Clinicians have long sought to identify a reliable threshold for the administration of red blood cells (RBCs), a term best known as the transfusion trigger. Nevertheless, despite a wealth of research in people, a specific clearly defined transfusion trigger has not been determined. Clinical signs such as altered mentation, weakness, tachycardia, tachypnea, cool extremities, decreased pulse pressure, anuria, and pale mucous membranes (Figures 23.1 and 23.2) have been used to evaluate the hemodynamic stability of food and fiber animals (Hunt and Wood 1999; Divers 2005). In addition to clinical signs, laboratory tests are frequently used as an adjunct to evaluate tissue oxygenation. Tests most often used include packed cell volume (PCV) or hematocrit (HCT), hemoglobin (HGB) concentration, protein concentration, blood lactate concentration, anion gap, base excess, mixed venous oxygen saturation (S_vO₂), central venous oxygen saturation (S_cvO₂), and oxygen extraction ratio (O₂ER) (Table 23.1) (Hunt and Wood 1999; Gutierrez et al. 2004; Divers 2005; Hurcombe et al. 2007; Carson et al. 2012a; Dellinger et al. 2013; Balcomb and Foster 2014; Nemeth et al. 2014).

Historically, HGB concentration has been used as the principal transfusion trigger in human medicine and much work has revolved around establishing the appropriate HGB concentration at which to initiate a blood transfusion (Divers 2005; Carson et al. 2012a,b). Experimentally, myocardial oxygenation and cognitive ability become impaired at a HGB concentration <5 g/dL (50 g/L) or a PCV of approximately 15% (Madjdpour and Spahn 2007). At this point the risk of global tissue hypoxia and subsequent organ compromise increases. Due to risks of morbidity and mortality associated with transfusions, several studies in human medicine have evaluated liberal (HGB < 10 g/dL [100 g/L]) versus restrictive (HGB < 7 g/dL [70 g/L]) transfusion strategies, with the majority of studies favoring more restrictive criteria (Hebert et al. 1999; Vincent et al. 2002; Vallet et al. 2007; Klein et al. 2007; Napolitano et al. 2009; Carson and Kuriyan 2010; Lelubre and Vincent 2011; Carson et al. 2012a; Salpeter et al. 2014). Thus, an HGB concentration of <7 g/dL (70 g/L) is commonly used as the transfusion trigger for humans (Hebert and Tinmouth 2011).

Similar studies to determine the most appropriate HGB concentration transfusion trigger have not been performed in food and fiber animals. It has been suggested that a HGB concentration of <5 g/dL (50 g/L) could serve as a transfusion trigger in food and fiber animals (Divers 2005). Another potentially useful test for determining the need for transfusion is PCV. Clinical experience and expert opinion have led to recommendations for RBC transfusions when the PCV decreases below 15–20% in animals with acute blood loss and 10–12% in animals with chronic blood loss (Hunt and Moore 1990; Hunt and Wood 1999; Divers 2005; Balcomb and Foster 2014). The recommended PCV is lower for chronic blood loss due to adaptations the body is able to make to improve cardiac output and oxygen delivery (Divers 2005). It is important to remember that in cases of acute blood loss it can take several hours for the PCV and plasma protein to decrease, therefore these laboratory measurements might not accurately reflect the overall clinical picture or need for intervention (Mudge 2014).

In addition to the lack of information regarding appropriate HGB concentration and PCV thresholds for RBC transfusions, other laboratory tests used routinely in people have not been evaluated for use in food and fiber animals, are cost prohibitive, and are not easily adaptable to field situations because of the specialized equipment required. However, with handheld blood gas and biochemical analyzers becoming more widely available, certain parameters (e.g., base excess and lactate) might become more widely used. Table 23.2 provides some suggested cut-off values for selected clinicopathologic triggers for transfusion in food and fiber animals.

PCV, packed cell volume; HGB, hemoglobin; S_vO₂, mixed venous oxygen saturation; S_cvO₂, central venous oxygen saturation; O₂ER, oxygen extraction ratio.

Animals with comorbidities such as sepsis, cardiovascular disease, respiratory disease, or pregnancy can require transfusions at a different trigger than an animal that is otherwise healthy (Madjdpour and Spahn 2007). Therefore, the decision to transfuse RBCs should be based on a combination of factors that include the animal’s history, clinical signs, and laboratory data. Furthermore, serial measurements of the aforementioned parameters will have more value in determining the need for transfusion than measurements performed at a single time point (Tennent-Brown 2014). Thus, regular re-evaluation of the patient’s clinical status is an important part of the therapeutic plan.

