Viral diseases

18 Neoplastic conditions

A neoplasm is an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of the normal tissue and persists in the same excessive manner after cessation of the stimuli which evoked the change.

(Sir Rupert Willis 1952)

	Benign	Malignant
Extent of cell differentiation/Anaplasia	Limited/absentWell-differentiated cells (recognizably similar to normal tissue type)	MarkedLack of differentiation with anaplasia; atypical cell structure/form
Local invasion	LimitedUsually cohesive with well-demarcated margins that do not invade local normal tissues. This does not necessarily equate with encapsulation	ExtensiveLocally invasive with ill-defined margins. Some are more structured and expand locally
Rate of growth/expansion	Slow progressionPeriods of rapid/slow expansion or static, and some periods of apparent regression	Rapid (or variable/slow)Erratic growth patterns
Mitotic index	LowFew mitotic figures and these are normal in appearance	HighNumerous mitotic figures and possibly abnormal/bizarre mitoses
Blood supply	Variable	Variable (usually high)
Metastasis	No	Yes (frequently present)The more undifferentiated the tumour the more likely are metastases

2. Functional disability when the tumour interferes with the normal function of any adjacent organ. Usually this is a result of space-occupying properties. For example, a large perineal melanoma can obstruct the rectum and result in tenesmus, obstructive constipation (obstipation) and colic as a result of non-strangulating small colon obstruction (Fig. 18.4). Although penile carcinoma is singularly rare in (working) stallions (as opposed to geldings), when it does occur it can be very aggressive with devastating effects on the animal’s breeding capacity (Fig. 18.5). The location of the tumour and its nature will clearly influence the effects it has on the adjacent structures. For example, a melanoma in the jugular groove can seriously affect the vago-sympathetic trunk and produce Horner’s syndrome and laryngeal paralysis. Even head oedema and swallowing difficulties can arise. Tumours that involve the eyelid may have serious effects on the eye itself (Fig. 18.6). This is the reason why a very careful and exhaustive clinical assessment is essential in all cases where neoplastic disease is suspected. The presenting sign may be dramatic but secondary to a less obvious tumour state. Functional loss can present in a misleading way. For example, the main (most obvious) external sign in a horse with multicentric lymphosarcoma may be diarrhoea. A careful clinical examination might identify significant cutaneous nodules. In the same way a single cutaneous haemangioma/haemangiosarcoma lesion may be missed in a horse with unilateral hind limb lameness, which could be due to disseminated tumours. It is always worth establishing the whole range of presenting signs before making a tentative diagnosis.

3. Metabolic deficiencies such as anaemia and hypoproteinaemia as a result of continued plasma and blood loss. Ulcerated and/or exposed tumours that can easily be traumatized can cause major blood and protein losses (Fig. 18.7).

4. Secondary infection/infestation: open ulcerated tumours are very liable to infection and myiasis in particular. Whilst the blood supply can be considerable it does appear that local immunity can be severely compromised by the tumour’s existence.

Figure 18.4 A large perineal melanoma, which had extended into the pelvis resulting in an extramural small colon obstruction. This is turn resulted in distress and finally in colic signs relating to a complete obstruction. Options for treatment in such cases are very limited and in this case colostomy was considered but ultrasonographic examination revealed many intra-abdominal and thoracic tumours and so the pony was destroyed.

Figure 18.5 This 17-year-old working stallion developed a highly malignant, aggressive, proliferative squamous cell carcinoma. This is a very rare condition in entire stallions but precluded his use for stud purposes and he was destroyed. Post-mortem examination showed inguinal and iliac lymph node involvement.

