Basic Caprine Hematology

Anatomic Considerations

The principal organ of hematopoiesis in the goat is the bone marrow. The spleen of the goat is located in the left craniodorsal abdomen, between the diaphragm and the dorsal sac of the rumen.

Goat Medicine, Third Edition. Mary C. Smith and David M. Sherman. © 2023 John Wiley & Sons, Inc. Published 2023 by John Wiley & Sons, Inc.

Bone Marrow

The myeloid to erythroid (M : E) ratio of the bone marrow of the goat has been reported as 0.69, indicating more active erythrocyte than granulocyte production. The proportions of cell types found in normal caprine marrow are reported as follows: myeloblasts, 0.58%; promyelocytes,

0. 79%; neutrophilic myelocytes, 2.69%; metamyelocytes, 8.25%; band neutrophils, 8.88%; segmented neutrophils, 9.98%; eosinophils, 1.79%; basophils, 0.06%; monocytes, 0.02%; lymphocytes, 7.49%; and nucleated erythrocytes, 56.33% (Coles 1986). Megakaryocytes were not included in the differential count.

Bone marrow collection procedures should be performed aseptically and, at the minimum, with local anesthesia at the collection site. Bone marrow biopsies and aspirates from the goat can be conveniently obtained from the iliac

crest using a 13-gauge Styletted needle (Akram et al. 2017). In addition, an aspirate can be obtained from the dorsal 5-10 cm of ribs using sturdy, 3.8 cm, 16- or 18-gauge stylet- ted needles (Weber 1969).

Blood and Plasma Parameters

Mean blood volume as a percentage of bodyweight (bw) is approximately 7%, with a range of 5.7-9% (Fletcher et al. 1964; Brooks et al. 1984), or 70-85.9 mL/kg bw (Jain 1986). Blood volume increases in response to increased altitude (Bianca 1969). The mean specific gravity of whole blood is 1.042, with a range of 1.036-1.050 (Fletcher et al. 1964; Brooks et al. 1984). The osmotic pressure of serum colloids is 300 mm H₂O.

Mean plasma volumes range from 4.2% to 7.5% of bw, or 53-60.2 mL/kg bw (Jain 1986). Mean plasma specific gravity is 1.022, with a range of 1.018-1.026.

Erythrocyte Parameters

Erythrocyte parameters in the goat are labile, changing markedly over the goat's lifetime (Holman and Dew 1964, 1965a; Edjtehadi 1978). So-called normal or average values from the clinical laboratory, then, must be interpreted in the context of the animal's age. The decreases in hemo­globin (Hb) and packed cell volume (PCV) that occur dur­ing the first month of life may be due to iron deficiency anemia associated with a milk diet. The decrease can be prevented in kids by administering 150 mg of iron as iron dextran at birth (Holman and Dew 1966). Other age-related variations seen in erythrocyte parameters are shown in Table 7.1.

Seasonal changes are also noted in erythrocyte parame­ters, with greater red blood cell (RBC), Hb, and PCV values in late summer and fall than in winter and spring. Male goats tend to have higher RBC counts than females. Pregnancy has little effect on erythrocyte parameters, but PCV can decrease during the first five months of lactation.

Table 7.1 Erythrocyte parameters in the normal goat from selected reports worldwide.

States

Hb, hemoglobin; MCH, mean corpuscular hemoglobin; MCHC, mean corpuscular hemoglobin concentration; MCV, mean corpuscular volume; PCV, packed cell volume; RBC, red blood cell.

