Avian haematology and biochemistry are vital diagnostic tools in the assessment of a sick bird and in the routine assessment of healthy and new birds. Blood profiles should be interpreted alongside other fundamental pieces of information in order to form an accurate diagnosis. A good foundation often includes a detailed history, clinical examination, additional clinical pathology (such as faecal analysis) and imaging, frequently in the form of orthogonal whole-body radiographs.
This article discusses how to approach venepuncture in the avian patient, interprets the complete blood cell count and basic biochemistry parameters, and highlights the significant differences from mammalian blood profiles.
Care must be taken regarding the volume of blood that can be safely withdrawn from a patient, especially in small sick birds (Samour, 2006). The total blood volume of a bird equals 10 percent of its body weight on average, therefore 10 percent of the total blood volume can be withdrawn safely from circulation (ie 1 percent of the bird’s body weight in grams) (Samour, 2006). For example, a 100g bird has a total blood volume of approximately 10ml, therefore 1ml can be safely withdrawn as a sample. However, the volume withdrawn should be reduced for sick or dehydrated birds (Samour et al., 2015). The possibility of haematoma formation must also be taken into account when considering the volume withdrawn (Forbes, 2008).
The total blood volume of a bird equals 10 percent of its body weight on average, therefore 10 percent of the total blood volume can be withdrawn safely
Several sites can be used for blood collection in avian patients. Site selection depends on the species involved and whether chemical restraint is required. The right jugular vein (Figures 1 and 2) is often sampled as it is larger than the left jugular vein in most avian species (Campbell and Ellis, 2007). Relatively large volumes can be drawn, although haematomas can often form due to the lack of surrounding subcutaneous tissue (Samour, 2006). The jugular can be found under a featherless tract of skin called the apterium, which allows for good visualisation in most species; however, the jugular apterium is noticeably absent in pigeons and waterfowl (Chitty and Lierz, 2008).
The basilic vein is another site commonly used for blood collection in avian patients (Samour et al., 2015). It can be found on the ventral surface of the elbow joint, which is easily visible under the skin (Samour et al., 2015). The fragile nature of this vein means haematomas can readily form following blood collection (Campbell and Ellis, 2007).
The medial metatarsal vein can be found on the medial aspect of the tibiotarsus and can be useful where conscious restraint is required or for long-legged birds (Chitty and Lierz, 2008). However, it can be more difficult to sample from psittacine species where the tarsometatarsus is shorter (Samour, 2006).
In general, blood is stored in lithium heparin tubes for biochemistry and EDTA tubes for haematology (Figure 3). EDTA tubes should be used with caution as haemolysis can occur in some species, such as corvids and cranes (Samour, 2006). If you are testing for heavy metals, attention should be paid to the container of choice for zinc analysis and the chosen anticoagulant for lead analysis – this is because chelation can occur, leading to inaccurate results (Chitty, 2002). Similarly to mammalian blood analysis, it is important to note any artefacts, such as haemolysis or lipaemia, which can have significant effects on the results (Forbes, 2008).
Avian haematology differs from mammalian haematology in several aspects. Avian erythrocytes and thrombocytes are nucleated (Figure 4), so automated analysers cannot be used (Astill et al., 2022). Avian erythrocytes have, on average, a shorter half-life than mammalian erythrocytes, so some polychromasia from circulating immature red blood cells is considered normal (Campbell and Ellis, 2007). Avian leucocytes are similar to mammalian lines except in the fact that avian heterophils are equivalent to mammalian neutrophils (Campbell and Ellis, 2007).
It is worth noting that there are substantial differences in normal heterophil to lymphocyte ratios between avian species (Forbes, 2008).
Packed cell volume/haematocrit
The haematocrit (HCT) or packed cell volume (PCV) can be decreased due to similar aetiologies as those seen in mammals. Regenerative anaemia may occur as a result of haemorrhage or toxicosis from lead or zinc poisoning, while haemoparasites such as Plasmodium can also lead to regenerative anaemia (English, 2008). Non-regenerative anaemia can be seen as a consequence of chronic inflammatory or infectious disease and bone marrow pathology (English, 2008). Overhydration can also cause a relative decrease in the red blood cell count (Samour, 2006).
Primary polycythaemias are rare, while secondary polycythaemias can occur because of chronic circulatory or respiratory diseases, such as hypersensitivity pneumonitis seen in macaws (also known as “macaw asthma”) (Baldrey, 2012). It can also be seen with iron storage disease, which is more commonly seen in toucans (English, 2008). Dehydration can cause a relative increase in PCV/HCT (Jones, 2015).
Leukopenia can be seen alongside chronic inflammatory disease processes or with severe bacterial or viral infections (Chitty, 2018). Circovirus (psittacine beak and feather disease) should be a differential if significant leukopenia is seen (Campbell and Ellis, 2007). On the other hand, leukocytosis can occur as a result of stress, inflammation, infection, tissue damage or neoplasia (Chitty, 2018). The differential white blood cell count is important for distinguishing between these causes (Baldrey, 2012).