Figure 18.6 (A) This 14-year-old cream pony was presented rather later than it should have been with multiple tumours on its eyelids. The melanomas were obvious clinically (black arrows) and the ulcerative squamous cell carcinoma on the lower lid (blue arrow) was confirmed by biopsy. The larger proliferative mass was thought to be a component of one or other of these but was in fact a sarcoid (open arrow). There is no reason why animals cannot have more than one condition at the same time! In spite of the severity and complexity the pony was not seriously disabled by them. (B) This highly destructive carcinoma resulted in significant disability and secondary eye complications. Metastatic tumours were present in the parotid lymph nodes and the floor of the guttural pouch.

Figure 18.7 This large type 2 (sessile) fibroblastic sarcoid on this donkey’s chin bled and exuded serum incessantly and resulted in clinically significant hypoproteinaemia and anaemia. Metabolic signs caused by tumours can be primary or secondary as in this case.

The paraneoplastic syndrome

This is a syndrome that has a complex and poorly understood pathogenesis characterized by clinical and metabolic events that cannot be directly explained by the presence of the tumour (either locally or by metastatic spread) or by the formation of naturally occurring bioactive mediators and hormones in the cell type concerned. Although in horses these syndromes are less common and in skin tumours they are even rarer, they are important. Quite often it seems that they are overlooked in the process of diagnosis of tumours in general but they may be the very first evidence of a serious neoplastic disease. Also the syndromes themselves may be fatal and then the death may be attributed to causes other than cancer. For the most part the syndromes are very complex and difficult to treat without removing the primary cause, i.e. the neoplastic disease.

There is also a clinical risk that animals showing signs consistent with the paraneoplastic syndrome (weight loss, anaemia, pica, etc.) may have other non-neoplastic reasons for these signs.

Cancer should be regarded as a parasitic state but in the case of the paraneoplastic syndrome it is even more than this. It is generally considered to be a complex multi-endocrinological effect from the secretion of false endocrinologically active metabolites of the tumour. Some tumours have a much higher propensity for inducing these changes. From a clinical perspective it appears as generalized metabolic effects resulting in disproportionate weight loss, inappetence, recurrent fever and sometimes anaemia. There are few primary skin tumour types that have well-recognized paraneoplastic consequences but, for example, the common equine sarcoid can be associated with debilitating effects and with a disproportionate metabolic effect reducing performance and possibly resulting in weight loss. Furthermore there are dermatological consequences of other tumour types such as haemangiosarcoma and lymphosarcoma that are included in the paraneoplastic syndromes. Various forms of immune-mediated vasculitis including paraneoplastic pemphigus (see p. 267) and paraneoplastic pruritus are recognized (Williams et al 1995, Anhalt 1997). Often the clinical effects are most obvious in the oral mucosa but changes can occur in the skin. Histologically subepidermal clefting, and vesicle formation with accumulation of lymphocytes, plasma cells and neutrophils were reported. These changes are dependent on the tumour and so removal of a causative mass can result in resolution of the dermal problem.

The approach to a suspected skin tumour

Clinicians tend to underestimate the contribution they make in the diagnosis of neoplastic disease, often preferring to rely almost totally on the pathologist. However, if the clinician provides a really careful dossier of information for the pathologists the likelihood of an early diagnosis is far greater. The role of the pathologist is critical, if only to make a clear differentiation between abnormal accumulations of normal cells and the various stages of tumour development. Clinically, early tumours can often easily be mistaken for hyperplastic tissue responses and vice versa (Fig. 18.8). Careful review of both the clinical and pathological findings may change the diagnosis.

Figure 18.8 (A, B) This 5-year-old Shetland pony stallion developed this large exophytic mass on its tail with extensive hair loss and exudation. Initial reports from biopsies suggested that it was a neoplastic state but a review of the histology, and several new biopsies, confirmed that it was a reactive state to free keratin in the dermis, probably as a result of rupture of follicular cysts. Advice on treatment was limited because no previous reports of a similar state could be found.

