ABSTRACT: The aim of this paper is to explain the common morphological changes seen in canine and feline leukocytes. It covers topics such as toxic change in neutrophils, as well as some of the inclusion bodies sometimes encountered on routine haematology. It also gives a brief overview of some of the common conditions that can cause these changes, knowledge of which will aid rapid diagnoses.

Canine and feline leukocytes are classified into two groups: the granulocytes (or polymorphonuclear leukocytes) and the agranulocytes (mononuclear leukocytes).

The granulocytes are:




These leukocytes contain small red- to purple-staining granules, called lysosomes, within their cytoplasm [See ‘Common morphological changes seen in canine and feline haematology – red blood cells’, VNJ Vol 26: pp56-58 for further information on identifying leukocytes]. Lysosomes contain antibacterial agents and other infection controlling compounds.

The agranulocytes:

   lymphocytes monocytes.

These cells do contain granules in their cytoplasm, but only in very small numbers, and are not normally seen on routine haematological examinations. 

Throughout this article we will concentrate mainly on neutrophils and lymphocytes, as these are the most common cells examined during routine haematology.


The most common changes in routine haematology will be seen within the neutrophil population (Figure 1).

Figure 1: Normal neutrophils

A regularly seen change in neutrophil morphology is the increase in the number of immature neutrophils, called ‘bands’ (Figure 2). These cells are the same size as normal neutrophils, but have a purple-staining, hyposegmented or horseshoe nucleus. Bands are released early from the bone marrow in times of chronic inflammation, and will be reported as a ‘left shift’ – an increase in the number of juvenile neutrophils seen on the film – on a haematology report.

Figure 2: 'Band' neutrophils.

When an inflammatory reaction is taking place, neutrophils may also undergo a process called ‘toxic’ change. There are varying degrees of toxic change within neutrophils and they will normally be reported as a 1+, 2+ … up to 5+ toxic changes on a haemotology report. This article will, however concentrate on the changes in morphology that may be observed.

Foamy Basophilic Cytoplasm

Normal neutrophils have a light pink, evenly staining cytoplasm but during times of severe bacterial infection, the cytoplasm stains varying shades of blue (basophilic) and will appear foamy with vacuoles (Figure 3).

Figure 3: Foamy basophilic 'band' and mature neutrophils

 Dohle Bodies

Dohle bodies are small, irregular, bluish aggregates found in the cytoplasm of toxic neutrophils (Figure 4). They are small segments of retained endoplasmic reticulum; it is important not to confuse them with bacteria (Figure 5) or other cytoplasmic inclusions.

Figure 4: Dohle bodies

Figure 5: Bacteria

Toxic granulation

This is a rarely seen toxic change in cats and dogs, in which there is the presence of magenta-staining granules within the basophilic cytoplasm of neutrophils. These granules are normally seen in cases of severe toxaemia, but can be a normal finding in some Birman cats!


This is often referred to as a ‘right’ shift – an increase in the number of mature neutrophils on the blood film. These are neutrophils which have five or more segments to their nucleus, and may often occur as part of the normal cell ageing process, or may be seen within cells that have been in circulation for longer than normal (Figure 6).

Figure 6: Normal and hypersegmented neutrophils


The major morphological changes in agranulocytes are seen within the lymphocyte population. Most lympho-proliferative disorders cause an increase in the number of lymphocytes within the blood stream.

Reactive lymphocytes

The most common variation seen are reactive lymphocytes, sometimes known as immunocytes. These reactive lymphocytes have a round nucleus with a dark blue-staining cytoplasm (Figure 7). They are sometimes larger than normal lymphocytes and contain increased amounts of cytoplasm.

Figures 7a and 7b: A normal feline lymphocyte and a reactive feline lymphocyte

Low numbers of reactive lymphocytes can be seen in a normal animal’s blood film, but increased numbers can be seen in animals that are antigenically stimulated.

Atypical lymphocytes

The term ‘atypical lymphocytes present’ is sometimes seen on haematology reports and is used to cover a broad spectrum of scenarios.

Some haematologists use this term to describe any lymphocyte that is not normal, whilst others use it to describe neoplastic lymphocytes, such as those seen in lymphoma or some leukaemias. Morphologically atypical (or neoplastic) lymphocytes have a very similar appearance to reactive lymphocytes – the main differences are seen within the nucleus.

Atypical lymphocytes can have heavily cleaved or indented nuclei and sometimes have one or two nucleoli present (Figure 8). These
cells are usually large, with a dark blue-staining cytoplasm, and great care should be taken when differentiating between a neoplastic lymphocyte and a reactive lymphocyte, especially in cats.

Figure 8: Atypical lymphocytes in a canine blood film. Note the nucleoli

A small percentage of lymphocytes on the blood film may include small, red- or purple-staining granules within the cytoplasm. The cells are generally slightly larger than normal lymphocytes, and are thought to be natural killer or T cells.

These can be considered normal in small numbers and a response to a non neoplastic inflammatory disorder – a blood-borne parasitic infection, for example. However, large numbers of these cells can be a sign of leukaemia, whilst large red-purple staining granules in feline samples can be indicative of large granular lymphoma.

There are many different types of lympho-proliferative disease and, consequendy, many different morphological changes may be seen. To try and cover them all in one article would be impossible but the basic details given above, should aid you and the veterinary surgeon in finding the correct diagnoses for your patients.

Miscellaneous findings

The final section will introduce you to another cell sometimes seen on haematology slides – mast cells.

Mast cells are not normally found in the blood stream, but will appear in low numbers in cases of chronic inflammation or in large numbers as part of a neoplastic process. They are recognised by their abundant deep purple-staining granules contained within the cytoplasm. It is often impossible to visualise the nucleus, as it is completely covered by the granules (Figure 9). Mast cells should not be mistaken for basophils, although they do share similar characteristics.

Figure 9: Mast cells seen in a feline blood film

You should now have a basic understanding of white 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.

The final instalment of this series will explore individual case studies using all of the information in the haematology articles.

The third article in the series will appear in the April issue of VNJ. 


Matthew Garland certNatsd (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.2045-0648.2010.00024.x or Veterinary Nursing Journal Vol 26 pp 85-87

Further reading

BSAVA Manual of Clinical Pathology ISecond Edition] (2005) Eds Elizabeth Villers & Laura Blackwood. 

Veterinary Nursing Journal • VOL 26 • March 2011 •