ABSTRACT: The aim of this paper is to explain the common morphological changes seen in canine and feline erythrocytes. It covers the size and shape of red blood cells, but also changes such as agglutination, rouleaux and some of the inclusion bodies sometimes encountered on routine haematology. As an aid to rapid diagnosis, it also gives a brief overview of some of the common conditions that can cause these changes.

The aim of this and subsequent articles is to show some of the common morphological changes in red and white blood cells seen on routine blood smears. Identifying these changes can be instrumental in diagnosing certain conditions and can help the veterinary surgeon to choose the correct course of treatment.

Erythrocyte morphology

Erythrocytes from cats and dogs are anucleated and stain pink. When looking at red cell morphology, six areas should be evaluated:







With these six factors in mind we will now look at some of the common morphological changes seen in canine and feline haematology. Some changes are considered normal in some species, while others are considered abnormal.


Normal canine erythrocytes have a wide rim of haemoglobin with an area of central pallor; this is not seen in feline red blood cells.


Polychromatic cells (also known as reticulocytes) are larger than normal red blood cells, and normally stain blue-grey. They are juvenile cells and represent the level of regeneration in anaemia cases (reported as polychromasia on haematology profiles). Normally only very small numbers of polychromatophils are seen in cats and dogs (less than 1.5 per cent of all red blood cells seen in a film).


The presence of red blood cells with decreased levels of haemoglobin and a larger area of central pallor is called hypochromasia (Figure 1). The large central pallor is surrounded by a thin rim of pale staining haemoglobin, and this is commonly seen in cases of iron deficiency (resulting from chronic haemorrhage, for example).

Figure 1: Canine erythrocytes showing hypochromasia


A variation in size of red blood cells is referred to as anisocytosis (Figure 2). It is increased when different populations of cells are present, for example in a regenerative anaemia when large populations of polychromatophils are present.

Figure 2: Polychromasia and anisocytosis on a canine blood film


Poikilocytosis is a non-specific term used to describe the presence of abnormally shaped red blood cells, of which there are many. Some can relate to a metabolic or organ disorder. Listed below are some of the commonly found changes in erythrocyte shape.


These are red blood cells with irregular (rounded) shaped projections from the surface of the cell (Figure 3). These are commonly seen in liver disease, but can also be caused by tumours, such as haemangiosarcomas.

Figure 3: Canine acanthocyte

Echinocytes (crenated red blood cells) Echinocytes have numerous short, evenly spaced; normally equal-sized surface projections (Figure 4) and should not be confused with acanthocytes. These are normally an artifactual change caused by sample ageing, but can also be seen in cases of dehydration and renal disease.

Figure 4: Canine echinocytes

Codocytes (or Target cells)

These cells are thin flexible red blood cells which have an increased surface area, and look like ‘targets’ (Figure 5). They have a narrow rim of haemoglobin, with a wider area of central pallor and a small central area of haemoglobin. They are a non-specific finding but can be associated with liver disease and iron deficiency.

Figure 5: Canine codocytes and schistocytes


These are irregularly shaped red blood cells fragments (Figure 5). These can be caused by several conditions, such as liver disease and tumours, such as haemangiosarcomas.


Spherocytes are small, round, deep- staining red blood cells, which lack the central pallor (Figure 6). They are caused by the cell membrane being partially phagocytosed (eaten) by macrophages, which leaves a smaller compact cell. They are an indicative feature of immune- mediated haemolytic anaemia (IMHA), but may also be seen in low numbers in normal blood films. They can be very difficult to distinguish in cats and other species, as their red blood cells lack the central pallor.

Figure 6: Canine spherocytes


There are many inclusions that can be seen in red blood cells – they are remnants of nuclear material to pathogenic organism. Here we will concentrate on the two most common seen on routine blood films, Howell-Jolly bodies and Heinz bodies.

Howell-Jolly bodies

Howell-Jolly bodies are small, single, round, deep blue-staining inclusions seen within the cellular membrane (Figure 7). They are nuclear remnants and are often seen in regenerative anaemia, in both dogs and cats.

Figure 7: Heinz Body and Howell-Jolly body

Heinz bodies

Heinz bodies are round inclusions lying along the inner surface of the red blood cell membrane (Figure 7). These are quite normal findings in small numbers in cats, but can be associated with anaemias or exposure to oxidant chemicals or drugs such as paracetamol and onions!


This is the name given to the stacking or branching of red blood cells (Figure 8) and should not be mistaken for agglutination. It is a common finding on normal feline blood films, and is also a very prominent finding in equine haematology. Increased rouleaux formation in cats and dogs is normally indicative of inflammatory disease and hyperglobinaemia.

Figure 8: Rouleaux


This is the irregular clumping of red blood cells and is caused by large amounts of antibodies on the surface of the cells causing them to stick together (Figure 9). This is commonly seen in cats and dogs with IMHA.

Figure 9: Agglutination

If agglutination is suspected, then this can be checked microscopically by mixing one drop of blood with one drop of saline on a glass slide, and checking for clumps of red blood cells using the x40 objective lens. If this test is positive then this is called auto-agglutination.

Miscellaneous findings

A common finding, often seen in cases of strongly regenerative anaemia, is nucleated red blood cells (Figure 10). These are juvenile red blood cells which have been released from the bone marrow early to help satisfy the body’s demand.

Figure 10: Nucleated red blood cell


You should now have a basic understanding of red cell morphological changes seen on routine blood film examinations, and have an insight into some of the conditions that can cause them. Identifying these changes can facilitate the rapid diagnosis of conditions or help rule out others. We shall move on to white blood cell morphological changes in subsequent issues. 


Matthew Garland CertNatSci(Open) VN

After qualifying as a VN in 2004, Matthew worked in small animal practice before moving to Torrance Diamond Diagnostic services in 2006. Now, as laboratory manager of TDDS-Ringwood, he has developed a strong interest in haematology and biochemistry.

To cite this article use either

DOI: 10.1111/j.2045-0648.2010.00016.x or Veterinary Nursing Journal Vol 26 pp 56-58

Further reading

BSAVA Manual of Clinical Pathology ISecond Edition) Eds Elizabeth Villers and Laura Blackwood

• VOL 26 • February 2011 • Veterinary Nursing Journal