Lymphopenia

Introduction

Lymphopenia is a reduction in the numbers of lymphocytes in the blood. This phenomenon is most commonly associated with stress (as part of the stress leucogram) or with viral infection. Causes include:

Stress leucogram, characterised by neutrophilia, monocytosis, eosinopenia and lymphopenia. A similar pattern may occur with hyperadrenocorticism (Cushing’s disease) or exogenous corticosteroid therapy as these hormones selectively suppress lymphocytic activity and the inflammatory response in general.
Viral infection with canine distemper, Infectious canine hepatitis, feline infectious enteritis or bovine viral diarrhoea (BVD)
Bone marrow suppression caused by chemotherapy or radiotherapy. Since neutrophils have a much shorter circulating half life, they are the first cell line to fall with any insult to the bone marrow but lymphopenia may occur as part of a more severe pancytopenia.
Inherited or acquired immunodeficiency syndromes, including SCID (Severe Combined Immunodeficiency Syndrome) in Arab foals and feline immunodeficiency virus infection.
Leakage of lymphatic fluid may result in loss of lymphocytes, as occurs with lymphangiectasia or chylothorax.