The goat has a small erythrocyte, with a diameter of 3.2-4.2 μm. As a result, standard methods of determining the PCV may overestimate it by as much as 10-15% because of inadequate centrifugation time. For the microhemato­crit method, centrifugation for a minimum of 10 minutes at 14 000 G is recommended for accurate results. The stand­ard Wintrobe method cannot generate sufficient relative centrifugal force to completely pack goat RBCs, even when centrifugation time is extended. As much as 20% of plasma can be trapped with the erythrocytes (Jain 1986). The standard erythrocyte sedimentation rate (ESR) is not appli­cable to the goat, because there is no settling of goat RBCs within the one-hour interval prescribed for the Wintrobe method. However, a decrease of 2-2.5 mm can be observed in normal goat blood held for 24 hours. It has been noted that when using automated, impedance-based blood ana­lyzers that are calibrated primarily for dogs or human beings, small goat RBCs may be miscounted as platelets, leading to the erroneous recording of RBC parameters sug­gestive of anemia (Jones and Allison 2007).

The small size of the goat RBC is also associated with the highest osmotic fragility among the domestic species. When exposed to hypotonic saline solutions, RBC hemoly­sis begins at concentrations of 0.62-0.74% and complete hemolysis occurs in saline concentrations of 0.48-0.60%. Pygmy goat erythrocytes are more fragile than those of Toggenburg goats on the basis of susceptibility to osmotic lysis, possibly because of differences in membrane compo­sition (Fairley et al. 1988).

Goat RBCs have only slight biconcavity. In blood smears, they lack a zone of central pallor and do not exhibit rouleaux formation, except perhaps at the edge of a thick smear. Normal cell shape is variable and poikilo- cytosis is common, with triangular, rod-shaped, pear­shaped, and elliptical cells frequently seen, especially in goats younger than 3 months of age. Sickle-shaped cells can occur in Angora goats, similar to those seen in deer (Jain et al.

Nucleated RBCs may be seen in newborn goats as old as 6 weeks of age, but are uncommon afterward. Reticulocytes are always absent or rare in healthy goats. Basophilic stippling of goat erythrocytes has been reported in experi­mental lead poisoning (Davis et al. 1976). The normal life span of circulating RBCs in the domestic goat, Capra hir- cus, is an average of 125 days, but as long as 165 days in a wild goat, the Himalayan Tahr (Hemitragus jemlaicus) (Kaneko and Cornelius 1962).

Hemoglobin

The goat has one embryonic Hb type that occurs very early in fetal development in erythroid precursors. A single dis­tinct fetal Hb (HbF) that comprises alpha and gamma glo­bin chains is present by approximately 40 days of fetal life and persists through birth, diminishing to zero by approxi­mately 50 days of age. Replacing HbF in the young growing goat is HbC, an Hb type peculiar to sheep, goats, and other members of the Caprini family, including the aoudad, Barbary sheep, and the mouflon. In the goat, by 50 days of age, from 80 to 100% of Hb is the HbC type. Hemoglobin C demonstrates decreased oxygen affinity compared with HbF. It is postulated that HbC is a physiologic adaptation that allows the relatively fast-growing kid to satisfactorily oxygenate tissues during rapid growth (Huisman et al. 1969). The marked poikilocytosis commonly observed in kids between 1 and 3 months of age may be associated with the switch to HbC during this period.

By 120 days, the final adult types of Hb, comprising alpha globin chains together with beta globin chains, have largely replaced HbC.

In addition to small amounts of HbC, three adult hemo­globins are commonly recognized and identified as A, B, and D, although a later publication identified the three adult hemoglobins as A, D, and E (Garrick and Garrick 1983). Adult hemoglobins are composed of both alpha and beta chains and globin chain production is controlled by at least five structural genes. Polymorphism of goat Hb occurs from variation in either beta or alpha chains. Phenotypic varia­tion occurs in the goat, with phenotypes AA, AB, BB, and AD previously reported as most common (Huisman 1974). Some breeds of goats may possess only a single adult hemo­globin phenotype, e.g., homozygous HbA, as reported in two breeds of Omani goats (Johnson et al. 2002). Another Hb type, HbD_malta, has been identified in high frequency in goats on Malta (Bannister et al. 1979). Hemoglobin poly­morphisms in goats, sheep and cattle have been reviewed (Pieragostini et al. 2010). An association between certain Hb phenotypes and disease resistance has been observed in the goat in relation to helminthiasis (Buvanendran et al. 1981). In a Finnish study of milking goats, increased hemoglobin concentration correlated with increased milk yields and decreased somatic cell counts, though there was no direct explanation for the relationship (Atroshi et al. 1986).

