Dr Lizzie Youens BSc(Hons) BVSc MRCVS 

Rabbits are prey species and proficient at hiding clinical signs of illness and disease. Young rabbits are vulnerable and may need urgent veterinary intervention if unwell to prevent rapid deterioration of their health status. 

Objective measures of health, such as serum biochemistry and hematology, augment medical history and clinical examination findings to determine the best course of action for a patient. Interpretation of this data, however, needs to be accurate and relevant to the individual. Juvenile rabbits are still developing their organ systems, enzymatic processes, and metabolism. This maturing physiology can affect diagnostic results such as biochemical parameters, so care must be taken to interpret results with age-related changes in mind, following relevant guidelines and reference ranges. 

Read on to learn more about what age variations veterinarians should consider when working with rabbit chemistries.

When might I need a chemistry profile?

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As with any sick patient, a thorough medical history and clinical examination are essential. Alongside these, clinical data from chemistry and hematology profiles are often needed to provide essential diagnostic findings. Biochemistry profiles provide information on the health and function of many organ systems and can provide insights into a rabbit’s internal health.

Obtaining a chemistry sample

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The requirement for a biochemistry panel is usually a small amount of venous blood, which is either allowed to clot and then separated for serum, or a heparinized sample spun down to collect plasma. Around 0.25ml of serum or plasma is usually required. An average rabbit usually has a blood volume of around 60 ml/kg, and up to 10% can be collected if required.

Handling can be very stressful for pet rabbits, and therefore venipuncture should be performed calmly, swiftly, and gently. Obtaining a useful sample can be challenging. 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 pre-heparinized butterfly catheters or needles, although it is important to flush the line or needle with air following dilute heparin flush to avoid sample artifact.

Interpreting a rabbit biochemistry sample

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Chemistry results are usually interpreted through the use of reference intervals. These ranges have been developed using data from peer-reviewed studies and published by animal diagnostic labs as a guide of what is considered ‘normal’ for a healthy animal. 

However, 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. Biochemistry values are often tightly regulated, with small variations in results being potentially meaningful. Anything which alters physiology, such as being a young, growing animal, may well alter the standard values we accept as a ‘normal rabbit’. Results from a young rabbit, therefore, need to be interpreted with the individual animal in mind in order to gain meaningful conclusions.

What’s the evidence that age alters biochemistry results?

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A study looking at growth factors in New Zealand White Rabbits (Attia et al., 2013) found that use of growth promoters affected both biochemical and hematological values. Although this may not be applicable for veterinarians assessing a sick rabbit, it indicates that growth can affect serum chemistry results. 

Olayemi & Nottidge (2007) studied the effect of age on various biochemical parameters in New Zealand White Rabbits. They found that 4-8 week-old rabbits had age-related changes such as higher levels of creatinine, glucose, alkaline phosphatase (ALP), alanine transaminase (ALT) and total protein than adult rabbits, although the sample size was low with only 18 rabbits used. Young animals were also found to have lower globulin levels. 

A larger study (Korn et al., 2018) surveyed blood samples from 122 rabbits comparing biochemical results at weaning (8 weeks) and at adulthood. Again, higher values for ALP and glucose were found in young animals, alongside cholesterol, triglycerides and GGT. However, the 8-week-old rabbits were found to have lower albumin, globulin, ALT, creatinine, and urea. 

Multiple sources corroborate the lower protein levels in young rabbits, including Chineke et al. (2003) and Washington & Van Hoosier (2012), which suggests that the small study by Olayemi & Nottidge (2007) may be an outlier.

Reference ranges: Rabbit biochemical parameters

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Adult ranges from Melilo (2007).

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How do these values affect interpretation?

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Biochemical parameters are tightly controlled within the body, and small deviations can be highly significant. When interpreting data from juvenile rabbits, veterinarians should be aware that values outside of normal ranges may be due to physiological change around immaturity rather than disease.

Here are some of the key take-aways:

• ALP is high in young rabbits due to osteoblastic growth.
• Total protein levels are low in juveniles, likely due to an immature immune system and rapid growth. Albumin and globulin levels may both be low.
• Glucose levels are often higher in neonatal rabbits, alongside cholesterol and triglycerides. 
• Creatinine values may be lower in the young due to a lack of defined muscle mass.

These points are vital to remember, as changes to these clinical pathology parameters can lead to analytical and diagnostic confusion.

Here are some practical examples of how age may alter clinical analysis:

• A cholesterol value of 95mg/dL in a young rabbit may be completely normal, whereas in an adult it would be seen as a potential indicator of disease and a potential inflammatory marker.
• ALP of 125 IU/L would perhaps flag concern for bone or hepatic disease. In a juvenile, however, this value is physiologically normal.
• A total protein level of 7.5mg/dL would be within normal limits for an adult rabbit. However, if this result is seen in a neonate it would be classed as hyperproteinemic, perhaps due to dehydration, and therefore important not to miss. 
• A higher glucose reading can be perfectly normal in a young rabbit, whereas in an adult it would indicate stress or pathology such as GI disease. 

The evidence shows that age can alter many biochemical variables for rabbits. Understanding these key physiological variances and how they affect interpretation is essential for accurate diagnosis for young rabbit patients. 

References

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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 different growth promoters.’ Journal of Animal Physiology and Animal Nutrition 98(1) pp.128

Chineke, C., Adeniran, F., Ologon, A. & Ikeobi, C. (2003) ‘Analysis of effects of breed, sex and age on some serum biochemical parameters in rabbits’ Trop. J. Anim. Sci 6(2) pp.85-90

Melillo, A. (2007) ‘Rabbit clinical pathology’ Journal of Exotic Pet Medicine 16(3) pp. 135-145

Olayemi, O. & Nottidge, H. (2007) ‘Effect of Age on the Blood Profiles of the New Zealand Rabbit in Nigeria’ African Journal of Biomedical Research 10: 73-76

Korn, A., Bauer, N., Moritz, A. & Erhardt, G. (2018) ‘An update on clinical biochemical RIs of rabbits with special consideration for age, gender and size’ Veterinary Clinical Pathology 47(2) pp.233-245

Sharma, D. Hill, A. & Christopher, M. (2018) ‘Hypercholesterolemia and hypertriglyceridemia as biochemical markers of disease in companion rabbits’ Veterinary Clinical Pathology 47(4) pp.589-602

Washington, I. & Van Hoosier, G. (2012) ‘The Laboratory rabbit, guinea pig, hamster and other rodents’ American College of Laboratory Animal Medicine pp. 57-116