Clinical pathology of rabbits – interpretation of biochemistry, haematology and urinalysis results

Domestic rabbits originate from the European rabbit (Oryctolagus cuniculus), and are an increasingly popular family pet, with an estimate of at least 900,000 pet rabbits in the UK (PDSA and YouGov, 2019). In this article, we will look at the physiological considerations, biochemical considerations, haematology and urinalysis of rabbits.

Physiological considerations

The digestive system of the domestic rabbit is adapted for processing large quantities of fibrous food. Rabbits have a large caecum where microbial fermentation of food takes place, producing a soft paste containing amino acids, volatile fatty acids and vitamins. This material is expelled as pellets called caecotrophs, which rabbits ingest as an additional source of nutrients in a process called caecotrophy. Due to caecotrophy, it is not possible to obtain a truly fasting blood sample from a rabbit.

Rabbit blood samples may clot and haemolyse easily, therefore careful handling of blood samples is required to ensure accurate laboratory test results. Blood samples from laboratory rabbits are often used for the calculation of laboratory reference intervals; however, pet rabbits are usually more genetically diverse, which is expected to influence reference intervals.

Stress associated with transport or handling can affect parameters such as glycaemia and leukocyte distribution

Several biochemistry parameters such as urea, cholesterol and bile acids vary according to a circadian rhythm. This also affects the total and differential white cell count. Stress associated with transport or handling can affect parameters such as glycaemia and leukocyte distribution. Anaesthesia, blood collection techniques, intravenous fluid therapy and time of day may influence some blood test results. Also, pregnancy affects haematocrit, total protein, glucose, triglycerides, cholesterol, sodium, calcium and phosphate.

Biochemistry

Glucose

Glucose metabolism is markedly influenced by stress in rabbits. Stress may cause marked hyperglycaemia, and glucose levels up to 15mmol/l are reported with handling and levels higher than 20mmol/l with painful conditions. Hypoglycaemia may be caused by anorexia, starvation or hepatic dysfunction. Furthermore, it leads to the release of free fatty acids from adipose tissue, causing ketoacidosis and hepatic lipidosis.

Urea

Urea and creatinine may increase due to renal disease but may also reflect prerenal azotaemia due to dehydration or water deprivation. Urea may also increase with high protein intake, vigorous exercise and time of sampling. Hyperphosphataemia may be seen with chronic renal failure. Causes of renal disease in rabbits are nephrolithiasis, renal neoplasia, interstitial nephritis and nephrotoxicity. Low serum urea levels may indicate hepatic dysfunction.

Calcium

In rabbits, calcium is absorbed from the intestine in proportion to dietary concentrations, and calcium homeostasis is predominantly regulated by the kidneys. Serum calcium levels in rabbits are higher than in other mammals and not as tightly regulated. Variations are more marked in mature rabbits than in juvenile rabbits. Calcium deficiency causes poor mineralisation of the bones and teeth. Excessive dietary calcium causes large amounts of calcium carbonate sediment in the urine and predisposes to the formation of urinary sludge and cystitis.

Proteins and amino acids

Total protein may increase secondary to dehydration. Hypoproteinaemia may be seen with liver disease, chronic enteropathy, starvation or malnutrition and, less commonly, with protein-losing nephropathy or exudative skin lesions.

Alanine aminotransferase (ALT) activity is lower and less organ-specific in rabbits than in other mammals. ALT may be released from hepatocytes or myocytes. An increase in ALT indicates possible hepatocyte damage; however, the degree of the increase does not correlate with the severity of hepatic disease and is not a prognostic indicator.

Hypoproteinaemia may be seen with liver disease, chronic enteropathy, starvation or malnutrition and, less commonly, with protein-losing nephropathy or exudative skin lesions

Aspartate aminotransferase (AST) is found predominantly in myocytes and hepatocytes, and weight loss, tissue damage during blood collection and liver disease may elevate AST levels.

Alkaline phosphatase (ALP) is found particularly in the bones, liver and intestinal wall, and increases in plasma activity are usually due to the isoenzymes released from liver and bone. Similar to other species, higher concentrations of ALP are found in juvenile rabbits than in adults due to increased osteoblastic activity.

Gamma-glutaryl transferase (GGT) originates from the bile duct epithelium and increases significantly in obstructive hepatic disease.

Serum amylase levels are lower in rabbits than in other species, but they may increase with pancreatitis, corticosteroid therapy and decreased GFR.