Response to Anemia

The goat demonstrates only a mild to moderate reticulo­cyte response to anemia. Red cell parameters have been measured after experimental induction of hemorrhagic anemia via controlled venisection that reduced mean RBC counts by more than 50% (Dorr et al. 1986). Reticulocyte counts before bleeding were between 0 and 0.5%. Maximum reticulocyte counts occurred six to eight days after bleed­ing, and ranged from 3.2 to 7.7%. A second reticulocyte peak was seen between 11 and 90 days, with maximum counts between 2.9 and 6.3%. Anisocytosis and persistence of macrocytes for four weeks after bleeding were more dra­matic indicators of regeneration than was reticulocytosis. The large size of newly released erythrocytes was reflected in a progressive increase in the mean corpuscular volume (MCV), which was highest from 25 to 29 days after bleed­ing, but did not always exceed the normal range of MCV reported for the species. Therefore, the MCV must be inter­preted carefully in establishing the presence of regenera­tive anemia. Hemoglobin and PCV levels returned to pre-bleeding levels by five weeks.

Jain et al. (1980) reported a marked poikilocytosis in response to experimentally induced hemorrhagic anemia in normal Angora goats exhibiting increased percentages of erythrocyte sickling. The percentage of fusiform (sickle) cells decreased as new distinct poikilocytic forms developed. Poikilocytosis was maximal between 8 and 13 weeks after bleeding. The change in erythrocyte mor­phology was attributed to the presence of HbC in regen­erative cells.

Blood Types

Most references to the caprine blood grouping system compare it to the sheep system, but specific similarities and differences are not always clear. Seven blood groups are known to exist in the sheep, identified as R-O, A, B, C, D, M, and X. The antigens of the R-O group are soluble substances, and naturally occurring anti-R antibodies may be found in R-negative sheep. The M system in the sheep is associated with variation in intracellular RBC potassium concentration, with animals homozygous for the Ma allele having greater intracellular potassium levels. High- potassium and low-potassium red cell types have also been recorded in goats, but no clear association with the M blood group system has been established, despite attempts to do so (Ellory and Tucker 1983). The B system is most complex, with at least 52 alleles involved in expression of B-group antigens. Using heteroimmune sheep reagents, at least five of the seven sheep blood groups have been confirmed in goats; A, B, C, M, and R. Multiple phenogroups occurred in the B system, similar to sheep (Nguyen 1977). However, additional studies using caprine alloimmune reagents have revealed additional blood groups in goats, including E and F (Nguyen 1990) as well as G, H, I, J, K, L, N, and Q (Vankan and Bell 1993).

As with other ruminant species, hemolytic testing is preferred to agglutination testing for blood typing work because of the inherent inagglutinability of erythrocytes in some individuals (Andresen 1984). Neonatal isoeryth- rolysis is not known in goats. However, hemolytic dis­ease has been reported in kids 1 week of age that had received bovine colostrum at birth (Perrin et al. 1988). This has also been observed in lambs receiving bovine colostrum.

In 1992, the American Dairy Goat Association (ADGA) instituted a voluntary blood typing program to assist in identification and parentage verification of registered goats. In 1998, ADGA replaced identification of individual goats and parentage by blood typing with DNA typing, because of the superiority of DNA technology and the lim­ited availability of antisera available for blood typing.

Transfusions

In sheep, it has been advised that R-positive blood not be used for transfusion to avoid early donor RBC destruction by isoantibodies (Andresen 1984). In practice, however, cross-matching of goat or sheep blood before single trans­fusions is not usually performed (Bennett 1983). Nevertheless, naturally occurring antibodies to RBCs can be present in goats and cross-matching may be advisable. Transfusion reaction rates of 2-3% have been reported (Fletcher et al. 1964). Cross-matching is indicated when­ever multiple transfusions are anticipated.

