EQUINE CLINICAL PATHOLOGY - Rossdale & Partners

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T h e B e a u f o r t c o t t a g e l a b o r a t o r i e s Chronic haemorrhage, e.g. following castration, may cause an anaemia and selfperpetuating thrombocytopenia. Platelet transfusion may result in haemostasis without further surgical interference. Guttural pouch mycosis may cause internal carotid artery ulceration, resulting in profound acute and sometimes fatal epistaxis and blood-loss anaemia. Gastric ulceration, not uncommonly seen in performance horses, can cause anaemia from chronic haemorrhage. Examination of faecal samples for the presence of occult blood can be performed but results are frequently positive, most commonly because of activity of intestinal parasites, even in well-managed horses, and are therefore not reliably diagnostic of gastric ulceration. Unfortunately, serum pepsinogen assays are not reliably diagnostic of gastric ulceration in horses. Where suspected, the diagnosis must be made and the significance assessed by gastroscopic examinations. Intravascular haemolysis, i.e. erythrocyte destruction resulting in haemolytic anaemia, has a variety of different causes, which require specific diagnostic testing. Equine Infectious Anaemia (Coggins’ agar gel immunodiffusion test), autoimmune haemolytic anaemia (Coombs’ antiglobulin test), Piroplasmosis (Babesiosis) (complement fixation test), disseminated intravascular coagulopathy (DIC) (prolonged prothrombin and activated partial thromboplastin times) and a variety of plant and environmental toxins are some causes of intravascular haemolysis. Equine anaemia is most commonly macrocytic (raised McV), reflecting splenic replacement with juvenile erythrocytes, as seen with blood loss, infections and parasitism. Reticulocytes are not commonly seen in equine blood samples and so their presence or absence is not a reliable means of determining regeneration or non-regeneration, as in other species. Normocytic (McV within normal range) anaemia is sometimes seen in horses who are being challenged by respiratory viruses but who show no clinical signs. Microcytic anaemia (low McV) is uncommon but is sometimes seen in immature individuals and has been reported in horses with iron/ folate deficiency. Leucocytes (WBC & Differential leucocyte count) Leucocytosis (raised total leucocyte count) and neutrophilia (raised segmented neutrophil count) most commonly occurs in performance horses in association with septic and non-septic inflammatory conditions. Septic inflammation is most commonly associated with bacterial infection. In performance horses this may occur following an injury, e.g. penetrating wounds followed by cellulitis, septic arthritis or tenosynovitis, upper respiratory infections and less commonly 14
G u i d e t o e q u i n e c l i n i c a l p a t h o l o g y in systemic bacterial infections. Non-septic inflammation can occur following nonpenetrating injury, e.g. bruising of soft tissues, tendons, ligaments or periosteum and traumatic arthritis or tenosynovitis, sometimes associated with degenerative joint disease. Marked leucocytosis is seen in foals responding to bacterial infections, notably Rhodococcus equi (‘summer pneumonia’) and Streptococcus equi (‘strangles’) infections. Neutrophilic ‘shifts to the left’ (appearance of juvenile neutrophils or ‘band’ neutrophils), while diagnostically helpful in foal-hood infections, are seldom seen in adult horses unless severely and acutely infected with conditions such as acute cellulitis or lymphangitis. Some cases of acute salmonellosis, endotoxaemia, peracute enterocolitis, peritonitis and pleuritis have neutrophilic shifts to the left, more commonly associated with leucopenia and neutropenia. Leucopenia (low total leucocyte count) and neutropenia (low segmented neutrophil count) is most commonly seen in adult performance horses during the acute phase of a viral challenge, when there may or may not be clinical signs. These may include lethargy, pyrexia, nasal discharge, coughing and oedematous legs. Leucocytic changes seen with infection very much depend upon sampling time in relation to the stage of the disease process (see Fig.1). If the early acute infectious phase has passed then the leucopenic phase will be missed and haematologic examinations will reflect repair and sometimes-secondary bacterial involvement with leucocytosis and neutrophilia (see page 14). Therefore, blood samples should be taken from symptomless stablemates where viral infections are suspected, in order to help with diagnosis, epidemiology and assessment of recovery. Unfortunately, unless Equine Influenza, Fig.1: equine blood leucocyte response to viral challenge 14 Circulating blood leucocytes x 10^9/l 12 10 8 6 4 2 0 1 2 3 4 5 6 7 8 Days – Viral challenge starts on day1. Basal leucocyte count is 8 x 10 9 /l 15
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EQUINE CLINICAL PATHOLOGY - Rossdale & Partners

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