Blood and cholesterol

Bilirubin levels in rabbits are normally very low. Marked increase in bilirubin is expected in rabbits with biliary obstruction, eg secondary to hepatic coccidiosis (mostly in young rabbits) or hepatic neoplasia (mostly in older rabbits). Viral haemorrhagic disease (VHD) and aflatoxicosis due to ingestion of mouldy food may cause severe hepatic disease in rabbits.

Large individual variations in blood cholesterol and triglyceride values can occur in rabbits. Male rabbits have lower cholesterol levels than females and there is also a diurnal variation with higher levels of cholesterol reported in the afternoon.

Triglycerides and cholesterol may be increased in rabbits with hepatic lipidosis (usually as a consequence of intestinal stasis). In anorexic rabbits, a lipaemic sample is a poor prognostic indicator as it reflects impaired lipid metabolism and the presence of hepatic lipidosis.

Electrolytes

Electrolyte disturbances are commonly associated with abnormal electrolyte losses from the intestine. Hypernatraemia may be caused by water deprivation, while hyponatraemia may occur as a result of chronic renal failure. Hypokalaemia has been noted in rabbits in conjunction with muscular weakness.

Haematology

Anisocytosis and reticulocytes (2 to 4 percent) are considered normal findings when examining rabbit blood smears. However, in rabbits less than two months of age, the reticulocyte percentage may be up to 12 percent of the red cell count. Occasional nucleated red blood cells and Howell-Jolly bodies may be seen on blood smears from healthy rabbits.

Occasional nucleated red blood cells and Howell-Jolly bodies may be seen on blood smears from healthy rabbits

Anaemia occurs most often secondary to haemorrhage, such as with trauma, flea infestation and uterine adenocarcinoma. Haemolytic anaemia is rarely reported in pet rabbits. Red cell regeneration is characterised by increased anisocytosis, polychromasia, nucleated erythrocytes and the presence of Howell-Jolly bodies. Non-regenerative anaemia may be seen with chronic disease.

Granulocytes

Rabbit neutrophils are usually called heterophils and may be mistaken for eosinophils. Heterophils have a multilobulated nucleus. Their cytoplasm stains pink with Romanowsky stains due to fusion of primary acidophilic granules. Variable numbers of large intracytoplasmic eosinophilic granules are also seen.

Rabbit eosinophils are larger than neutrophils and have large cytoplasmic granules that stain pink-orange. They usually have more granules than heterophils, and their nucleus is bilobed or U-shaped.

Rabbit basophils have numerous purple intracytoplasmic granules (similar to other mammals). Healthy rabbits usually have more circulating basophils than other mammals: around 5 percent, but up to 30 percent of the circulating leukocyte population.

Lymphocytes, leukocytes and monocytes

Rabbit lymphocyte and monocyte morphology is similar to other mammals. Small lymphocytes predominate in circulation; however, large lymphocytes may also be present. These large lymphocytes commonly have intracytoplasmic azurophilic granules.

Regarding leukocyte concentrations, there is a circadian leukocyte variation with lower lymphocyte concentration noted in the evening. The neutrophil:lymphocyte ratio is approximately 1:1 in adult rabbits. Changes in this ratio may be associated with stress (heterophilia and lymphopenia). Rabbits do not usually develop a significant leukocytosis with bacterial infection; however, a reversal of the heterophil:lymphocyte ratio is commonly noted.

Monocytosis may occur in association with chronic infection, and eosinophilia may be seen with parasitic diseases (except infection with Encephalitozoon cuniculi).

Urinalysis

Normal rabbit urine is turbid and may be coloured red due to presence of plant pigments. Dipsticks can be used to differentiate haematuria from red urine. In rabbits, urine specific gravity (USG) varies between 1.003 and 1.036; however, USG may be difficult to measure accurately due to urinary crystals. Urine pH varies between 7.6 and 8.8.

Normal rabbit urine is turbid and may be coloured red due to presence of plant pigments. Dipsticks can be used to differentiate haematuria from red urine

Small quantities of albumin may be found in the urine of healthy rabbits, especially juveniles. Normal rabbit urine may contain small amounts of calcium carbonate, triple phosphate and calcium oxalate crystals, and low numbers of leukocytes and erythrocytes may be a normal finding.

Urolithiasis is a common finding. Haematuria may occur with the following conditions: cystitis, urolithiasis, uterine adenocarcinomas and uterine polyps.