Thymus
The thymus is a bilobed primary lymphoid organ located in the cranial mediastinum (thoracic portion) with extensions along the ventral trachea in the neck (cervical portion). It is essential for the development and education of T-lymphocytes: immature thymocytes from the bone marrow undergo positive and negative selection in the thymic cortex and medulla to produce self-tolerant, MHC-restricted T-cells. The thymus is largest and most active in the fetus and neonate, reaching maximum size in the first months of life before undergoing gradual involution. By adulthood it is largely replaced by adipose tissue, though small islands of functional thymic tissue persist.
Thymic disease is uncommon in adult horses. Relevant mainly in foals for immune development. Persistent thymic tissue can occasionally be relevant in imaging or pathology of the cranial mediastinum.
## Overview Thymoma is a neoplasm of thymic epithelial cells and the most common cranial mediastinal mass in adult horses. It can compress the cranial vena cava, causing ventral edema of the head and
## Overview Thymic lymphosarcoma is lymphoid neoplasia involving the thymus, typically in young horses. It produces a rapidly enlarging cranial mediastinal mass, pleural effusion, respiratory distres
## Overview Severe combined immunodeficiency (SCID) is a lethal autosomal recessive disorder of Arabian foals (and breeds carrying Arabian bloodlines). A mutation in the DNA-dependent protein kinase
Not a structure of practical concern for owners under normal circumstances. Immune function in foals is supported by colostrum and appropriate management.
## Overview Thymoma is a neoplasm of thymic epithelial cells and the most common cranial mediastinal mass in adult horses. It can compress the cranial vena cava, causing ventral edema of the head and forelimbs, jugular distension, pleural effusion, and dyspnea. Paraneoplastic syndromes may occur. Weight loss is common. ## Causes Neoplastic. ## Diagnosis Thoracic ultrasound or radiographs, cytology or biopsy. ## Treatment Surgical removal when feasible. Palliative corticosteroids. Prognosis depends on invasiveness. ## Recovery and Prognosis Variable. Non-invasive thymomas have a better outlook than invasive ones. ## Prevention None. Signs include: pleural effusion; ventral edema of head and forelimbs; dyspnea; jugular distension; weight loss
## Overview Thymic lymphosarcoma is lymphoid neoplasia involving the thymus, typically in young horses. It produces a rapidly enlarging cranial mediastinal mass, pleural effusion, respiratory distress, and sometimes cranial vena cava syndrome (swelling of the head and neck). Cytology shows a monomorphic lymphoid population, distinguishing it from thymoma. Corticosteroids may give temporary improvement. Prognosis is poor. ## Causes Lymphoid neoplasia. ## Diagnosis Ultrasound or radiographs of the chest, thoracocentesis, and cytology. ## Treatment Palliative corticosteroids. Chemotherapy protocols exist but survival remains short. ## Recovery and Prognosis Poor. Survival is typically weeks to a few months. ## Prevention None. Signs include: rapidly enlarging cranial mediastinal mass; pleural effusion; respiratory distress; cranial vena cava syndrome; partial response to corticosteroids
## Overview Severe combined immunodeficiency (SCID) is a lethal autosomal recessive disorder of Arabian foals (and breeds carrying Arabian bloodlines). A mutation in the DNA-dependent protein kinase catalytic subunit gene prevents development of functional T and B lymphocytes. Foals are normal at birth but develop fatal infections once maternal antibodies wane, usually between 2 and 5 months of age. Death typically occurs by 5 months. ## Causes Inherited DNA-PKcs mutation. ## Diagnosis Lymphopenia, absence of IgM, and DNA testing. Affected foals and carriers can be identified by genetic test. ## Treatment None effective. Supportive care only. Euthanasia is humane once infections begin. ## Recovery and Prognosis Fatal. ## Prevention Genetic testing of breeding Arabians to avoid carrier-to-carrier matings. Signs include: recurrent infections starting at 2 to 5 months; pneumonia; diarrhea; failure to thrive; death by 5 months; no response to antimicrobials
Thymic disease is uncommon in adult horses. Relevant mainly in foals for immune development. Persistent thymic tissue can occasionally be relevant in imaging or pathology of the cranial mediastinum.
Not a structure of practical concern for owners under normal circumstances. Immune function in foals is supported by colostrum and appropriate management.
See the Thymus on our interactive 3D horse model. Open 3D Model