Leucogram

Assessing Changes in the Leucogram EDTA
Interpretation of white blood cell data can be somewhat subjective rather than based on objective
evidence. When samples are submitted they are initially analysed with an automated machine and
then results are confirmed via microscopy by our haematologists.
It is more meaningful to consider the absolute numbers of the different types of leucocytes rather
than their relative percentages. For example, cases 1 and 2 below have the same differential
percentages but case 1 shows a neutropaenia whereas case 2 shows a lymphocytosis and eosinophilia.
These will have different interpretations. Also cases 3 and 4 have the same percentages but it is only
case 4 that shows a neutrophilia. It is also important to interpret the results in light of other
inflammatory markers such as Globulins, Serum Amyloid A and Serum Iron.

Case 1 Case 2 Case 3 Case 4

Total WBC count 5.6 x 109

/L 9.8 x 109

/L 7.2 x 109

/L 11.1 x 109
/L

Neutrophils 2.1 x 109

/L (38%) 3.7 x 109

/L (38%) 5.5 x 109

/L (77%) 8.5 x 109
/L (77%)

Lymphocytes 3.0 x 109

/L (54%) 5.3 x 109

/L (54%) 1.7 x 109

/L (23%) 2.6 x 109
/L (23%)

Eosinophils 0.4 x 109

/L (8%) 0.8 x 109

/L (8%) 0.0 x 109

/L (0%) 0.0 x 109
/L (0%)
The relative concentrations of cells within the circulating and peripheral pools will be influenced by
factors such as stress and excitement, therefore the conditions of sampling must be considered when
interpreting results. Samples should ideally be collected at least six hours after exercise or stressful
incidents and not immediately post feeding. If this is not possible, the results should be interpreted
cautiously.
The typical glucocorticoid-mediated response that accompanies the stress of exercise and competition
(as well as exogenous corticosteroid administration) results in neutrophilia, lymphopaenia and
eosinopaenia and an overall increase in leucocyte numbers. However, this effect may be balanced
during high-intensity exercise or short-term excitement (such as during venepuncture) by increased
blood volume and lymphocyte release secondary to splenic contraction.
NEUTROPHILIA

• Inflammatory Diseases
• Endogenous (stress, or exogenous glucocorticoids)
• Catecholamines (acute excitement/fear)
  Neutrophilia is usually the result of an acute or chronic inflammatory response. This may be caused
  by infectious (viral, bacterial or parasitic) or non-infectious disease (e.g. myopathies, surgical trauma,
  immune-mediated disease, neoplasia). Given that stress is a cause of neutrophilia, cross-checking
  against acute phase markers (Serum Amyloid A, serum iron and globulins) may help determine the
  “inflammatory versus stress” question.
  A left shift indicates the release of juvenile neutrophils (Band neutrophils or in extreme cases
  metamyelocytes). This indicates an acute infectious/inflammatory condition which can include
  cellulitis/lymphangitis, colitis, endotoxaemia peritonitis and pleuritis.

NEUTROPAENIA

• Excessive demand/sequestration (e.g. septicaemia, bacterial peritonitis, pleuritis, colitis)
• Endotoxaemia
• Viral infection (mid-late phase response)
• Myelophthisis/bone marrow suppression
  Neutropaenia is often seen in horses showing signs of relatively mild lethargy and suboptimal
  performance and is frequently attributed to viral challenge. Neutropaenia is also commonly seen in
  cases of liver disease in horses and GGT (at least) should be checked in such cases. In horses showing
  more marked signs of illness such as tachycardia and pyrexia then severe bacterial sepsis,
  endotoxaemia, loss of neutrophils into an effusion (e.g. peritonitis, pleuritis) or into inflamed bowel
  (e.g. colitis) are further possible common causes of neutropaenia.
  Neutropaenia is an occasional consequence of bone marrow suppression. As the leucocyte with the
  shortest lifespan, the neutrophil population may be the first to be noticeably reduced by bone marrow
  failure before reductions in platelets and red cells are observed.
  LYMPHOCYTOSIS
• Infectious diseases (primarily mid-late phase viral)
• Catecholamines (acute excitement/fear)
• Dramatic elevations (e.g. 20-100 x 109

