Exotic small mammals such as ferrets, rabbits, and rodents are popular companion animals. Due to their popularity as household pets, veterinary professionals are often called upon to provide advanced veterinary care including, but not limited to, administering blood transfusions. Given the paucity of information in the veterinary literature pertaining to transfusion medicine in exotic species, common principles of blood transfusion medicine and blood substitute therapy in mammals such as dogs and cats can sometimes be extrapolated to exotic small mammals. However, the difficulties of catheter placement, shortage of blood products, and challenging blood collection techniques discourage many clinicians from performing transfusions in exotic pets. Further studies are needed to gather information about exotic animal transfusion medicine. Although there is sometimes little choice, caution should be taken when using blood substitutes and providing whole blood transfusions to these animals.

Blood transfusions are generally indicated to replace a deficiency of blood constituents such as red blood cells (RBCs), plasma, clotting factors, and platelets. Frequently, the need for blood transfusions is acute, such as with hemorrhage due to trauma, surgery, or massive hemorrhage resulting in hypovolemia. Currently, the only option for exotic small mammals is a whole blood transfusion. Acute blood loss between 20% and 30% of the total blood volume in most patients is considered critical and will likely necessitate a transfusion. Other conditions sometimes requiring blood transfusions include, but are not limited to, coagulopathies, hypoproteinemia, decreased oxygen-carrying capacity, hemolysis secondary to hemolytic anemia (e.g., lead toxicosis), immune-mediated diseases, reduced hematopoiesis associated with chronic kidney disease, toxin exposure, and neoplasia (Box 25.1).

Proper donor selection is pertinent for a successful transfusion. It is important to ensure that the donor is a healthy young to middle-aged adult in good body condition. Ideally, donors are male or nulliparous females. The donor should have no previous history of blood transfusions or transmissible diseases. With the exception of ferrets, most exotic small mammals do not have standard vaccination protocols. Donor ferrets should be fully vaccinated prior to donation. Lastly, and arguably most importantly, the donor should be the same genus and species as the recipient. RBC destruction and transfusion reactions are greatly increased when blood from one species is transfused into another species (e.g., rabbit to mouse) (Lichtenberger 2004). Proper donor selection in any species will minimize the risk of transfusion-associated complications, including reactions.

A thorough physical examination, complete blood count, and biochemistry profile should be performed prior to donation. Blood values should be within normal limits, including a PCV >40% (Lichtenberger 2004). In some cases, a patient might be too small to perform a complete blood count and biochemistry panel prior to donation. When this is the case, minimally, a PCV, total solids, blood glucose, and blood urea nitrogen should be performed.

A comprehensive physical examination should be performed prior to blood collection. The exotic small mammal physical examination is performed similarly to that in dogs and cats. All items needed to perform the physical examination should be within arm’s reach before the patient is removed from the enclosure. Exotic small mammals can become easily stressed, therefore it is essential to keep the “time in hand” to a minimum. A visual physical examination should be performed prior to removing the patient from the cage or carrier. The visual examination will allow assessment of the overall mentation of the animal.

The physical examination is often started by first obtaining the patient’s weight and temperature. Many exotic small mammal patients are less than 2–4 kg, therefore it is best to use a gram or pediatric scale that can provide accurate weights to 1 gram. Obtaining a temperature can be challenging, but a rapid digital thermometer is generally the quickest and best method (unless the patient is too small).

Blood types are defined by inherited antigens located on the surface of RBCs. The antigens are made up of various substances such as carbohydrates, proteins, glycoproteins, and glycolipids. These genetic markers are species-specific and vary in immunogenicity and clinical significance. Antigens contribute to the recognition of self. They elicit the production of antibodies once introduced into the circulation of an animal lacking these antigens. It is ultimately the antibodies in the serum of the recipient that have the ability to bind to these antigens, which will lead to acute or delayed hemolytic transfusion reactions.

To date, many species have documented blood types including primates, birds, dogs, and cats. While a great deal of information exists in these species, research is lacking in most exotic animals. The domestic chicken has been vastly studied, with 28 blood groups identified in this species (Hohenhaus 2004). Unfortunately, blood types have not been described in other companion or wild birds, nor have they been described in most exotic small mammals. Blood groups have not been vastly studied or identified in rabbits, guinea pigs, chinchillas, or rats.

The European or domestic ferret (Mustela putorius furo) is unique among the mammals that have been studied. The serum and RBCs of 212 randomly paired ferrets was examined and revealed no evidence of naturally acquired blood group antibodies. Six pairs of ferrets were transfused twice, 34 days apart, and all ferrets were tested 21 days after the first transfusion and then 10 and 30 days after the second transfusion. At no point were RBC antibodies detected; therefore, blood groups found in other animals either do not exist in the domestic ferret or represent antigen systems too weak to elicit measurable responses. This information suggests that transfusions in ferrets pose little clinical risk, even without crossmatching, because they have no detectable blood group antigens or antibodies (Manning and Bell 1990).

Because blood transfusions have the potential to introduce foreign antigens upon administration to the recipient, crossmatching should ideally be performed in all patients prior to transfusion. Due to the lack of knowledge regarding blood types in most species of exotic small mammals, crossmatching is recommended prior to transfusion, especially when previous transfusion history is unknown.

Crossmatches are agglutination and hemolysis tests performed to assess donor–recipient compatibility, observing interactions between the donor and recipient RBCs and antibodies contained in the plasma. A major crossmatch detects the presence of pre-existing antibodies in the recipient’s plasma that could produce an immunologic reaction. Pre-existing antibodies could be either naturally occurring or antibodies produced after a previous incompatible transfusion. In ferrets, attempts have been made to induce an antibody response experimentally, with no success (Manning and Bell 1990; Hillyer 1995). Sensitization after blood transfusion has not been well studied in other exotic small mammals.

Since naturally occurring alloantibodies or induced antibodies have not been identified in the domestic ferret, an immunologic transfusion reaction is unlikely to occur, and multiple transfusions from the same donor can be administered without a crossmatch. Because other exotic small mammal species have not been well studied, it is advisable to perform a crossmatch prior to administering any blood transfusion. A compatible crossmatch reduces, but does not eliminate, the risk of a transfusion reaction; therefore, patients must still be monitored closely during and immediately after the transfusion.