Transfused autologous RBCs have a half survival time of 8 days, while transfused allogeneic RBCs have a half sur­vival time of only 2.4-5.1 days (Gulliani et al. 1975). Sheep RBCs transfused into goats had a maximum average life span of 4.6 days (Clark and Kiesel 1963). Cross-species transfusions in clinical practice are not advised, though a recent preliminary study suggests that xenotransfusion of goats with cattle blood may be a therapeutic modality for the treatment of caprine anemia (Smith et al. 2021).

Goats are considered as a “large-spleened” domestic species. As such, the healthy goat can accommodate a blood loss of up to 25% of the red cell mass acutely, and up to a 50% loss over a 24-hour period. In such cases, fluid replacement is of more concern than RBC replace­ment. If the PCV drops below 20% in acute blood loss, then transfusion of whole blood or packed RBCs should be considered.

In chronic blood loss, as often occurs with parasitic dis­eases, the PCV may reach levels as low as 9% without overt clinical manifestations of anemia, so long as the animal is not decompensated by stress, activity, or concurrent dis­ease. Treatment of the underlying cause of anemia may be sufficient therapy without the need for blood transfusion. If the anemia is accompanied by profound hypoproteine­mia with clinical signs of edema and ascites, plasma trans­fusions may be indicated.

For transfusions, the safe volume of blood that can be collected from a healthy goat has been reported from as low as 6 mL/kg (Mitruka and Rawnsley 1981) to as high as 15 mL/kg bw (Bennett 1983). In practice, 10 mL/kg bw is a reasonable volume. A 4% solution of sodium citrate is a suitable anticoagulant for blood collection, using 50-100 mL per 400 mL of blood collected. Collection bags or bottles should be swirled continuously during blood collection, and blood administered through a filtered system to remove possible clots. Blood can be safely given to recipients in volumes of 10-20 mL/kg bw. Initially the rate of administration should not exceed 1 mL/kg/hr for the first 20 minutes, but then can be increased to 20 mL/ kg/hr if no complications are observed. A review of indi­cations, products, and procedures for transfusion in live - stock, including goats, has been published (Credille and Epstein 2016).

Leukocyte Parameters

The mean white blood cell (WBC) count of the goat is gen­erally reported to be 9000 cells/pL, with a range of 4000-14 000. However, total WBC counts and differential cell counts may vary significantly with age, as shown in Table 7.2. This is also true of the neutrophil-to-lymphocyte (N : L) ratio. The following mean N : L ratios have been reported for normal goats at different ages: 1 day old, 1.6 : 1; 1 week old, 0.8 : 1; 1 month old, 0.6 : 1; 3 months old, 0.3 : 1; 2 years of age, 1.1 : 1; and 3 years of age and older, 1.0 : 1 (Holman and Dew 1965b). These changes are par­tially due to a lower neutrophil count at birth that peaks at 1 month of age and then returns to and remains at birth levels after 3 months of age.

More significant is the dramatic two- to threefold increase in lymphocyte numbers that occurs from birth to 3 months of age. Lymphocyte numbers begin to decline again and remain roughly equivalent to neutrophil numbers throughout adulthood. Eosinophils, basophils, and monocyte counts did not change notably with age in this study (Holman and Dew 1965b). However, in a field study with samplings of 1000 goats in Mexico, eosinophil counts in kids younger than 7 weeks of age averaged 0.5% of the WBC count, but 4.3% in adults (Earl and Carranza 1980).

Table 7.2 Total leukocyte numbers and differential counts reported in normal goats.

WBC, white blood cell.

