By Dr Lizzie Youens BSc(Hons) BVSc MRCVS
A complete blood count (CBC) is an essential diagnostic tool in clinical practice. Hematological values are used in multiple species, including pet rabbits, to provide valuable insight into cell numbers, distribution and morphology to interpret alongside other key data from the clinical history, physical exam and other diagnostic tests such as biochemistry.
When interpreting a CBC, it is imperative that the results are interpreted with expertise and can be relied upon to avoid misinterpretation or misdiagnosis. Neonatal animals have a maturing physiology, which can alter both hematological and biochemical values. Age-specific reference intervals are therefore important for accurate evaluation when it comes to assessing the health status of pet rabbits.
Much can be gained from a thorough history and full clinical exam of any sick patient. However, further diagnostic tools are often needed to provide objective data alongside subjective assessments. A CBC, along with a serum biochemistry, are often the first tools to be used as part of an essential minimum database.
A complete blood count looks at the cell count and cell distribution of red and white blood cells and can therefore provide information into potential pathologies involving multiple pathways, including inflammation, infection, and immune-mediated disorders. However, a neonatal rabbit may well have differing blood values to an adult, and clinicians will need to know which variations are normal, and which are potentially due to illness or disease.
Handling and sampling can induce great stress in pet rabbits, and therefore blood sampling should be performed calmly, swiftly, and gently. A normal rabbit usually has a blood volume of around 60 ml/kg, and up to 10% can be collected if required.
The requirement for a CBC is usually a small amount of venous blood. Obtaining a useful sample can be challenging in rabbits. Recommended sites include the lateral saphenous, cephalic, or marginal ear veins. As rabbit blood coagulates quickly, care should be taken to avoid clotted samples. Some clinicians prefer to use preheparinized butterfly catheters or needles. It is important to flush the line or needle with air following dilute heparin flush to avoid sample artifact.
Hematological values are usually interpreted by use of reference levels to see if an individual patient’s results are within the normal range expected of a healthy animal. These reference ranges are produced by veterinary pathologists and diagnostic laboratories using peer-reviewed data from a sample population.
It is important to know that the data used to produce these reference levels often comes from a closed colony of adult rabbits: usually of the same sex, breed, and age-range. Anything which alters physiology, such as being a young, growing animal, may well alter the standard values we accept as for a ‘normal rabbit’. CBC results, therefore, need to be interpreted in context, with an understanding of how basic differences such as growth may change the range of normal hematological figures.
A study using growth promoters in rabbits (Attia et al., 2013) found that use of growth promoters affected both red and white blood cell counts. Although this may not be applicable for veterinarians assessing a sick rabbit, it indicates that periods of growth can cause hematological variance.
Jeklova et al. (2009) assessed hematological data in New Zealand White rabbits from one day old to 20 weeks of age. They found considerable variation from adult values in total counts of both red and white blood cells, and changes to the ratios of white blood cells, including raised eosinophils and basophils. Age-related changes had disappeared by 6 weeks of age. This corroborates early studies such as Laird et al. (1970) who found that both RBC and WBC counts are low in neonatal rabbits and slowly rise to adult levels. Their estimation of hematological adult-hood is slightly later, at around 90 days.
Melillo (2007) notes that rabbits under 12 weeks of age have lowered values for both red and white blood cells. It is also noted that there is significant diurnal variation throughout the day, with leukocyte numbers being particularly affected, dipping to their lowest level around late afternoon to evening. This is corroborated by Fox & Laird (1970). Stress can also have a major impact on some hematological parameters, but usually only with chronic stress, such as with transportation or chronic pain, rather than in handling for a sample.
Adult ranges based on Leineweber et al. (2018) and Moore et al. (2010).
Performing a CBC on a patient reveals valuable information about their health status. It is important that this information is assessed accurately for the individual patient.
There are some key points to remember when assessing the clinical significance of blood results from a neonatal rabbit (under 3 months).
Does this affect our interpretation? Certainly! Take these examples…
A low total RBC count in an adult rabbit may indicate acute infection or anemia, such as through blood loss or chronic disease like renal failure. However, a lower RBC count may in fact be perfectly normal in young pet rabbits.
Infectious disease in rabbits tends not to present with a raised white blood cell count but with a shift from a lymphocyte-dominant composition to a more neutrophil-heavy count, or a decreased total count. Neonatal rabbits already have an altered lymphocyte/neutrophil ratio and lower total WBCs, so they may appear to have an infection or chronic stress when they are actually healthy.
A high eosinophil count in a neonate may be falsely assumed to be due to parasitic infection, but is actually a common finding in a young rabbit.
As prey animals, rabbits often mask signs of illness, pain, and disease. A thorough history and physical exam, including heart rate, respiratory rate, hydration assessment, and core temperature are important in the assessment of any patient. However, further diagnostic tests such as a hematological assessment may be needed. Being able to interpret these results by considering individual patient factors such as age is essential for accurate diagnosis and prompt treatment.
Attia, Y., El-Hanoun, A., Bovera, F. & Monastra, G. (2013) ‘Growth performance, carcass quality, biochemical and haematological traits and immune response of growing rabbits as affected by difference growth promoters.’ Journal of Animal Physiology and Animal Nutrition 98(1) pp.128
Jeklova, E., Leva, L., Knotigova, P. & Faldyna, M. (2009) ‘Age-related changes in selected haematological parameters in rabbits’ Research in Veterinary Science 86(3) pp.525-528
Laird, C., Fox, R., Mitchell, B., Blau, E. & Schultz, H. (1970) ‘Effect of strain and age on some hematological parameters in the rabbit’ American Journal of Physiology 218(6)
Fox, R. & Laird, C. (1970) ‘Diurnal variations in rabbits: hematological parameters.’ American Journal of Physiology 218(6)
Melillo, A. (2007) ‘Rabbit clinical pathology’ Journal of Exotic Pet Medicine 16(3) pp. 135-145
Leineweber, C., Muller, E. & Marschang, R. (2018) ‘Blood reference intervals for rabbits (Oryctolagus cuniculus) from routine diagnostic samples’ Tierarztl Prax Ausg Kleintiere Heimtiere 46(6)
Moore, D., Zimmerman, K. & Smith, S. (2010) ‘Hematology of laboratory rabbits’ Veterinary Hematology