Leukopenia can be seen alongside chronic inflammatory disease processes or with severe bacterial or viral infections
Heteropenia is usually caused by artefacts (Samour et al., 2015); if pathological, it is often accompanied by a degenerative left shift as a result of decreased production or overwhelming demand (Stacy et al., 2022). A decrease can also be seen as a consequence of sequestration (Chitty, 2018). Heterophilia can result from a stress response or infectious or non-infectious causes of inflammation (Chitty, 2018). A left shift and toxic changes can be seen in cases of heterophilia with more severe inflammatory disease, for example bacterial sepsis (Campbell and Ellis, 2007).
A monocytosis is commonly seen due to chronic infectious diseases. These include aspergillosis, avian tuberculosis and Chlamydia psittaci infections (Samour, 2006).
Lymphopenia can be seen with viraemia or as part of a stress response (Jones, 2015), whereas lymphocytosis can result from chronic infectious or inflammatory disease and, in rare cases, lymphoid leukaemia (Jones, 2015).
Eosinophils and basophils
Eosinophils and basophils are difficult to interpret in birds due to a lack of data. Eosinophilia has been loosely associated with parasitic disease and cases with marked tissue damage (Jones, 2015). Basophilia has also been observed in birds with respiratory disease and hypersensitivity reactions (Jones, 2015).
Alanine transaminase (ALT) and alkaline phosphatase (ALP) have little value in recognising liver disease in birds (Forbes, 2008). Aspartate aminotransferase (AST) can be elevated in liver disease but is found in other tissues, including skeletal muscle, so is non-specific (Harr, 2006). It also has a long half-life, so levels are slow to rise and fall following tissue damage (Forbes, 2008). Gamma-glutamyl transferase (GGT) can increase due to cholestasis but lacks sensitivity (Chitty, 2018). Similarly, glutamate dehydrogenase (GLDH) lacks sensitivity; however, it is more specific as a marker of hepatocellular disease (Chitty, 2018).
Elevated bile acids are considered a sensitive and specific marker of liver dysfunction, and cholesterol can increase or decrease as a result of liver disease (Harr, 2006). In cases of hepatic lipidosis, cholesterol is often increased alongside triglycerides and the liver markers discussed above (Samour et al., 2015). Other causes of high cholesterol include reproductive or endocrine disease, biliary obstruction and atherosclerosis (Harr, 2006). Triglycerides can also increase due to atherosclerosis (Beaufrère et al., 2013). The major bile pigment in birds is biliverdin, which is not metabolised to bilirubin (Samour et al., 2015). Due to this, bilirubin has little value in diagnosing liver disease in birds, unlike in mammals (Samour et al., 2015).
Unlike in mammals, where excess nitrogen is converted to urea, the major product of nitrogen catabolism in birds is uric acid (Baldrey, 2012). Birds produce small amounts of urea that are reabsorbed depending on hydration status (Samour et al., 2015). Elevated urea is not a useful indicator of renal disease but can be indicative of dehydration (Samour et al., 2015). Increased uric acid is commonly due to renal disease in birds (Harr, 2006).
It is crucial to consider species-specific reference ranges with hyperuricaemia as carnivorous birds have higher uric acid levels than grain-eating birds
Significant kidney damage is needed before hyperuricaemia is observed; therefore, normal uric acid levels do not rule out kidney disease (Jones, 2008). It is crucial to consider species-specific reference ranges with hyperuricaemia as carnivorous birds have higher uric acid levels than grain-eating birds (Samour et al., 2015). Additionally, post-prandial physiological hyperuricaemia is seen in carnivorous birds, so a fasting period before sampling is recommended in these species (Chitty, 2018).
Similarly to mammals, a third of calcium is found bound to plasma protein in birds, so total calcium will be directly affected by plasma protein levels (Samour et al., 2015). Therefore, evaluating ionised calcium is considered the most accurate reflection of the calcium status of avian patients (Forbes, 2008).
Evaluating ionised calcium is considered the most accurate reflection of the calcium status of avian patients
Ionised calcium can be increased due to normal reproductive activity in female birds (for example, egg laying) or due to reproductive pathology (Harr, 2006). Hypocalcaemia alongside an abnormal calcium to phosphorus ratio often indicates malnutrition. It is frequently seen in parrots fed a seed-based diet deficient in calcium and vitamin D with excess phosphorus (Baldrey, 2012). This can lead to secondary nutritional hyperparathyroidism. Reduced ionised calcium can be observed in renal disease, but unlike in mammals, high phosphorus is not considered a consistent finding in birds with kidney dysfunction (Jones, 2008).
Serum protein electrophoresis is recommended to accurately evaluate protein distribution and better characterise the inflammatory response (Samour et al., 2015). Albumin can be increased because of dehydration and in reproductively active females (Harr, 2006). Albumin is decreased due to reduced production resulting from liver disease, increased loss through kidney or gastrointestinal tract disease, and increased use in chronic inflammation, for example (Harr, 2006). Elevated alpha and beta globulins usually indicate acute inflammation or infection, whereas elevated gamma globulins can indicate a more chronic inflammatory or infectious process (Samour et al., 2015).
Haematology and biochemistry parameters can form a vital part of investigations in the avian patient. Hopefully, interpreting results using species-specific reference ranges can help prioritise future tests. This will ensure the best chance of achieving a diagnosis while minimising the risk of unnecessary handling stress and general anaesthesia for the patient.