Intestinal adenocarcinoma is a common tumour of older sheep (Maxie 2007, Radostits et al. 2007). Most are discovered at slaughter as incidental findings; if clinical signs are present, weight loss and occasionally ascites are usually the only findings (Maxie 2007). The gross appearance of the tumour is pathognomonic and means that differentiation from ovine Johne's disease (OJD) is quite easy grossly. This case of intestinal adenocarcinoma is a typical presentation and an example of how the disease can mimic OJD.
Six of 2,300 5-6 year old first cross ewes run on improved pastures in the Cowra district of NSW were noticed to be unusually thin in late October 2014. As ovine Johne's disease was considered a possibility one of these ewes was euthanased and necropsied on the 26 November 2014.
A 5 year old ewe was examined. The ewe lagged to the back of the mob at mustering but was bright, alert and responsive. The ewe was in fat score 1.5 with pink mucous membranes and no other detectable abnormalities.
The serosa of the small intestine, caecum and colon was covered in miliary to focally extensive firm, white, raised plaques. In some areas, the plaques were isolated, pinpoint lesions, whereas in other areas they coalesced to cover as much as 90 percent of the serosal surface.
Caecum: The lamina propria, submucosa, muscularis mucosa and serosa are markedly expanded by a poorly demarcated, unencapsulated, infiltrative mass composed of two populations of neoplastic cells supported by abundant fibrovascular stroma. The neoplastic cells occur singly and in loose packets, lobules and cords, and are supported by stroma of varying density composed of fibrocytes, fibroblasts and a light basophilic matrix. The predominant type of neoplastic cell is round to ovoid, 20-50 μm in diameter, with distinct cell borders, abundant finely stippled basophilic cytoplasm, peripherally located oval nuclei with lightly stippled hypochromatic chromatin and single nucleoli. The secondary type of neoplastic cell is round to oval, 15-30 μm in diameter, with distinct cell borders, moderate amounts of cytoplasm containing small eosinophilic granules, single peripherally located large nuclei with coarse chromatin and single nucleoli. Both sub-populations of neoplastic cells exhibit moderate anisocytosis and anisonucleosis. Mitoses are rare, averaging < 1 per HPF. Neoplastic cells are present within all layers of the caecum, as well as in the adventitia of large vessels. Villi are fused and blunted, and within the mucosa are multifocal crypt abscesses composed of non-degenerate neutrophils, with rare macrophages and lymphocytes. The lamina propria is diffusely expanded by an infiltrate of moderate numbers of lymphocytes and plasma, with lesser numbers of macrophages and rare eosinophils.
McDonald et al. (1965) found that 0.4% of all sheep and 2% of sheep over 5 years of age had lesions consistent with intestinal adenocarcinoma on one property. A 1980 southern NSW abattoir survey found 3 sheep affected per 1,000 (0.272%), with a regional variation in rates of between 0.2 % and 1.5% (Ross 1980).
Differential diagnoses that should be considered in cases of wasting include OJD, malnutrition, chronic infections such as pneumonia or caseous lymphadenitis, hepatic disease, internal parasitism and fasciolosis. These differentials should be kept in mind even in OJD-infected flocks. Bush et al. (2006) found that up to one third of cases of wasting in OJD-infected flocks could be attributed to causes other than OJD.