Interpretation of white blood cell data can often be somewhat subjective and based on opinion rather than hard and fast objective evidence. Nevertheless there are certain clear rules that can be followed to help in the initial interpretation and subjective views can be superimposed in an attempt to refine the final interpretation. The following discussion considers some of the aspects important in reaching a final interpretation of a leucogram.
In the first instance it is much more meaningful to consider individually the absolute numbers of the different types of leucocytes rather than their relative percentages. It is only when the total individual leucocyte numbers are normal that the relative differential cell counts might be helpful. For example, the two cases below have the same differential percentages but case 1 shows a neutropaenia only whereas case 2 shows a lymphocytosis and eosinophilia only.
Case 1 | Case 2 | |
WBC | 6.7 x 109/l | 9.8 x 109/l |
PMN | 2.5 x 109/l (38%) | 3.7 x 109/l (38%) |
LC | 3.6 x 109/l (54%) | 5.3 x 109/l (54%) |
Eos | 0.5 x 109/l (8%) | 0.8 x 109/l (8%) |
The three most common leucogram abnormalities are neutrophilia, eosinophilia and monocytosis. Neutrophilia (PMN > 6.5 x 109/l) is the commonest of these. 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 (eg. azoturia, surgical trauma, immune-mediated disease, neoplasia) disease. Another potential cause of neutrophilia in the horse is stress associated with concurrent disease, overtraining or poor management. The typical effect of endogenous or exogenous corticosteroid on the leucogram is neutrophilia associated with a lymphopaenia and eosinopaenia. Therefore this type of leucocyte pattern in sick horses may be a non-specific result of the primary condition rather than a specific indicator of an inflammatory aetiology. Short term excitement perhaps associated with venepuncture of a nervous horse or transport immediately before sampling can also result in neutrophilia but lymphocytosis may be more likely in these cases.
Eosinophilia (Eos > 0.8 x 109/l) 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. Hence intra-arterial S.vulgaris larvae (although very rare) may well be associated with a peripheral eosinophilia, lungworm infection is usually associated with an eosinophilia in tracheal washes or bronchoalveolar lavage samples, encysted cyathostomes are associated with eosinophilic infiltrates detectable in caecal, colonic and sometimes rectal biopsies. Eosinophilia is unfortunately a fairly non-specific finding as the eosinophil has many general roles in host defence and eosinophilia is often seen as a non-specific component of a systemic inflammatory reaction (eg. viral infections). Eosinophils are attracted by mast cell degranulation and have therefore been associated with antigen-antibody interactions in tissues rich in mast cells such as skin, respiratory tract and intestine and peripheral eosinophilia is certainly seen fairly consistently in association with hypersensitivity reactions such as sweet itch.
Monocytosis (Monos > 0.5 x 109/l), similar to neutrophilia and eosinophilia, is a non-specific inflammatory indicator seen to rise in both acute and chronic inflammatory conditions and tissue damage.
Is it possible to differentiate bacterial and viral disease on the basis of haematology?
The most consistent haematologic finding associated with the early stages of viral infections (ie. the time when we are usually asked to examine the horse and take a diagnostic blood sample) is a neutrophilia (PMN > 6.5 x109/l). 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 then mild neutropaenia and possibly lymphocytosis and monocytosis would typify viral disease, whereas bacterial infections more typically remain neutrophilic with, possibly, a monocytosis, unless severe whereupon neutropaenia may occur:
| early | mid-late |
|
VIRUS (typical): | neutrophilia | - neutropaenia/lymphocytosis/monocytosis | - recovery |
BACTERIAL (typical): | neutrophilia | - neutrophilia/monocytosis | - recovery |
BACTERIAL (severe): | neutrophilia/neutropaenia | - neutrophilia/neutropaenia | - recovery (?) |
Is it possible to diagnose parasitism on the basis of haematology or serum proteins?
Nematode infections in the adult horse were once typified by intra-luminal adult worms and larval migration associated with Strongylus vulgaris. These were often associated with an eosinophilia detectable in blood samples in response to intra-arterial larvae and also, in some instances, a detectable increase in b1-globulins (especially IgG(T)). Parasitological surveys in more recent years have consistently confirmed the decline of S. vulgaris (almost to the point of extinction in many instances) in conjunction with a domination of parasitic burdens by members of the cyathostome genus (small strongyles) which consistently comprise almost 100% of nematode eggs detected in equine faecal samples in this country. Cyathostome infection results in encystment of larvae locally in the caecal and colonic wall but is not associated with larval parasitic migration. An eosinophilia is not associated with cyathostome infections and a raised b1-globulin fraction is a very inconsistent and non-specific finding.
Several research studies have failed to confirm any clinically useful relationship between serum protein electrophoresis and parasitism in horses. Normal concentrations of IgG(T) and b1-globulins are usually found in parasitised adult horses and ponies although changes may be more likely in young horses. In a recent investigation of horses with chronic diarrhoea, 55% of horses with parasitic diarrhoea did not have raised b1-globulins and 37% of horses with raised b1-globulins had no evidence of parasitism. Importantly, statistics would imply that serum protein electrophoresis would be even less predictive of parasitism in blood samples taken from non-diarrhoeic horses. ‘Cyathostomosis’, the acute diarrhoea and weight loss syndrome associated with en masse larval emergence, is consistently associated with a neutrophilia, hypoalbuminaemia and hyperfibrinogenaemia (all unfortunately non-specific findings), but other than this blood samples taken from parasitised horses show no consistent abnormalities in haematology or protein analyses.
Prime considerations in interpretation of the leucogram
These lists are not intended to be exhaustive and comprehensive but rather to serve as an aide memoire for the more common causes of abnormalities of the leucogram seen in the adult horse.
Neutrophilia (PMN > 6.5 x 109/l) |
|
|
- Inflammatory disease | - infectious | - viral (early phase response) |
|
| - acute/chronic bacterial |
|
| - parasitic |
| - non-infectious | - tissue damage (eg. azoturia, surgery etc..) |
|
| - neoplasia (eg. lymphosarcoma) |
|
| - immune-mediated diseases |
- Endogenous/exogenous corticosteroid (stress, equine Cushing’s disease) | ||
- Catecholamines (acute excitement/fear) | ||
Neutropaenia (PMN < 2.5 x 109/l)
- Excessive demand/sequestration (eg. septicaemia, bacterial peritonitis, pleuritis)
- Endotoxaemia
- Viral infection (mid-late phase response)
Lymphocytosis (LC > 5.0 x 109/l)
- Infectious diseases (primarily mid-late phase viral)
- Catecholamines (acute excitement/fear)
Lymphopaenia (LC < 1.5 x 109/l)
- Infectious diseases (eg. early EHV)
- Endogenous/exogenous corticosteroid (stress, equine Cushing’s disease)
Eosinophilia (Eos > 0.8 x 109/l)
- Hypersensitivity diseases (eg. sweet itch, urticaria)
- Inflammatory diseases (see neutrophilia)
- Parasitic larval migration (rare)
Monocytosis (Monos > 0.5 x 109/l)
- Inflammatory diseases (see neutrophilia)
© The Liphook Equine Hospital 2009