There is nothing special about the diagnostic approach to a cutaneous mass! The normal clinical investigative procedure should always be carried out – sometimes an aggressive looking lesion turns out to be simple and benign and possibly even not neoplastic, while another may look innocuous and actually be highly sinister. An instant diagnosis may sometimes be available through the recognition of definitive clinical features (such as melanoma) but an intuitive ‘guess’ can lead to the wrong interpretation even if the tumour is ‘recognizable’. For example, does a single, localized, darkly coloured mass on a grey horse have to be a melanoma? Where the mass is not sufficiently recognizable as to allow a definitive diagnosis to be made immediately, biopsy may be suggested. Biopsy is usually definitive provided that a suitably diagnostic specimen is obtained, because most tumour types are now recognized pathologically. The various techniques for biopsy/tissue sampling are described in Chapter 3 and typically some methods are more or less applicable to particular tumour types. However, the potential dangers of interference with any skin tumour must be considered carefully before performing any biopsy procedure. The least traumatic methods are fine needle aspiration and impression smears of ulcerated tumours. In both cases the ease and safety are somewhat offset by their variable clinical ‘return’. In many cases a reasoned judgement has to be made on the likely nature of the mass. Frozen sections examined immediately after removal may provide a rapid diagnosis, but this is often not feasible. In the case of very small tumours, a total excisional biopsy (surgical removal) may be undertaken with some degree of safety. Where biopsy is considered unavoidable, suitable therapeutic options should be available for treatment as soon as the results are known. There is no point in taking a biopsy if its results will not influence the clinical approach.

Pathologists are usually highly skilled in recognizing the features of tumours. Laboratory confirmation should always be undertaken if there is doubt and then a positive diagnosis can usually be achieved. Where surgical removal is performed, all tissue removed should be submitted to the pathologist; a pathologist will be better able to decide how and where to cut the tissue blocks to obtain the best specimens. Special stains and immunohistochemistry can give improved diagnostic information. It is sometimes very helpful to supply a photograph of the lesion and the site of the biopsy and it may be even better to consult with the pathologist before taking the samples.

Aspects of malignancy are clearly important (even if only retrospectively). Skilled pathologists are usually able to establish whether there is an adequate margin or whether excision has not been complete. A report of a ‘safe margin of excision’ is reassuring for the surgeon. Many tumours have local variations within the mass of tissue and examination of a relatively small piece may be misleading. For example, benign granulation tissue can be interspersed amongst a fibroblastic sarcoid; there may be an area of occult sarcoid around a smaller nodular or fibroblastic sarcoid; and there may be precancerous changes surrounding a squamous cell carcinoma lesion. A limited biopsy may provide the wrong information but extensive biopsy may be undesirable. The clinical examination and the experience of the clinician are therefore important.

Haematological responses including white cell and protein responses are generally not helpful in cutaneous neoplasia of any type. Cancer cachexia, characterized by weight loss, depression, anorexia, metabolic disturbances and general malaise which resolve after tumour treatment, is only rarely encountered in primary cutaneous neoplasia. However, where it does exist, it suggests that neoplastic lesions have significant secondary effects and sometimes these can be identified from blood. It is also important to remember that there are some cutaneous manifestations of internal tumours such as cutaneous metastases, immune-mediated vasculitis, and paraneoplastic pemphigus and in the case of pituitary pars intermedia dysfunction (PPID/Cushing’s disease/pituitary adenoma) even hirsutism, polydipsia-polyuria and laminitis.

Note

Pituitary adenoma (adenoma of the pars intermedia) is an internal neoplastic tumour with skin manifestations recognized as pituitary pars intermedia dysfunction (Cushing’s disease). However, there remains considerable controversy as to whether this is a true neoplasm or a secondary hyperplasia of the pars intermedia following loss of dopaminergic inhibition. Pathological enlargement of the pituitary gland (and the pars intermedia in particular) is a frequent finding during necropsy examinations of old horses. The pathological features suggest that this is a hyperplastic condition rather than a true neoplasm; the mitotic index is low, cells are well differentiated and often fully functional. The cutaneous manifestations (hirsutism, sweating, abnormal shedding patterns, ectoparasitism and cutaneous abscessation) are not in themselves strongly suggestive of a tumour origin but are typical and easily recognized in most cases, especially when considered with other characteristic signs such as polydipsia/polyuria, supraorbital bulging, weight loss, laminitis and neurological signs or central blindness (amaurosis).