Serum

Serum is the component of blood separated after clotting has occurred. Unfortunately, little work has been done to evaluate the effect of serum or serum products on the health of food and fiber animals. Several available products contain antibodies against Pasteurella spp., Trueperella pyogenes, E. coli, Salmonella spp., and Clostridium perfringens type C and D (Anonymous 2013). However, there is little data to confirm the effectiveness of these products in clinical situations. Studies performed in calves in the late 1980s using LPS infusions found that an antiserum directed at a mutant E. coli did not mitigate the effects of LPS on any clinical parameters tested (Morris et al. 1986a,b). More recently, a study evaluated the ability of serum to increase IgG in calves with FPT and found that the administration of 500 mL of a commercially available serum product significantly increased serum protein concentrations, but had no effect on serum IgG concentration. In addition, the proportion of calves with FPT after transfusion was 82%, making this product an unacceptable source of IgG for calves with FPT (Chigerwe and Tyler 2010). Therefore, the use of serum or hyperimmune serum products in food and fiber animals cannot be supported.

Blood group antigens are glycoproteins and glycolipids located on the surface of RBCs (Reid and Mohandas 2004). These antigens provide membrane structural integrity and membrane molecular transport as receptors for extracellular ligands, as adhesion molecules, for recognition of self by the immune system, and, in some cases, as genetic markers (Reid and Mohandas 2004). In ruminants, blood groups are detected by hemolytic tests because the RBCs of these species are not prone to agglutination (Divers 2005; Andrews and Penedo 2010). Porcine blood groups can be detected by either agglutination or hemolytic tests (Andrews and Penedo 2010). Food and fiber animals rarely have naturally occurring alloantibodies to the species blood groups that the individual animal does not have, making the risk of a reaction to a first transfusion in these patients low (Divers 2005; Andrews and Penedo 2010; Balcomb and Foster 2014). Nevertheless, transfusion reactions do occur and an understanding of the various blood groups and types in food and fiber animals is necessary. Indeed, should a patient require additional transfusions during treatment, crossmatching is recommended as sensitization does occur and can affect the lifespan of the transfused cells, the systemic health of the patient, or both. Historically, a single letter designation has been used to denote particular blood group systems in livestock species. The more recent nomenclature convention is to precede the specific system with the letters EA (erythrocyte antigen) and this is the terminology used in the rest of the chapter. Table 23.3 summarizes blood groups recognized in food and fiber animals.

EA, erythrocyte antigen.

Cattle

Studies investigating the blood groups of cattle began in the 1940s and served as a model for the investigation and determination of blood groups in other food and fiber species. Eleven major blood groups have been identified in cattle: EAA, EAB, EAC, EAF, EAJ, EAL, EAM, EAR, EAS, EAT, and EAZ (Stormont 1962; Andrews and Penedo 2010). In addition to the 11 blood groups, more than 70 blood group factors have been identified, with most of the variation found in the EAB and EAC systems (Andrews and Penedo 2010). The EAJ antigen, unlike other blood group antigens, is a glycolipid antigen found in plasma, only attaching to RBCs when sufficiently high concentrations are present (Andrews and Penedo 2010; Balcomb and Foster 2014). It is important to note that cattle that are J antigen negative can have naturally occurring anti-J antibodies in circulation and should an EAJ negative cow be transfused with blood from an EAJ positive donor significant reactions could occur (Divers 2005).

Sheep

Eight blood group systems have been described in sheep including EAA, EAB, EAC, EAD, EAM, EAR, and EAX (Stormont 1982; Andrews and Penedo 2010). Additionally, 22 different blood group factors are recognized, with the most variation found in the EAB system. The R antigen, like the J antigen in cattle, is found in plasma and only bound to the RBC membrane when in high concentrations (Andrews and Penedo 2010).

Goats

Blood group systems in goats are far less understood than those in other food and fiber animal species. Six blood group systems have been identified so far: EAA, EAB, EAC, EAE, EAF, and EAR. The R antigen is analogous to the R antigen found in sheep (Nguyen 1990).

Pigs

Sixteen blood groups systems have been identified in pigs: EAA, EAB, EAC, EAD, EAE, EAF, EAG, EAH, EAI, EAJ, EAK, EAL, EAM, EAN, EAO, and EAP (Stormont 1982; Andrews and Penedo 2010). Groups E and M are similar to groups B and C in cattle in that the majority of antigen diversity is found in these groups. Antigens A and O are soluble compounds in the serum and saliva of A and O positive animals, respectively. They attach to the RBC membrane within the first few weeks after birth (Andrews and Penedo 2010).

New World camelids: llamas and alpacas

Six blood groups systems have been identified in New World camelids: EAA, EAB, EAC, EAD, EAE, and EAF (Penedo et al. 1988). Unfortunately, little is known about blood group variation in these animals.