(1)
Office of Research and Development, United States Environmental Protection Agency, Washington DC, USA
Abstract
As difficult as clinical pathology interpretation may be for humans, it is far more complex when numerous other species are considered. Clinical pathology diagnosis is generally similar across species but there are many species-specific differences that must be considered to avoid erroneous diagnosis. For example, birds have nucleated red blood cells but mammals do not. It is normal for red blood cells to resemble a stack of coins (rouleaux) in cats, dogs, guinea pigs, and rabbits, but this is a sign of disease in other species. Mouse leukocyte, lymphocyte, and eosinophil counts can be threefold higher in the morning than in the evening. Alkaline phosphatase is a good indicator of cholestasis except in cats. Manual restraint of rats during sampling can cause increases in plasma protein, calcium, and magnesium; and decreases in blood pH and glucose. The urine of healthy cats tends to be cloudy. Rabbit urine tends to be basic and turbid and it is high in volume because rabbits do not concentrate their urine as well as other animals. Only by understanding peculiarities for a given species can one avoid calling adverse what is actually a normal finding. This chapter provides specific information for the species most commonly encountered in toxicology studies.
4.1 Birds
Hematology
Birds have lower RBCs and higher WBCs than mammals. Erythrocytes are nucleated, and typically elliptical and biconvex, with as many as 10 % of them being reticulocytes in healthy birds. Chicks have low RBCs, but they reach adult levels within a week. Lymphocytes are the most numerous leukocytes. Basophils are common, especially in pheasant. All hematopoiesis occurs in the bone marrow but B-lymphocytes differentiate in the bursa of Fabricius, which is located near the cloaca. Mammals do not have an analogous organ to the bursa. Thrombocyte counts are slightly lower in adults, particularly in males. The speed of clotting in birds and mammals is similar. Thrombocytes are nucleated and function similarly to mammalian cells, although they do have phagocytic abilities. Avian thrombocytes are never called platelets. Birds lack clotting factors XI and XII. Clotting relies primarily on the extrinsic pathways. Heterophils (the avian equivalent to neutrophils) and eosinophils look similar, and lymphocytes, monocytes, and thrombocytes look similar.
Automatic counting of erythrocytes is complicated by the presence of nuclei. Cyanmethemoglobin measurements of hemoglobin will be falsely elevated because of light scattering by the RBC nuclei unless the reagent blood mixture is centrifuged. Hematopoietic neoplasia is common in birds. Bone marrow cannot be collected from live birds.
Clinical Chemistry
Alkaline phosphatase (ALP) increases due to liver disease in carnivorous birds but not in herbivorous birds.
4.2 Cats
Hematology
Male and female cats have similar values. RBC counts and Hb levels are lower, and reticulocytes counts are higher in kittens under 3 months old. Nucleated RBCs are found in kittens (≤12 weeks old), but rarely in cats (except those with myeloproliferative disorders). HCT and Hb decrease during the first months of life and there is microcythemia. RBCs, HCT, and Hb gradually increase to adult levels. Reticulocyte counts are very low except during recovery from blood loss anemia. RBCs, HCT, and Hb decrease during late pregnancy, then quickly rebound at postpartum. Erythrocytes tend to be anisocytotic (varying in size), and they become smaller and more numerous with age. Erythrocyte diameters range from 5.4 to 6.5 μm and rouleaux formation is common. Abnormal erythrocyte shapes (poikilocytosis) are common in disease states, such as blister cells caused by oxidative injury. Heinz bodies and Howell Jolly bodies may be seen in small numbers.
WBC counts vary widely since healthy cats can draw on a marginal pool containing three times the number of circulating neutrophils. Neutrophils are the most numerous leukocytes. Band neutrophils and basophils are rare in healthy cats, and metamyelocytes are never found. Eosinophils have rod-shaped granules. Leukocyte and monocyte counts are lower in kittens under 3 months old.
Excitement (such as during venipuncture) stimulates splenic contraction and the release of erythrocytes and platelets. Fear, apprehension, rage, and muscular activity cause an increase in neutrophils and lymphocytes. Because of difficulties in drawing blood, clotting may interfere with analysis, especially with electronic counters. Crenation is a common problem. The H. Felis parasite causes feline infectious anemia followed by macrocytic hemolytic anemia, and erythrocytes may lose their normal biconcave shape and become spherocytes or stomatospherocytes. ACTH and corticosteroid therapy cause neutrophil and monocyte counts to increase, and lymphocyte and eosinophil counts to decease.
Clinical Chemistry
Excitement can significantly affect chemistry values for an individual cat over time. Age and sex have little effect. Total plasma protein increases in old cats, mostly due to increased gamma globulin. Alkaline phosphatase is a poor indicator of cholestasis in cats. The half-life of the hepatic alkaline phosphatase isoenzyme is approximately 10 % that of the dog.
Urinalysis
Feline urine tends to be acidic. Bilirubinuria is rare in cats. The urine of healthy cats tends to be cloudy due to lipid droplets.
4.3 Dogs
Hematology
Male and female values are similar. Erythrocyte counts, HCT, and Hb decrease during the first months of life, but then gradually increase to adult levels. Erythrocytes tend to be anisocytotic (varying in size), and they become smaller and more numerous with age. Erythrocyte diameters range from 6.9 to 7.3 μm. Macrocytosis is common in poodles. Some rouleaux formation occurs in dogs. Abnormal erythrocyte shapes (poikilocytosis) are common in disease states. Reticulocytes are <3 % (except in puppies) and nucleated red blood cells are rare. Fear, such as during venipuncture, can cause splenic contraction yielding a significant increase in erythrocytes (erythrocytosis). Hemoglobin levels are increased by diets containing vitamin B-complex and other hematopoietic factors. Growing dogs have low fibrinogen levels, and thus will have negative corrected ESR values.
WBCs decrease with age (due to decreases in lymphocyte counts), and they fluctuate due to diurnal, seasonal, and physiologic effects. Neutrophil counts remain fairly constant and eosinophil counts increase somewhat throughout life. Leukocyte counts peak at pregnancy term. There is wide variation in WBC counts between breeds. Neutrophils are the most numerous leukocytes (three to fivefold higher than lymphocytes). Band neutrophils are in small numbers, and metamyelocytes are never found in the blood of healthy dogs. Monocytes can be easily mistaken for band neutrophils, but their cytoplasm stains bluish.
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