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Clinical pathology for exotic small mammals (Proceedings)

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Apr 01, 2009

Nontraditional mammalian species, such as ferrets, lagomorphs, marsupials, hedgehogs, and rodents, are stoic by nature and have evolved to mask their illness to avoid predation. This behavior can create a clinical challenge for the veterinarian and a false sense of security for an owner. In some cases, an animal that appears clinically normal may have a terminal illness. Hematology and plasma biochemistry analysis can be used to evaluate these species to characterize their true physiological status and aid in disease diagnosis.

When working with small species, it is important to determine the volume of blood that can be safely collected from the patient. The blood volume of a mammal is approximately 10% of the animal's body weight. In general, 1% of the total blood volume can be safely collected for sample processing, which is approximately 1.0-ml/100-gr BW. Because of the small size of some of the exotic mammals, veterinarians may be concerned that they cannot collect enough blood to perform a hematologic or chemistry profile; however, most laboratories or analyzers can perform a complete blood count (CBC) and plasma chemistry analysis on 1-ml of blood. It is important to consider the animal's history and physical examination findings when determining the volume of blood that can be safely collected. If an animal has experienced an acute blood loss, than a reduced volume of sample should be collected

Blood samples should be placed into appropriate collection tubes immediately after collection. Hematological samples may be placed into ethylenediaminetetraacetic acid (EDTA) or lithium heparin. EDTA is generally preferred for complete blood counts. Blood for plasma chemistry analysis may be placed into a lithium heparin vial. The plasma chemistry samples should be centrifuged immediately to separate the cells from the plasma. If the blood cells are not separated from the cell fraction, than glucose levels may be lower than normal, and phosphorus and potassium levels higher than normal. There are some newer pieces of equipment, such as the Abaxis VetScan (Union City, CA), that can perform a plasma chemistry analysis on a small volume (100 microliters) of whole blood.

The complete blood count routinely includes a packed cell volume, total white blood cell count, and a differential white cell count. The packed cell volume can be used to assess a patient's general health and hydration status. In general, exotic small mammal hematocrits are 30-55. Interestingly, ferrets, which are one of the least active mammals found in captivity, generally have packed cell volumes >40-45. Anemia in exotic species may be attributed to acute blood loss, chronic infections, malnutrition and toxicities. The erythron of an anemic animal should be evaluated to estimate prognosis. Most laboratories will report alterations in the red blood cell numbers by including information on possible difference in erythrocyte cytoplasmic color (polychromasia) or cell size (anisocytosis). Laboratories generally provide this information using an ordinal scale: 1-4, with 1 being minimal and 4 being a large amount. .

Unlike the lower vertebrates (e.g., birds, reptiles, amphibians, and fish), complete blood cell counts in exotic small mammals can be determined using an automated cell counter. When making differential blood smears, the author prefers to mix 1 drop of 22% bovine albumin (Gamma Biologicals, Inc, Houston, TX, 77092) to 5 drops of blood prior to making a smear to stabilize the cell membranes and improve cell visualization. White blood cell counts for most mammals fall between 5-15 x 103 cells/microliter; however, white blood cell counts for rabbits (4-10 x 103 ) and geriatric ferrets (3-6 x 103 ) are generally lower. In both of these species, inflammatory responses are not necessarily characterized by elevated peripheral cell counts, but instead alterations in the cell differential.

Exotic small mammals have the same type of white blood cells seen in domestic mammals: neutrophil, lymphocyte, monocyte, eosinophil, and basophil. One exception is the rabbit, which has a heterophil instead of a neutrophil. The neutrophil (or heterophil in rabbits) is associated with the acute inflammatory response. Monocytes are a common finding in a chronic inflammatory response. In general, the function of the other white blood cells is considered to be similar to those described for other mammals. When interpreting a differential it is important to first determine the primary leukocyte for that species, which will be either the neutrophil or lymphocyte. The proportion of the predominant to the secondary cell will range from 1:1 to 3:1. Alterations in this ratio would be suggestive of an abnormal differential.