Introduction

Clinical pathology holds the key to unravelling much of the mystery surrounding the avian patient. Although many avian diseases present with identical clinical signs, laboratory data can often distinguish between infectious and metabolic disease, bacterial versus fungal, renal versus hepatic, etc. In a clinical situation, an appropriately broad selection of laboratory tests offers the best odds of quickly determining the nature of the patient's problem.

Complete Blood Count (CBC)

The complete blood count (CBC) is one of the most important components of an avian diagnostic panel. For analytical purposes the CBC may be divided into components describing: the volume and character of the red blood cells; the numbers, percentages, and characteristics of the white blood cells; the concentration of solids in the plasma; the relative number of thrombocytes; and the presence or absence of blood-borne parasites. While many other tests provide information not demonstrated by the CBC, no other single test provides such a broad range of information. Differences exist between avian and mammalian blood but, once these differences are recognized, the similarity in functions of the various components becomes evident.

Unlike dogs and cats, cell counts of birds may vary widely among members of a given species. To determine normal cell count for an individual, baseline data must be collected during periods of apparent good health. Reference values for various species have been published, but these tables should only be used as rough guidelines. Published ranges will typically be wide, therefore, subtle patient variations may not be apparent.

Beyond these differences, the functions of the various cellular components in avian blood are roughly comparable to those of mammals. Infections, non-infectious inflammation, necrosis, neoplasia, etc. may cause a leucocytosis. A moderate heterophilia often indicates the presence of bacterial infections or cellular necrosis, and extremely high heterophil counts often accompany pansystemic illness such as chlamydiosis, aspergillosis, or tuberculosis. These changes are usually characterized by varying degrees of toxic changes in the white cells. Subtle-to-moderate heterophilias, without toxic changes in the white cells, may reflect stress leucograms.

An overwhelming bacterial infection, sepsis, or a severe viral infection may result in a leucopenia with a heteropenia or occasionally a lymphopenia. The leucopenia may be due to decreased production or increased consumption of the cell line. Increased consumption is evidenced by the presence of immature and toxic cells, findings not present with decreased production.

A lymphopenia may occur in severe viral infections, such as avian circovirus in young African grey parrots. (This presentation, in fact, usually demonstrates a pancytopenia.)

A monocytosis implies the presence of chronic infection, granulomatous disease, or extensive necrosis in which a large amount of phagocytosis is occurring. Classic examples of this include chronic forms of aspergillosis, tuberculosis, and chlamydiosis.

Eosinophil functions have not been clearly defined. Intestinal parasitism may produce an eosinophilia, but not consistently. Peripheral eosinophilia does not appear to occur in allergic conditions.

Basophils are uncommon findings in normal avian haemograms. Conditions that cause their appearance include respiratory infections, resolution of tissue damage, parasitism, and some chlamydial infections.

Serum Biochemistries

Aspartate Aminotransferase (AST, SGOT)

The intracellular enzyme most useful for diagnosing hepatocellular disruption in avian species is aspartate aminotransferase. Although present in liver, skeletal muscle, kidney, heart, and brain, elevations are frequently associated with liver disease or muscle damage.

Whenever an elevation in AST is detected, the creatine kinase (CK) level should be reviewed. An elevated AST without a concurrent elevation in CK is highly suggestive of hepatocellular disruption. It should be emphasized that this does not confirm liver disease; nor does a normal AST positively rule out liver disease. As with all diagnostics, the AST provides evidence towards a diagnosis but does not in itself determine the diagnosis. Also, the AST in no way indicates the functional capacity of the liver. The bile acids test, discussed shortly, is more appropriately used to evaluate hepatic function.

Bile Acids

Bile acids are produced by the liver to aid in the digestion of fats. After excretion into the intestinal tract, bile acids are reabsorbed and returned to the liver via the portal circulation. The liver then extracts the bile acids from the blood for recycling. Elevation of bile acids in the general circulation implies decreased ability of the liver to extract the bile acids from the portal circulation, and therefore suggests impaired liver function.

