CHAPTER 33 The Hematologic and Lymphoid Systems
Sample Collection
Collection of blood samples from neonatal and young animals for hematological and coagulation evaluation can be challenging because of small vessels and relatively small quantities of blood available for testing. Blood collection tubes are available from Becton, Dickinson and Company (Franklin Lakes, NJ; www.bd.com/vacutainer/pdfs/VS7629_ProductCat.pdf, BD Microtainer Blood Collection Tubes, BD Microtainer Plastic Clad Micro-Hematocrit Tubes, and 1.8-ml draw BD Vacutainer Citrate Tubes) that will permit collection of sufficient quantities of blood for the various hematological (0.6 ml) and coagulation (<2 ml) testing discussed in the following text.
The Hemogram
Normal Hematologic Values for the Puppy and the Kitten
Unfortunately, hemogram data have not been reported for mixed-breed puppies and kittens younger than 6 months of age, but some studies have reported data for specific breeds. A recent study reported comparison data for Beagles and Labrador Retrievers. These data were limited but did show some significant differences between these two breeds in the white blood cell (WBC) count, red blood cell (RBC) count, hemoglobin, and hematocrit during the first year, and these differences were particularly prominent during the first 8 weeks of life. The more complete body of data has been obtained on animals from closed colonies of selected breeds, and Tables 33-1 and 33-2 are derived from the values reported in those studies. Although these data represent a more complete dataset, factors such as nutrition, environmental conditions, and health in a closed colony (i.e., research colony) may not adequately reflect the general populations of young animals. If, however, the hematologic values obtained for a puppy or kitten are outside the range of values presented here and the reference values for adult dogs and cats obtained from a reference laboratory, they can be considered abnormal with confidence.
Generally WBC counts for kittens and puppies are within the normal adult reference intervals from birth to about 6 to 8 weeks of age and vary only slightly during that period. For the dog, the WBC, segmented neutrophils, and lymphocytes are usually within the adult normal reference interval at birth, although lymphocytes may be low in some animals. These values increase between 1 and 3 months of age and may be greater than the normal adult reference interval during this time. These values will then decrease gradually over the next several months (see Table 33-1). Band neutrophils have been reported in one study to be increased at about 1 week of age and then decreased to within the adult normal reference interval thereafter. Like the dog, the WBC, segmented neutrophil, and lymphocyte counts for kittens are within the normal adult reference intervals at birth, but the WBC and lymphocyte counts increase above the normal adult reference interval between 2 and 4 months of age. These values then return to within the normal adult reference interval by about 5 to 6 months of age (see Table 33-2). It has been proposed that this increase in WBC and lymphocyte counts in kittens is associated with excitement caused during blood collection, although an increase in mature neutrophils was not documented during this period.
Erythrogram Evaluation
BOX 33-1 Causes of anemia in the puppy and kitten
The number of reticulocytes, or polychromatophilic RBCs, necessary to determine a regenerative response depends on the PCV; the lower the PCV, the more reticulocytes (or polychromatophils) are necessary to support an interpretation of a regenerative response. The criteria used to determine adequate regeneration for adult dogs and cats can probably be used for puppies and kittens more than 4 months of age (Table 33-3), but young animals less than 4 months of age should mount a more vigorous response than the older animals. Ideally, an absolute reticulocyte concentration (Abs RC) should be used to determine regeneration (Abs RC = RBC/µl × % reticulocytes = retics/µl). For most puppies and kittens, a regenerative response can also be assessed using a corrected reticulocyte percentage (cRP = RP × patient Hct/avg Hct). Normal canine values for corrected reticulocyte counts during the first year of life can be found in Table 33-1.
In both kittens and puppies, hemolytic anemia can have a number of causes (see Box 33-1). To date, there is no evidence that kittens and puppies are more prone to hemolytic anemia than adult animals. Although not common, neonatal isoerythrolysis is the most common type of immune-mediated hemolytic anemia in newborn kittens and is related to their blood type (types A, B, AB). Neonatal isoerythrolysis is covered in greater detail in Chapter 2. Briefly, hemolysis does not manifest itself during gestation or at birth, but results when a blood type A kitten receives colostrum containing anti-type A alloantibodies from the blood type B queen. Affected kittens may exhibit lethargy progressing to depression, lost suckle reflex, anemia, icterus, and hemoglobinuria followed by death in 2 to 3 days if not treated. Not all type A kittens born to type B queens will necessarily be affected, however. Specific blood types are associated with specific breeds and specific areas of the world. Neonatal isoerythrolysis is most common in Cornish and Devon Rex, Exotic, British Shorthair, and Persian cats, but has also been reported in Himalayan and in domestic shorthair and longhair cats. Neonatal isoerythrolysis is rare in puppies.
TABLE 33-4 Morphologic features of RBCs useful for diagnosis of hemolytic anemias
Cause of hemolysis | RBC feature | Description of RBC abnormality |
---|---|---|
Immune-mediated | Agglutination | Small to large clumps of RBCs that are not dispersed with an equal volume of normal saline |
Spherocytes | Small RBCs with no central pallor (may be difficult to distinguish from normal RBCs in the cat) | |
Hemoglobin oxidation | Heinz bodies | One to several spherical structures within or protruding from the RBC membrane; nonstaining and refractile with DifQuik, nonstaining to pale staining to normal hemoglobin staining with Wright’s stain, blue with new methylene blue stain |
Microangiopathies | Schistocytes | Irregular fragments of RBCs |
Helmet cells (keratocytes) | Football helmet-shaped RBCs with strap-like extensions of RBC membrane from each side | |
Blister cells | RBCs with eccentric vacuoles, often at the periphery of the cell | |
Hemoparasites | Mycoplasma spp. | Dark staining, very small, round to ring-shaped to rod-shaped organisms, usually dotted epicellularly along the periphery or across the surface of the RBC |
Babesia | Large pear-shaped (piriform) structures, present singly, in pairs, or tetrads in the RBC | |
Cytauxzoon | Tiny, densely stained to piriplasm-type organisms within the RBC; usually single, but tetrads have been reported |
Leukogram Evaluation
Many texts present information regarding normal and abnormal features of the various leukocytes (neutrophils, lymphocytes, monocytes, eosinophils, and basophils) seen on a blood film. The changes seen in the cells in response to numerous diseases do not differ from the responses seen in adult animals and so will not be covered in detail here. To aid the clinician in generating differential diagnoses, Boxes 33-2 to 33-5 are provided as quick references for causes of neutrophilia/neutropenia, monocytosis, eosinophilia, and basophilia, respectively. A similar table will be provided for lymphocytes in a later section of this chapter.
BOX 33-2 Causes of neutrophilia and neutropenia
†Usually associated with primary diseases, such as renal failure, endocrinopathies, inflammation, infection (especially viruses), and neoplastic disease.
BOX 33-4 Causes of eosinophilia*
Parasites
Idiopathic Eosinophilic Syndromes
Hypoadrenocorticism (Caused by Maldevelopment of the Adrenal Cortex)
Note: The list serves only to provide examples of possible causes.
Heritable Diseases of Puppies and Kittens Resulting in Hemogram Changes
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