Band neutrophils may represent up to 2.5% of the neu­trophils in neonates as old as 6 weeks of age, but are rare or absent in health after this time. Lymphocytes in goats exhibit three distinct sizes: small, medium, and large. Young goats may exhibit two and a half times more small lymphocytes than large. It is postulated that the small lym­phocytes represent thymocytes and that small lymphocyte numbers decrease as thymic involution progresses (Earl and Carranza 1980). Morphologically, large lymphocytes may be confused with monocytes. The large lymphocyte is distinguished by a nuclear chromatin that is condensed in large, irregularly shaped clumps, compared with the looser, stringier chromatin pattern of monocytes.

Interpretation of Leukogram

Guidelines for interpretation of caprine leukograms have been suggested (Coles 1986). A WBC count more than 13 000/pL constitutes leukocytosis and a count less than 4000/pL, leukopenia. A neutrophil count greater than 7200/pL represents neutrophilia and a count less than 1200/pL, neutropenia. More than 100 band neutrophils/pL constitutes a left shift. Lymphocytosis is interpreted as a lymphocyte count greater than 9000/pL and lymphopenia as a count less than 2000/pL. Monocytosis is represented by a monocyte count more than 550/pL and eosinophilia by an eosinophil count more than 650/pL. Inflammatory responses in the goat often produce a neutrophilia with total WBC counts in the range of 22 000-27 000/pL. The maximum WBC count reported in a goat is 36 300/pL, observed in association with a kidney abscess (Jain 1986).

A mature neutrophilia is often seen in association with stress and chronic infections, particularly when abscesses develop, as in caseous lymphadenitis or mastitis. Acute bacterial infections can produce a moderate to severe neu­trophilia with some degree of left shift. Acute, severe coc- cidiosis in young kids is a frequent cause of marked neutrophilia and left shift.

Leukopenia is a common finding in infectious condi­tions of goats associated with the release of endotoxin. Transient leukopenia has been reported with experimental administration of staphylococcal enterotoxin B and Escherichia coli endotoxin (Van Miert et al. 1986). Leukopenia also occurs in theileriosis and tick-borne fever. Infection of WBCs is a fundamental part of the pathogen­esis of these diseases. In theileriosis, lysis of lymphocytes occurs when protozoal merozoites are released into the bloodstream and lymphopenia may be observed. In tick- borne fever, Ehrlichia phagocytophilia invade the cyto­plasm of granulocytes and monocytes, producing a marked leukopenia. Persistent leukopenia also has been reported in experimental heartwater disease (cowdriosis; Illemobade and Blotkamp 1978) and as a result of plant poisoning with Ipomoea carnea in goats in the Sudan (Tartour et al. 1974).

Neoplastic transformation of lymphocytes may occur in caprine lymphosarcoma. This condition is not common in goats, and leukemia is a rare clinical presentation when the disease does occur. Peripheral lymphadenopathy is the clinical presentation most likely to suggest the diagnosis of lymphosarcoma in goats, but it is not a consistent finding. Lymphosarcoma is discussed further in Chapter 3.

Platelet Parameters

The mean platelet count in the goat is generally considered to be 500 000/pL, with a range of 340 000-600 000/pL (Lewis 1976; Mitruka and Rawnsley 1981), though some clinical pathology laboratories suggest a platelet reference value range of 340 000-900 000/pL. One disparate study identified considerably lower counts in goats, with an aver­age count of 116 000/pL during the first 2 weeks of life, decreasing to 28 000/μL by 1.5 years of age and stabilizing at an average of 62 550/pL by 2 years of age (Holman and Dew 1965b). These findings have not been duplicated by others. Platelets appear in the peripheral blood, usually in clusters of varying size. The platelets themselves also vary in size and shape, but all contain prominent azurophilic granules that are evenly distributed throughout the cytoplasm.

Coagulation Parameters

Two studies of coagulation parameters in the goat have been reported (Breukink et al. 1972; Lewis 1976). Both gen­eral coagulation tests and specific coagulation factor assays were conducted. Reported values are summarized in Table 7.3.