Haematological tests in sheep health management

Abstract

The evaluation of sheep complete blood cell count (CBC) can be of significant aid to clinical examination. Accurate assessment of haematologic data depends on the proper collection, preparation and transportation of blood samples. Blood should be drawn from animals at rest, calm and well restrained at the time of sampling, in order to avoid artifactual changes, such as a stress leucogram caused by endogenous steroid or epinephrine release or haemolysis from traumatic collection. Proper venipuncture is important to avoid contamination by tissue thromboplastin, which encourages clot formation and invalidates haemostatic function tests. Tubes must be filled to capacity to ensure the proper blood–anticoagulant ratio. When haemostasis tests are indicated, blood should be carefully transferred to vials containing anticoagulant, in order to prevent haemolysis, which precludes the use of evacuated tubes. Blood samples for haematologic studies are best processed as soon as possible after collection, but if a delay is expected they should be refrigerated at 4 °C and for a maximum period of 24 h (with the exception of platelet counts which should be performed within 4–6 h after sample collection), while air-dried blood smears must be prepared immediately after sampling to be stained with Wright or any modified Romanovsky stain. Complete blood counts can be performed either manually or with the aid of sophisticated instrumentation. Automated haematology instruments provide rapid cell counts of blood, haemoglobin calculation, potentially useful erythrocyte indices, but they cannot detect morphologic abnormalities and haemoparasites. Red blood cell counting and morphological changes are useful when evaluating anaemia. The differential white blood cell count is more important than the total white blood cell count, because increases and decreases in individual cell types may occur simultaneously, leaving total count unchanged. Overall, CBC can provide valuable information to the sheep clinician. Moreover, evaluation of a well-prepared blood film is an essential adjunct to interpretation of all cell counts, allowing detection of significant morphologic abnormalities or presence of haemoparasites.