Ask your laboratory expert: haematology results in equine practice

Have you ever wanted to know what the most common bloodwork anomalies in horses with Fasciola hepatica infestation are? Have you wondered how you can check for regenerative anaemia in horses? Or perhaps bloodwork has come back with an interesting anomaly that you just can’t place. Queries like these crop up all the time in veterinary practice and there are always moments where we wish we could “ask the expert”. So, in this series, experts from NationWide Laboratories will answer these niggling questions, from pathology and haematology to biochemistry and serology, providing insights into the world of veterinary diagnostics.

In part one of this ask the laboratory expert series, Stacey Newton answers the haematology-based questions posed by equine vets during and after a recent webinar on the interpretation and significance of haematology and biochemistry results in equines (Newton, 2022).

Can equine infectious anaemia cause immune-mediated haemolytic anaemia?

In short, yes – any infectious disease process can result in a secondary immune-mediated haemolytic anaemia (IMHA).

Any infectious disease process can result in a secondary immune-mediated haemolytic anaemia (IMHA)

The clinical signs of equine infectious anaemia (EIA) are very variable and can be categorised as acute, chronic or subclinical in presentation. The acute form may present with pyrexia, depression, increased heart rate (HR) and respiratory rate (RR), haemorrhaging, bloody diarrhoea, ataxia, weight loss and jaundice. On the other hand, clinical signs of chronic EIA include recurring pyrexia, depression, anaemia, weight loss and weakness with normal periods. If the horse presents with severe unexplained anaemia, you should isolate it and test for EIA.

Which parameters of haematology could be helpful in the prognosis of colic?

There are several parameters that may be important in indicating prognosis in colic. However, the underlying cause of the colic should be considered as some causes will obviously have a much poorer prognosis than others; also, horses with surgical compared to medical colic will likely have a poorer prognosis.

A recent publication in Veterinary Clinical Pathology looked at some parameters and survival times for colic in a group of 17 horses (Nikvand et al., 2019). Though testing a relatively small group of animals, the study concluded that a combination of prolonged prothrombin time (PT), increased D-dimer and low fibrinogen could indicate that an individual is less likely to survive. Also, very high levels of renal and liver parameters would also have a poorer prognosis. This does appear to suggest these horses likely had disseminated intravascular coagulation (DIC).

Lactate is also a sign of inadequate tissue oxygenation and high levels may indicate a poor prognosis; however, this is a point-of-care or in-house test as it is too labile to send to an external lab. I would also say that cases with persistent high serum amyloid A (SAA), globulins (acute phase proteins) and low albumin may hold a poorer prognosis.

Why is polychromasia not seen in horses, and why, then, can we use mean corpuscular volume and mean corpuscular haemoglobin concentration?

Polychromasia is not seen in horses because their red blood cells (RBCs) are not released from the bone marrow until they are more mature than in other species. At this stage they will also have lost their RNA which gives the blue tinge to polychromatic cells with normal haematoxylin and eosin (H&E) stains. The RBCs are, however, larger in size and thus the mean corpuscular volume (MCV) is greater. This can be of use in some cases to suggest regeneration, depending on the time of sampling in relation to the incidence of haemorrhage or haemolysis. If increased, which in normal circumstances is not physically possible, mean corpuscular haemoglobin concentration (MCHC) suggests intravascular haemolysis, indicating that there is free haemoglobin as well as that measured in the RBCs.

Polychromasia is not seen in horses because their red blood cells (RBCs) are not released from the bone marrow until they are more mature than in other species

We know that acute inflammatory markers are useful to evaluate the kinetics of inflammation, but are there any other reasons to use them?

Acute phase proteins include globulins, fibrinogen, SAA, iron and albumin. As well as D-dimers, platelets, PT and activated partial thromboplastin time (APTT), fibrinogen can also be used as a marker of DIC in horses, as it is low in many cases of DIC. Iron (Fe) is reduced with inflammation but can also be used as a marker of Fe deficiency anaemia alongside a reduced MCV (microcytic). It may also decrease with haemolysis and chronic anaemia, so is not specific on its own. Albumin is a negative acute phase protein and when significantly decreased, suggests other causes such as protein loss, eg protein losing enteropathy, protein losing nephropathy and, though rare, hepatic insufficiency.