/L) can be seen in rare cases of leukaemia and in these
cases the lymphocytes will be assessed for neoplastic changes (see extreme thickness of
‘buffy coat’ in image)
LYMPHOPAENIA

• Infectious diseases (e.g. early EHV or severe bacterial infections)
• stress, or exogenous glucocorticoids
• In foals a severe lymphopaenia can be indicative of genetic immunodeficiency syndromes.
  EOSINOPHILIA
• Hypersensitivity diseases (e.g. sweet itch, urticaria)
• Diseases of the intestine skin and lung
• Inflammatory diseases (see neutrophilia)
  Eosinophils have many general roles in host defence and eosinophilia is often seen as a non-specific
  component of a systemic inflammatory reaction. Eosinophils are attracted by mast cell degranulation
  and have therefore been associated with antigen-antibody interactions in tissues rich in mast cells
  such as the skin, the respiratory tract and the intestine.
  Peripheral eosinophilia is seen fairly commonly in association with hypersensitivity reactions such as
  sweet itch. Eosinophilia is very rarely found in association with intestinal parasitism in horses.
  Eosinophils undoubtedly play a role in host defence against parasitic infections but are found local to
  the parasite. Lungworm infection (also very rare) is usually associated with an eosinophilia in tracheal
  washes or bronchoalveolar lavage samples. Encysted cyathostomins are associated with eosinophilic
  infiltrates in caecal, colonic and sometimes rectal biopsies.

MONOCYTOSIS
Monocytosis is a non-specific inflammatory indicator seen to rise in both acute and chronic
inflammatory conditions and tissue damage. Granulomatous diseases and chronic bacterial infections
can lead to monocytosis.
BASOPHILIA
Basophilia is very uncommon in the horse and when it does occur is attributed to non-specific
hypersensitivity responses.
THROMBOCYTOSIS
Platelets are a useful inflammatory marker as they tend to increase in the presence of inflammation
due to cytokine stimulation of the bone marrow. They do not act as an acute phase response but
increase in most cases of chronic, persistent inflammation. High platelet counts will frequently be seen
in association with abscessation (e.g. Rhodococcus) and also with non-infective inflammatory
conditions such as neoplasia.
THROMBOCYTOPAENIA
Thrombocytopaenia is often seen as an artefact following collection in EDTA. If this is the case then
there will frequently be clumping of the platelets and this will be commented on. If the
thrombocytopaenia is suspected to be real then it can be rechecked on a sodium citrate tube to which
improves accuracy. Genuine thrombocytopaenia can be due to lack of production (bone marrow
disease), consumption (DIC) or destruction (autoimmune). Clinical bleeding disorders generally
happen when platelet counts are < 20 x 109
/L.
COMMON CAUSES OF INFLAMMATION

Infection

Bacteria
Viruses
Parasites
Protozoa

Tissue Damage

Surgery
Traumatic injury
External rhabdomyolysis/Atypical myopathy

Immune-mediated disease

Lymphangitis/vasculitis
Immune-mediated haemolysis/thrombocytopaenia
Pemphigus foliaceus
Endotoxaemia Secondary to colitis or similar
Neoplasia
Other

Inflammatory bowel disease
NSAID toxicosis
Sand enteropathy

IS IT A BACTERIAL OR VIRAL INFECTION?
The most consistent haematologic finding associated with the early stages of viral infections (i.e. when
clinical signs are most marked and blood samples are most likely to be taken) is a neutrophilia. This
has been demonstrated in association with many types of viral infection in adult horses and is
indistinguishable from bacterial infections on the basis of haematology. However, later in the course
of disease mild neutropaenia and possibly lymphocytosis and monocytosis would typify viral disease,
whereas bacterial infections more typically remain neutrophilic and may develop a monocytosis. If,
however bacterial infection is severe then neutropaenia may occur. Chronic bacterial conditions are
frequently associated with a thrombocytosis by contrast to viral conditions. In addition, acute phase
protein responses tend
to be milder with viral diseases (e.g. SAA <50 mg/L) than with bacterial (e.g. >100 mg/L).