Diagnostic pathologists are in the privileged position of having a job full of variety where no day is the same. Our job is essential in providing diagnostic support to veterinarians in practice, as in many instances (especially in cases of neoplasia), a final diagnosis can only be made by histological examination of tissues under the microscope.
While most of the samples are from dogs and cats, we are also sent samples from small mammals, horses, donkeys, farm animals, laboratory animals, birds and zoo animals. Most biopsies we read are from the skin, oral cavity or mammary glands, but samples from any organ can arrive at our lab. This diversity of cases means we are constantly exposed to new material and keeps things interesting!
Usually, pathologists can make a definitive diagnosis by histological examination of biopsies. However, not all cases are straightforward, and some samples we receive cannot be assigned a final diagnosis. In these situations, we discuss the potential differential diagnoses to try to help the referring vet with management of the patient or suggest additional tests that may be useful.
Usually, pathologists can make a definitive diagnosis by histological examination of biopsies. However, not all cases are straightforward, and some samples we receive cannot be assigned a final diagnosis
The following are some examples of typical cases that a small animal diagnostic pathologist might see during their working day.
The first example we have is a seminoma (Figure 1), a tumour of germ cells of the testis. Figure 1A shows a large mass effacing the normal testis and neoplastic/tumour cells filling the tubules (arrows). These tumours can cause the testis to become greatly enlarged.
Infiltrates of lymphocytes (circled in Figure 1B) are a characteristic finding when these tumours are examined under the microscope. These tumours display features such as frequent mitotic figures and apoptotic cells, as well as very large nuclei with prominent nucleoli, seen in Figure 1C. In many tumours, these features indicate malignancy. However, seminomas usually display benign behaviour and castration is usually curative.
Female rabbits are very prone to uterine adenocarcinoma (Figure 2) – a malignant tumour of the glands lining the uterus – the mean age of affected rabbits is approximately six years (Walter et al., 2010). These tumours can spread to other parts of the body, but most notably the lungs. Spaying female rabbits prevents this tumour from developing.
Interstitial cell tumour
Interstitial cell tumours are common testicular tumours of dogs (Figure 3). These tumours used to be known as Leydig cell tumours. They are usually benign, and castration is generally curative. The tumours are well demarcated from the surrounding testis and are formed from sheets of large polygonal cells with bright eosinophilic (pink!) cytoplasm, which is often vacuolated.
“Mixed” mammary tumour
Mixed mammary tumour is common in female dogs and surgical removal is curative (Figure 4). This is known as a “mixed” mammary tumour due to the formation of bone and cartilage tissue within the mass.
Peripheral odontogenic fibroma
Peripheral odontogenic fibromas (Figure 5) are benign tumours of the periodontal ligament found in the mouth of dogs. These tumours are common and surgical removal is usually curative, although they can grow back if not fully removed. These tumours sometimes contain areas of bone and odontogenic epithelium, which is often thickened (hyperplastic).
Tubular mammary adenoma
Tubular mammary adenoma is a benign tumour of the mammary gland of dogs (Figure 6). They are well demarcated from the surrounding normal mammary tissue and are composed of small tubules lined by columnar epithelial cells. Full surgical removal is curative. Early neutering of female dogs greatly reduces the likelihood of development of both benign and malignant mammary tumours.
Bowenoid in-situ carcinoma
Bowenoid in-situ carcinoma (Figure 7) is a skin disease found in cats and consists of proliferation of the keratinocytes of the epidermis to form plaques. It is thought to be caused by infection with feline papillomavirus. Multiple lesions may occur, and any area of the body may be affected. The lesions are locally invasive and may become infected, but metastasis is thought to be very rare.
Sertoli cell tumour
Sertoli cell tumour is another common type of testicular tumour found in dogs (Figure 8). Approximately 25 to 50 percent of dogs show signs of elevated oestrogen (Sanpera et al., 2002), including mammary development, hair loss on the abdomen and bone marrow suppression. About 85 percent of these tumours are benign and in these cases, castration is curative.
The cross-section in Figure 9 is from a thyroid adenoma of a cat. Thyroid adenoma is a benign thyroid tumour which is common in older cats and often causes hyperthyroidism. Affected cats lose weight despite having an increased appetite, and they often drink and urinate more often.
This can be compared with Figure 10 which shows the adjacent normal thyroid gland. The tissue resembles that of the adenoma, but the cells are smaller, less densely packed and form round follicles. These contain pink material (colloid) which contains thyroglobulin, a thyroid hormone precursor.
Anaplastic mammary carcinoma
Anaplastic mammary carcinoma is a subtype of canine mammary tumour. These mammary tumours are unusual as the neoplastic cells are individualised and surrounded by a very dense fibrous stroma (Figure 11). This type of mammary tumour is very malignant, and frequently spreads via the lymphatics to the lungs.
Oral plasmacytomas are relatively common tumours found in the mouth in dogs (Figure 12). These tumours are benign and surgical removal is curative. They are formed by small round cells forming “nests” which sometimes have multiple nuclei as well as large, irregular nuclei. These tumours are also found in the skin. Older dogs are most often affected.
The images in Figure 13 are from the skin of a dog with calcinosis cutis, a condition in which the skin becomes mineralised. This most often occurs in dogs receiving long-term steroid medication or with hyperadrenocorticism (Cushing’s disease). Figure 13A shows mineralisation of the collagen of the dermis. Collagen fibres have a blue tinge and appear slightly jagged and fragmented. Figure 13B shows multinucleated giant cells, an inflammatory response to the mineralisation.
Figure 14 shows a biopsy from an eosinophilic granuloma in the mouth of a cat. The first image shows inflammatory cells surrounding bright pink (eosinophilic) material (Figure 14A), while in Figure 14B, we can see this bright pink material (known as “flame figures”) up close. In Figure 14C, eosinophils and macrophages (inflammatory cells) can be seen. These lesions are benign and arise in the mouth or on the skin due to hypersensitivity (allergy) to stimuli such as fleas, dietary components or environmental substances (eg pollen or mould).
Seasonal flank alopecia
Seasonal, or cyclic, flank alopecia is a poorly understood condition causing hair loss in dogs, usually in the early autumn to late spring. Affected breeds include Boxers, English Bulldogs, French Bulldogs and Miniature Schnauzers. Hair loss usually occurs on the flanks, and the hair coat can become dull and dry. Hair follicles become dilated with keratin (Figure 15) and there is hyperpigmentation of the epidermis. The distorted hair follicles can form irregular shapes resembling “crow’s feet”.
Figure 16 shows an example of polypoid cystitis in a dog. In this condition, the bladder wall forms irregular projections with areas of haemorrhage and the epithelium becomes thickened. Brunn’s nests (islands of epithelial cells) can form in the lamina propria. These must be differentiated from neoplasia (cancer) of the bladder. This condition may occur in dogs with bladder stones or chronic bacterial infection.