from ‘actinic’ sunburn dermatitis. Recognition that the horse has lost weight, has icteric mucous membranes and a plaque of ventral oedema, can be vital aids to a diagnosis. However, it is also true that some other signs can be misleading and that is when clinical excellence comes to the fore.

Once the problem list has been established, a broad differential diagnosis can be formulated. Careful elimination of the various options may lead directly to a convincing diagnosis but it is a fact that the broad scope for diagnosis and the rather narrow panel of available signs means that further tests are often required in dermatological diagnosis (see Chapter 3). It is also important to recognize that the use of further tests will not necessarily lead to a diagnosis and so sensible judgements have to be made at all stages in the procedure and the owner apprised of the difficulties and the ‘likely diagnosis’. The maxim ‘communication is all’ is particularly applicable to equine dermatology. An accurate diagnosis can, however, often be made.

Experience and training enable the veterinarian to provide a prognosis: Is the condition treatable or not, and is it worth treating? If a targeted, specific treatment is available, economic and effective this may be used. In some circumstances treatment may be worse than the disease and then other aspects should be considered. The overriding concern has to be the welfare of the animal. Often owners will wish for ‘cosmetic’ treatments but when this conflicts with the welfare needs of the horse such treatment may not be ethical.

Symptomatic treatment alone often masks important signs and so it is vital that any concurrent or recent therapy of any type is declared. The clinical signs of insect bite hypersensitivity may be significantly altered by concurrent steroid therapy for an airway obstruction.

When approaching a skin disorder the clinician must establish whether the condition is a primary dermatological disorder (even if there are secondary systemic signs and consequences) or whether the skin signs are secondary to a primary systemic disease. For example, failure to recognize that a severe photo/actinic dermatitis was the result of advanced liver disease might allow the veterinarian to suggest that the horse had a trivial disorder that simply required local topical treatment. Conversely it might in fact be a case of simple sunburn and then an unsupported conclusion that the horse had advanced liver failure could be equally dangerous. The problems may be primarily dermal, and some of these may have systemic signs which could easily mislead the clinician. Furthermore, significant skin disease can arise as a secondary manifestation of a primary systemic disease. A pruritic horse can rapidly cause a lesion at the site – the basic question to ask then is: Was the lesion present before the pruritus or was the pruritus the first sign with a lesion developing subsequent to self-trauma? In the former case the primary sign is the lesion while in the latter it is the pruritus. The investigation and differential diagnostic possibilities are very different.

History taking (anamnesis)

The history of both the animal and the presenting problem are important independently derived parts of all clinical procedures. These should preferably be established and recorded before any clinical assessment or judgement is made. At the outset the history should be directed at the general history of the horse rather than at the presenting complaint. This provides the clinician with background information about the duration of ownership, the management and disease history of the horse. During the collection of this information it is sometimes hard to keep the owner focused away from the presenting complaint. Failure to obtain an adequate history often leads to errors of interpretation and wrong diagnosis.

The value of an individually formulated examination sheet (such as is shown in Fig. 2.7) cannot be overstated – it allows a structured, thorough questioning without omissions. However, even this may not always be totally comprehensive and the use of a structured form can sometimes be too prescriptive for unusual circumstances and presentations so they should be used with care. No information is too trivial – subtle facts may ultimately be the most significant. It is important to avoid undue repetitions but it may be necessary to ask questions in different ways to obtain the most helpful and reliable answers.

The duration of ownership will carry an important message – recent purchase from an unwarranted sale with no ‘papers’ means that there will be no long-term information about the case and in fact it might have been sold because of the problem! The converse is the aged horse that the owner has owned since birth; everything should be known about it and the history may be so full that it becomes difficult to establish what is important and what is not. Nevertheless the extra information is far more helpful than the absence of a history. A pre-purchase examination at the time of acquisition should have identified if any problem was visible at that time. This examination can be assumed to be a complete and thorough examination of all body systems and the documentation may be supportive of a post-acquisition problem or it may have identified a significant or assumed insignificant problem even if the examination failed to make a diagnosis. Clinical descriptions can be very helpful in retrospect. For example, a small area of alopecia and hyperkeratosis on the medial thigh of a horse could be assumed to be a rub mark from a strap but if this corresponds some 2 years later with a proliferative fibroblastic mass it might well have been an occult/early sarcoid at the time. Descriptions are probably more help than a diagnosis that is wrong. If a diagnosis of ringworm (dermatophytosis) had been made in the example above the connection between the two lesions could have been overlooked because almost all dermatophyte lesions resolve spontaneously over some months.

One of the most overlooked yet important aspects of skin disease is the management system employed by the owner (Fig. 2.4). Changes in management immediately prior to the onset of a significant urticaria might suggest some aspect of the change could be responsible. Failure to ask the right historical questions can be very limiting. It is often helpful in long-standing cases to ask the client to provide a chronological list of the disease’s progress, variation of feed or environment or of rider tack or harness which has been used. Management changes such as feed, water, bedding, pasture and transport, or unusual circumstances such as access to toxic waste or preservative-treated fence posts, should be established early.

Figure 2.4 This horse presented with all the signs of insect bite hypersensitivity (IBHS/’sweet itch’) and the owner took strenuous measures to protect the horse from the Culicoides bites. However, its feed was placed directly adjacent to the overgrown, damp, forested area in which the midges bred. In this case the horse did not in fact have any hypersensitivity but was being bitten unmercifully by myriad Culicoides midges. The signs described were ‘typical’ of IBHS but the simple expedient of observing the horse in its normal environment provided very helpful information and a simple curative solution!

The history should establish all the relevant long-term information about the animal such as previous disease or illness, surgery, medication and the vaccination and worming status. Work and management details (including bedding types and feed regimens) provide useful information, particularly when considered with the time of onset of the condition.

Careful reviews of in-contact horses (or other species) and the environment are useful additions.

The concept of contagion between horses and between horses and the other species in the same place must be identified. Whilst some diseases may appear to be contagious because several animals are affected equally, there are some important non-infectious conditions that arise from management changes. For example the use of a strong chemical wash might affect all the horses and give the impression that the condition is highly contagious.

The history should include an assessment of in-contact horses as well as other species. Medication history and routine procedure details (vaccination, worming, dental and farriery) should be recorded. Consideration should be given to changes in management, peer group and contact species health and feeding. The long-term history should therefore be exhaustive and should include any significant abnormality of any body system at any time in the past.

The local environment is also important and it can be helpful to examine the horse in its own circumstance (see Fig. 2.4). Local disease surveillance can help enormously in some circumstances. For example, epizootic lymphangitis is endemic in some parts of Ethiopia but absent in others. In spite of the breed predilection of the Icelandic horse for insect bite hypersensitivity, the condition is very rare in Iceland because there are few (if any) Culicoides spp. insects there. Local knowledge is often helpful.

Owners may be inclined to blame environmental pollution for some skin diseases but these are few and far between and most such suggestions are born out of frustration and the desire to blame something/someone else. Local environmental conditions can also be important. Veterinarians may need, for example, to examine the pasture closely for photodynamic plants and toxic weeds; of course their recognition will require basic botanical skills also. Mimosa spp. and Leucaena spp. plants can cause specific dermatological problems. Seleniferous plants that concentrate selenium could be very significant if the horse(s) is already receiving selenium feed supplements.

Note