Transfusion support is also critical for the feline patient, most commonly to correct anemia and less often bleeding. Nevertheless, blood transfusions are overall still less frequently administered to cats than dogs for a variety of reasons. The peculiarities of feline blood types, blood collection, and transfusion will be presented.

Compared to canine transfusion medicine, cats can tolerate anemia better, they still get somewhat less medical attention, except for rodenticide toxicity and hepatopathies they bleed less severely, recruiting healthy donors is more difficult (occult heart disease, viral infections), blood collection requires sedation and special small bag collection systems, component therapy is less commonly practiced in clinics, cats have important naturally occurring alloantibodies and may experience life-threatening complications with a first transfusion, and the anemic cat is more sensitive to volume overload. There is no specific trigger PCV, but rather the overall clinical picture with a PCV of <20% is used.

Blood Typing

The major feline blood group system is known as the feline AB blood group system and contains 3 alleles: type A, type B, and the extremely rare type AB (except Ragdolls). Type A is dominant over B. Thus, cats with type A blood have the genotype a/a or a/b, and only homozyguos bib cats express the type B antigen on their erythrocytes. In the extremely rare AB cat, a third allele recessive to the A allele and/or codominant to B allele leads to the expression of both A and B substances. AB cats are not produced by mating of a type A to a type B cat unless the A cat carries the rare AB allele. Cats with type AB blood have been seen in many breeds and domestic shorthair cats.

Most domestic shorthair cats have type A blood, but the proportion of type B cats can be substantial in certain geographical areas. The frequency of A and B blood types varies greatly between different breeds, but likely not much geographically in purebred cats. Kitten losses due to A-B incompatibility and changes in breeding practices influence the frequency of A and B in various breeds. Most blood donors have type A blood, but some places also keep cats with the rare type B and type AB as donors. All blood donors must be typed. Naturally-occurring alloantibodies have been well documented in type A and type B cats and require that blood typing be performed prior to both blood transfusion and breeding to assure appropriate blood compatibility.

Cats have naturally-occurring alloantibodies. All type B cats have very strong naturally-occurring anti-A alloantibodies, which can be detected by hemolysis and hemagglutination assays. Kittens receive alloantibodies through the colostrum from type B queens and develop high alloantibody titers (>1:32) after a few weeks of age. These alloantibodies are strong hemolysins and hemagglutinins, and are of the IgM and, to a lesser extent, IgG classes. They are responsible for serious transfusion reactions and neonatal isoerythrolysis in type A or AB kittens born to type B queens. Type A cats have weak anti-B alloantibodies, and their alloantibody titer is usually very low (1:2), nevertheless they can also cause hemolytic transfusion reactions, but have not been associated with NI. Type AB cats have no alloantibodies. Furthermore, additional blood group systems have been identified such as the common Mik red cell antigen in domestic shorthair cats and Mik-negative cats may also produce naturally occurring alloantibodies.

Serological testing relies on identification of surface antigens, leading to agglutination and hence can distinguish A, AB, or B phenotypes. Several different reagents may be used but monoclonal antibodies against the type A and type B antigen are currently used in typing kits. A genetic test is also available for identification of the B allele, allowing identification of type B cats and carriers of this allele, but not distinguishing A or AB phenotypes.

Table. Blood type A and B frequency in cats in certain countries and breeds*

*Ignoring the rare AB cats in many breeds with type B cats

There are no universal donor cats. Donor and patient need to be typed, even if it is “only” a domestic shorthair cat. Simple AB blood typing cards (DMS Laboratories, 2 Darts Mill Road, Flemington, NJ) and chromatographic strip cartridges (Alvedia DME, Lyon, France and recently DMS) are available for in practice use.

Blood Crossmatching Tests

Blood incompatibilities have been recognized related to the AB blood group system and following blood transfusion through crossmatching cats or as a result of acute hemolytic transfusion reactions. Standard laboratory tube and gel column crossmatching techniques, but also in-clinic gel tube (DMS) and strip kits are now available. Screening feline blood donors and patients for the presence of naturally occurring alloantibodies (AB and Mik systems) prove necessary in clinical practice. The presence of autoagglutination or severe hemolysis may preclude the crossmatch testing.

