A two week old Arab foal with a history of protracted diarrhoea, weight loss and crusting exudate around the muzzle and tongue.
Example histopathological description
In the section of thymus, the lobules are reduced in size and separated by a prominent band of loose oedematous connective tissue and lipocytes. Within lobules there is mild congestion, dilated lymphatics and marked loss of differentiation between cortex and medulla. Each lobule is composed of a sparse population of lymphocytes, with scattered karyorrhexis, histiocytes, reticular cells, eosinophils, and occasional neutrophils, in a well vascularised stroma. Epithelial elements (Hassall’s corpuscles) are sparse and poorly keratinised. Macrophages are foamy, and a small number show erythropoiesis. There are sparse multiple foci of mineralisation and the mesothelium of the visceral pleura is plump (reactive).
The section of mucocutaneous junction (lip) includes epithelium, adnexal structures, collagenous connective tissue, large nerve bundles and striated muscle. The haired skin shows parakeratotic hyperkeratosis and acanthosis, with locally extensive erosion and epidermal necrosis. There is an overlying fibrino-cellular crust containing keratin, numerous yeast-like structures and occasional pockets of bacterial cocci. The superficial dermis has a mild mixed infiltrate of neutrophils, lymphocytes, plasma cells and a few eosinophils. There are yeast-like structures and neutrophils in some hair follicles. The deep muscle layers show refractile hyper-basophilic degeneration, necrosis and mineralisation. On the non-keratinised stratified (buccal) mucosa, the surface is largely intact, but the submucosa has a mild mixed inflammatory cell infiltrate. Mucous glands show mild to moderate degenerative changes and disruption with scattered pyknosis and lymphocyte infiltration. The yeast-like structures are poorly stained, predominantly spherical to ovoid, occasionally budding and have a diameter of 2-4 µm. Also present are filamentous hyphae and pseudohyphae, which are variably septate, irregularly branching and up to 30 µm in length.
Thymic hypoplasia; chronic, diffuse, severe, with marked lymphoid depletion.
Chelitis; ulcerative and exudative, subacute, locally extensive, severe, with intra-lesional yeasts (presumptively Candida spp), and underlying skeletal muscle degeneration.
Severe combined immunodeficiency disease (SCID).
SCID is a well known, inherited, autosomal recessive condition of Arab foals associated with a defect in B and T lymphocyte stem cell production. This produces marked hypoplasia of all lymphoid tissues, profound lymphopaenia and immunodeficiency. Secondary infections such as candidiasis (as in this case), and adenoviral infection are common. Foals usually die within the first three months of life despite intensive treatment. The protracted diarrhoea and weight loss mentioned in the history are commonly seen in cases of SCID. Note that SCID is presumed in this case as other lymphoid tissues were not examined, and acquired immuno-deficiencies (e.g. from septicaemias and agammaglobulinaemia) do occur.