Is it worth taking a blood sample when a horse is dull and you are unsure as to what it is, even if you cannot put it with a clinical picture?

I think in this case, blood tests can be helpful, and they may be helpful if there are obvious changes present. It is important to take a full history, signalment and duration of clinical signs (whether there is pyrexia or any colic signs, for example). Bear in mind that if several tests are done, you are more likely to have one or more come back abnormal, and it may not be significant depending on the test and value. Remember reference ranges only cover about 95 percent of the “normal” population.

Please can you explain the differences in reference ranges of haematology parameters between labs?

Different laboratories usually establish reference ranges from the population of patients that they receive samples from, which will vary between labs, regions and countries.

Can a high haematocrit with normal total protein be associated with cardiac and pulmonary diseases?

An increased packed cell volume (PCV)/haematocrit (HCT) with normal total protein (TP) may be seen with splenic contraction. This is a relative increase in relation to water, but TP does not increase. An absolute increase in RBC mass, which is usually stimulated by erythropoietin, can be secondary or primary. Primary erythrocytosis (or polycythaemia vera) is uncommon and is a diagnosis of exclusion. Secondary causes, however, may include those that affect the oxygenation of RBCs such as respiratory or cardiac disease.

How can we know the difference between bacterial neutrophilia and neutrophilia due to bacterial complications of a viral infection?

In most cases, this cannot be differentiated on haemograms. Therefore, correlation with other tests for underlying viral diseases is advised if appropriate, as determined by the clinical signs. Primary bacterial causes of respiratory disease, for instance, are uncommon compared to viral causes. So, if there is a primary viral cause with a secondary bacterial component, one may find that there is lymphocytosis and neutrophilia. If the bacterial infection becomes severe with a coexisting viral disease also present, we may then see a marked neutropenia with lymphocytosis with or without monocytosis. It may be more likely that thrombocytosis is seen with more severe bacterial infections.

In your experience, how high could stress-related leucocytosis/neutrophilia be?

I would say that most cases do not show a neutrophil count of greater than 15 x 10⁹/l, and it is usually less than this. No left shift is seen.

Is it possible to diagnose regenerative anaemia without a bone marrow biopsy?

Yes, though with repeated or serial haemograms if there is no clinical evidence of a haemorrhage (which is usually regenerative unless chronic). It can take three to five days for the bone marrow to respond to a single episode of haemorrhage or an uncomplicated regenerative haemolytic episode. The response to anaemia is more sluggish in horses in comparison to other species: it can take one to two months following severe haemolysis or two to three months following severe haemorrhage for a horse’s HCT/PCV to return to normal.

The response to anaemia is more sluggish in horses in comparison to other species: it can take one to two months following severe haemolysis or two to three months following severe haemorrhage for a horse’s HCT/PCV to return to normal

How do we check for regenerative anaemia in a horse?

First determine if there is an anaemia with PCV/HCT, RBCs and haemoglobin, then look at RBC parameters and morphology. Increased MCV may indicate regeneration but if absent, does not exclude regeneration (depending on the time frame of sampling and when the incident occurred). An increased MCHC may indicate intravascular haemolysis.

You may also see Howell–Jolly bodies, anisocytosis and occasionally reticulocytes although these are not commonly seen. Spherocytes may be seen but they are difficult to identify due to normal equine erythrocytes having no obvious central pallor. Other significant findings may include:

• Heinz bodies – might indicate oxidative damage and haemolysis, usually regenerative
• Schistocytes – can suggest underlying cause of haemolysis
• Parasites in or on RBCs and/or white blood cells (WBCs) – causes of haemolysis

You might also take sequential samples to see if the MCV or RBC mass increases, while bone marrow aspirate can be taken for cytology and biopsy for histology.