Table 18.2 lists common and unusual skin tumours.

Table 18.2 Common and rare skin tumours

Common skin tumours

Dermal melanoma/melanosarcoma

Fibroma/fibrosarcoma

Lymphosarcoma/lymphoma

Mast cell tumour (mastocytoma/equine cutaneous mastocytosis)

Neurofibroma (schwannoma)

Ossifying fibroma

Sarcoid

Squamous cell carcinoma

Rare/unusual skin tumours

Basal cell carcinoma

Epitheliotropic lymphoma(mycosis fungoides)

Giant cell tumour

Haemangioma/haemangiosarcoma

Histiocytoma (malignant/fibrous)

Keratoma/keratoacanthoma

Sebaceous gland tumour

Diagnostic procedures for neoplasia

The majority of skin tumours fall into the nodular diseases category. There are some important features of diagnosis and treatment which should be appreciated before any interference is undertaken. These include aspects of malignancy where interference could result in tumour dissemination, and the tendency to recurrence at the site.

Important diagnostic features include the following:

Physical examination

In all cases of cutaneous neoplasia a physical examination must be undertaken. This must include:

1. A detailed, systematic clinical examination (which will often include a rectal examination, endoscopic examination of the airways, radiology and ultrasonographic investigations).

2. Specific close examination of (all) the lesion(s) to establish individual extent and character.

3. Examination of as much of the surrounding tissue as possible and the appropriate draining lymph node(s).

All three of these have a strong influence on the diagnostic process as well as on treatment and prognosis. For example, a horse with a single, slowly expanding subcutaneous mass in the medial thigh might have gross enlargement of the ipsilateral subiliac lymph node – this might support a preliminary diagnosis of lymphoma but in any case would alter the outlook considerably. Whilst an exhaustive clinical examination may seem in many cases to be an unnecessary complication, there is little worse than making a totally wrong diagnosis that seriously affects both the patient and its owner. Time spent in diagnostic processes is never wasted and there are few genuine dermatological or oncological emergencies!

The body condition of the horse and the health (or normality) of its system/organ functions may be seriously affected by some neoplastic disorders but much less so by others. Local invasion of tumour tissue may result in local distortions, physical alterations or functional problems, e.g. palpebral tumours may cause eyelid problems with secondary corneal ulceration or nasolacrimal obstructions with epiphora. Diffuse, ill-defined tumours may suggest malignancy or at least the prospect of treatment limitations. Few lymph nodes are readily palpable in the horse and further procedures such as endoscopy, radiography, ultrasonography and rectal examination may be required.

The laboratory-based diagnosis of neoplastic conditions has become much more complex as more is understood about the various cell behaviour patterns. Normally diagnostic tests include:

1. histological examination of aspirates/cytology

2. partial or total excisional biopsy material

3. direct (impression smears)

4. haematological examinations (some tumour conditions do cause detectable, if largely non-specific, changes in plasma proteins, red and white cell counts and electrolytes and enzyme concentrations).

Cytological examination (see Chapter 3)

The histological diagnosis of most advanced equine tumours is not problematic. Where a tumour has a very benign, differentiated form and where the changes are so severe that cellular structure cannot easily be recognized, there are few difficulties. The intermediate stages are far more of a problem and specific stains or immunocytochemistry, molecular methods, flow cytometry and the detection of specific tumour markers will often help. Possibly the biggest help is the clinical description and the correct and appropriate sampling of tissues.