Confusion arises when it is noted that the liver is the organ of bile acid synthesis. It would seem logical that hepatic insufficiency would result in decreased production of bile acids, and therefore in decreased circulating levels. However, hepatic extraction of bile acids from the portal circulation is apparently more dependent on efficient liver function than is the synthesis of bile acids. It is reasonable to presume (and it does appear to happen) that at some point the production of bile acids does diminish and values fall. As with aspartate aminotransferase, a normal bile acid level does not absolutely rule out hepatic disease.

Calcium

Calcium levels are profoundly influenced by a number of normal as well as pathological conditions, and great care should be exercised in the interpretation of abnormal findings. Almost all pathological changes are secondary to conditions not associated with dietary levels. Because of the effectiveness of the parathyroid gland, dietary deficiencies of calcium will rarely cause obvious subnormal blood levels. Blood calcium levels are also directly linked to albumin levels. Hypoalbuminaemia will result in artifactual depression of measured calcium levels. Other causes of lowered blood calcium levels include hypoparathyroidism in African grey parrots, glucocorticoid administration, and insufficient exposure to full spectrum lighting.

Creatine Kinase (CK)

The primary sources of creatine kinase include skeletal muscle, cardiac muscle, and nervous tissue, and elevations are associated with significant disruptions of these tissues. The primary usefulness of this enzyme is in distinguishing between hepatic and non-hepatic causes of elevated aspartate aminotransferase (AST).

Any elevation in AST should be compared with the patient's CK level. If the CK is normal, it is relatively safe to conclude that the liver is the source of the elevated AST. If the CK is elevated along with the AST, muscle should be considered a possible source of the elevated AST. Other possibilities for the dual elevations would, of course, be concurrent liver and muscle or liver and neurological disease. Lastly, an elevated CK in the absence of other biochemical pathology is often the result of significant neurologic disease.

Glucose

Pathological changes in avian blood glucose levels principally involve elevations. Hypoglycemia is extremely rare in birds and, when present, is almost never associated with starvation. The primary cause of hypoglycemia in pet birds is septicemia.

Phosphorus

Elevated serum phosphorus is frequently observed in advanced renal failure. An elevated phosphorus level resulting from renal disease suggests chronicity and presents a guarded prognosis. Elevations are also observed in hypoparathyroidism and nutritional secondary hyperparathyroidism. Hemolysis may artifactually elevate serum levels. Malabsorption and vitamin D deficiencies may cause lowered blood phosphorus levels.

Total Protein (TP)

A total serum protein level must be evaluated in light of its components, albumin, and globulin. The total value is influenced by various factors but, as discussed previously, a normal value does not rule out abnormalities of the individual protein components. Overall, dehydration and immune stimulation may cause a hyperproteinemia. Hypoproteinemia may be caused by overhydration, protein loss in kidney disease, starvation, liver disease, or intestinal disease. As stated earlier, the protein should never be considered normal until the A:G ratio is known to be normal.

Uric Acid

The blood uric acid level is the primary indicator of renal function in birds. An elevated uric acid (UA) level is a reliable indicator that kidney function is impaired. With many tests, substantial elevations are necessary before there is reason for concern; however, even a subtle elevation in the uric acid warrants suspicion of renal disease. Conversely, serial determinations should be made after adequately hydrating the patient before concluding a diagnosis of renal disease. While it has been argued that dehydration has little effect on uric acid levels in birds, this is not the author's experience. Many patients with profoundly elevated UA levels have reverted to complete normalcy after fundamental rehydration.

An artifactual elevation of UA often occurs if blood is collected via nail trim due to contamination of the nail with the patient's droppings.

Summary

Laboratory testing is an essential component of avian medicine. When examining avian patients, both routinely and in the face of illness, it is necessary to utilize an appropriate assortment of tests in order to obtain a reasonably complete profile of the patient. Knowing how to interpret the findings is as important as knowing which tests to utilize. By employing the right tests, and correctly interpreting the meaning behind the findings, avian patients can be diagnosed and treated with maximal accuracy and effectiveness.