The major crossmatch tests for alloantibodies in the recipient’s plasma against donor cells, whereas the minor crossmatch test looks for alloantibodies in the donor’s plasma against the recipient’s red blood cells. Mixing a drop of donor/recipient blood with donor/recipient plasma will detect A-B incompatibilities if typing is not available. However, proper techniques for crossmatching and experience are required to detect other less severe incompatibilities. A major crossmatch incompatibility is of greatest importance because it predicts that the transfused donor cells will be attacked by the patient’s plasma, thereby causing a potentially life-threatening acute hemolytic transfusion reaction. As fatal reactions may occur with <1 ml of incompatible blood, compatibility testing by administering a small amount of blood is not appropriate. This has been shown in experimental studies to result in fatal reactions. The major and minor crossmatch can show incompatibility prior to any transfusion due to the presence of naturally occurring alloantibodies in cats, not only for the AB but also the Mik and possibly other blood group systems.

Previously transfused cats should always be crossmatched, even when receiving blood from the same donor. The time span between the initial transfusion and incompatibility reactions may be as short as 4 days and lasts for many years (i.e., years after the last transfusion alloantibodies may be present). Obviously, a blood donor should never have received a blood transfusion to avoid sensitization.

Xenotransfusion

Occasionally, anemic cats are given canine blood because either no feline blood is available or the feline blood is incompatible (AB, mik, and other mismatch). In a recent study by the author’s laboratory (Euler et al. 2016) we determined that canine blood is incompatible and very short-lived (<4 days) in cats. Therefore, we do not recommend such xenotransfusions (Euler et al. 2016). Apparently, Oxyglobin, a highly purified bovine hemoglobin solution should be again shortly available in the USA as it has been in Europe.

Feline Blood Donors

There are few commercial blood banks that offer feline blood products. Many hospitals have a small in-house colony and/or involve the staff’s or client’s cats. Healthy, young adult (1–8 years), good tempered cats of at least 4 kg lean body weight can be recruited. Due to the infectious disease risks indoor cats free of fleas and intestinal parasites are selected. A freely roaming cat in a veterinary hospital would not be a good donor candidate because of the potential of having acquired some infections from patients.

Blood donors must have no history of prior transfusion; have been regularly vaccinated and are healthy as determined by history, physical examination, and laboratory tests (complete blood cell count, chemistry screen, and fecal parasite examination every 6–12 months) as well as free of infectious diseases (testing depends on geographic area but may include regular FeLV, FIV, FIP, hemomycoplasma, Cytauxzoon testing. Donors should receive a well-balanced, high performance diet, and may be supplemented twice weekly with ferrous sulfate (Feosal, 10 mg/kg), if bled several times a year. Packed cell volume (PCV) or hemoglobin {Hb) should be >30% and >10 g/dl in cats.

Blood Collection

Cats are regularly sedated e.g., with a combination of ketamine (10 mg), diazepam (0.5 mg), and atropine (0.04 mg) by intravenous injection. Some sedatives, such as acepromazine, interfere with platelet function and induce hypotension, hence they should not be used. Blood is collected aseptically by gravity or blood bank vacuum pump from the jugular vein over 5 to 10-minute period. Large plastic syringe containing 1 ml CPD-A or 3.8% citrate per 9 ml blood and connected to a 19-gauge butterfly needle is commonly used for cats. This represents an open collection system in which connections allow exposure of blood to the environment; because of the potential risk for bacterial contamination, blood collected via an open system should not be stored for more than 48 hours. The maximal blood volume to be donated is 40–50 ml blood (one typical feline unit) per ≥5 kg cat. We have developed a closed blood collection system that permits component preparation into packed red blood cells and fresh frozen plasma as well as storage (28 days of red cells, 1-year FFP). Blood components are prepared from a single donation of blood by simple physical separation methods such as centrifugation generally within 4 hours from collection.

Blood Administration

The regular principles used in transfusing dogs are applied in cats. No food is given during the transfusion, and blood is administered separately without any drugs or other fluids. Because of the small volumes shorter tubing with a small filter are used instead of the large infusion sets. Despite assuring blood compatibility particular attention is given to the first few milliliters infused. Monitoring is done like in dogs. Transfusion reactions may be related to blood type incompatibilities but also allergic reactions, physical hemolysis, hypocalcemia, and infection.

Author’s studies were supported in part by grants from the National Institutes of Health (OD 010939) and the Winn Feline and other Foundations. The author is the director of the non-for-profit PennGen Laboratory which is offering genetic, hematological and blood typing, and compatibility testing.