The cell abnormalities encountered in aspirates are often subtle and difficult to interpret alongside artefactual alterations from sampling techniques. It is easy to over-interpret artefactual changes and indeed to overlook subtle but important diagnostic features. Incorrect sampling can easily occur and lead at best to a difficult interpretation and at worst to an error of diagnosis that could have been avoided (see p. 71). All smears/aspirates obtained from tumours must therefore be examined by an experienced cytopathologist. It is probably unacceptable to make a diagnosis without the help of such a specialist. Even recognizing ‘black’ cells in a suspect melanoma is not enough to establish useful information on pathological behaviour.

Malignant cells may be reported to exhibit some or all of the following:

• irregular/bizarre cell size/shape with uneven/unusual staining

• large obvious nucleoli within prominent/obvious nucleus with uneven staining and thickened nucleolar membranes

• high mitotic index (high proportion of cells in mitotic state).

Biopsy and histopathological examination (see Chapter 3)

There are several available sampling methods for use in equine oncology.

Biopsy of suspicious lesions is for the most part a sensible procedure. However, if the results of the procedure are not going to influence the management/treatment then there is little point in taking the biopsy at all. In some cases biopsy does not provide any more useful information – the common equine ‘melanoma’ probably does not warrant a biopsy because the clinical appearance is so ‘typical’. However, regular submissions even from such cases might add eventually to our understanding of the condition. Also, some tumours may be exacerbated by the process of biopsy – the equine sarcoid, for example, is probably best not subjected to biopsy unless an immediate therapeutic regimen can be applied because of the risk of exacerbation (Brostrom 1995). Therefore, presumptive diagnoses are often made and acted upon. It is only when there are complications or uncertainties that further testing is really required,

Biopsy of skin lesions is usually feasible under sedation and local analgesia alone. The interpretation of a biopsy relies heavily upon:

• correct collection of a representative (diagnostic) specimen

• correct processing (including fixation, section and staining)

• careful examination of the tissues by a qualified pathologist.

Histopathology is probably compulsory for all excised tissues. There is little or no excuse for not submitting such specimens. These specimens will provide important information including the true (undistorted) characteristics of the tissue and the presence or absence of a clear line of demarcation to normal tissue. Unless every single tumour cell is removed, regrowth is likely and so, if only as an assessment of prognosis, surgical pathology is vital. If excision was not complete the owner can be warned that recurrence is likely; it is not then a disappointing surprise when the tumour recurs. Most owners will accept this more readily if communication is maintained.

Haematology and serum biochemistry

These are almost invariably disappointing in skin neoplasia but may be significantly altered if:

1. There is extensive organ involvement (either primary or secondary).

2. There is significant secondary infection (either locally or systemically) as a result of impaired immune processes.

3. There are paraneoplastic syndromes.

There may be significant findings if the cutaneous condition is secondary to wider neoplastic processes and when the skin changes are the result of paraneoplastic changes due to internal neoplasia. The important principle is that when haematological changes are detected, they are interpreted with care.

Where specific tumour markers are available this is clearly a major help but so far there are no well-established markers in any tumour condition in horses. Only the melanoma appears to have any specific identifiable protein characteristics. The identification of bovine papillomavirus (BPV1/4) DNA genomic material by PCR in fibroblast cells taken from a suspected sarcoid can be good confirmatory evidence.

Radiology

This is sometimes useful in the horse but the anatomical difficulties make the benefit variable. Sinus and nasal involvement can be detected in some cases. Oblique and ventrodorsal projections are often required to establish the true extent but these are more difficult and in practice conditions may not be feasible. Thoracic radiography is sometimes indicated for highly malignant tumours but these are particularly unusual in equine skin neoplasia apart from the lymphosarcoma complex.

Ultrasonography

A detailed ultrasonographic study of the region and the mass itself is often useful and is an under-utilized aid. Some masses such as the equine melanoma have a very characteristic ultrasonographic character (Fig. CD18 • 1B). It may also be possible to use ultrasonography to guide biopsy instruments into typical or uncomplicated locations and so improve the outcome of the procedure. Space-occupying masses, fluid-filled cavities and displacements and distortions of the normal anatomy are easily recognized with experience. Ultrasonography can also provide a surgeon with useful information about the existence and/or the extent of a surrounding capsule (Fig. CD18 • 1C) as well as the proximity to other vital structures such as blood vessels and synovial structures (see Fig. CD18 • 21B).

Equine sarcoid

Profile

This is probably the most common cutaneous tumour of Equidae worldwide. All equids are liable to the condition including horses, mules, donkeys and zebras.

There is controversy over the aetiological role of papillomaviruses. A genome which closely resembles that of the bovine papillomavirus (BPV1/BPV2) has been identified consistently in sarcoid tissue from donkeys and horses (Reid & Smith 1992), but no vegetative virus particle has yet been conclusively demonstrated. Furthermore, the genome is apparently restricted to transformed fibroblasts. Keratinocytes do not appear to be involved directly. There are several examples where species-specific papillomaviruses are closely associated with non-papillomatosis neoplastic disease in that species, e.g. human papillomavirus (HPV) and cervical cancer and bovine papillomavirus (BPV) and bladder carcinoma. Nevertheless this is possibly the only circumstance when a papillomavirus from one species has any implication for another species.

The distribution of the lesions and the epidemiology strongly suggest that flies are an important part of the pathogenesis.

Clinically and pathologically, sarcoids present most of the features of a true neoplasm. The predominant cell type is a malignant/transformed fibroblast with characteristic in-vitro appearance and characteristics.

Sarcoids generally have a high capacity for local tissue invasion into the dermis and subcutis. However, true metastatic dissemination does not occur. Tumours can appear in freshly healing wounds in previously normal horses (Knottenbelt 2009), or reoccur at the same site following apparent complete surgical removal even up to 10 or more years later (Brostrom 1995).

Individual horses may be genetically susceptible and a familial tendency to sarcoid has been identified. An apparent predilection for Appaloosa and Arabians in America has led to the suggestion that this is a result of close contact with cattle (presumably infected with the bovine papillomavirus) but these breeds are not over-represented in any other populations of horse whether they are in contact with cattle or not. However, many horses may carry the genetic susceptibility to a varying extent, possibly explaining the variations in numbers and types seen on individuals. A familial tendency has also been identified, heavily supporting a genetic component. This susceptibility is almost certainly genetically orientated and is a property of certain MHC/ELA alleles. The presence of a W13 gene in the MHC complex appears to impart susceptibility; Lipizzaner horses do not appear to have a W13 gene and so the breed appears to be totally (genetically) resistant. The American Quarterhorse is apparently less affected than many other breeds. This is a complex issue with no definitive research progress.

Sex (geldings more commonly) and age (1–6 years) predilections have been proposed, but no horse can be considered to be totally exempt from the condition on the grounds of sex, breed, colour or age. Sarcoids are undoubtedly rare in very young horses but can occur from a few weeks of age.

Sarcoid is a locally aggressive, fibroblastic tumour occurring in six clinically recognizable forms (Fig. 18.9), all of which have a high propensity for recurrence (Knottenbelt & Pascoe 1994, Knottenbelt et al 1995, Knottenbelt 2005) . An individual horse may have more than one sarcoid type and for the purposes of clinical description, each lesion is classified individually. There are, however, some individual horses that have a predominant type – sometimes exclusively. Although each of these forms is commonly identifiable, it is important to recognize that the ‘less severe’ forms can rapidly progress to the more aggressive types, particularly if they are traumatized. Furthermore, the specific types may not be clearly identifiable in every case and then the’ mixed’ category is applied. It is, however, usually obvious that even the mildest forms are indeed sarcoid – in-vitro fibroblast cell cultures derived from these are typical and indistinguishable from those taken from the more aggressive lesions. These factors suggest that both cell and host factors are responsible in combination for the variety of forms.