Plus-Hex CLINICAL
Introduction
Blood transfusion is a life-saving intervention in clinical practice. A thorough understanding of feline blood groups is essential to ensure transfusions are carried out safely and to reduce the risk of adverse reactions in feline patients.
In veterinary medicine, transfusions are often used as an emergency measure in acute cases, such as acute blood loss due to trauma, as well as in the management of more chronic conditions such as immune-mediated haemolytic anaemia; they can also be considered in cases of neonatal isoerythrolysis [ 1 ].
Patients with a reduced circulating blood volume have a lower than normal red blood cell count, resulting in reduced oxygen delivery to the tissues [ 2 ]. If this is uncorrected or persists over a prolonged period, it can cause a hypoxic state, resulting in cell damage, multiple organ dysfunction and even death.
Several clinical signs can be evident in patients experiencing reduced tissue perfusion [ 2 ]. These can include the following:
• Reduced mentation
• Poor cardiovascular stability
• Tachycardia or bradycardia( specifically in cats)
• Hypotension and weak peripheral pulses
• Pale or white mucous membranes
• Delayed capillary refill time
• Hypothermia due to compensatory vasoconstriction
• Tachypnoea.
Blood types
Cats have three main blood types: 1. Type A 2. Type B 3. Type AB, also more recently known as Type C [ 3 ].
Type A is the most common and occurs in up to 99.7 % of domestic shorthair cats and most pedigree breeds, such as Siamese. Type B is the next most common blood group and is predominantly found in pedigree breeds such as the British shorthair and Birman, where it occurs in up to 30 % of their populations. To be classed as type B, a cat must have inherited the recessive B allele from both parents. Type C is considered to be a rare blood group, occurring in less than 1 % of the feline population, but is again more common in pedigree breeds, with up to 18 % of the ragdoll population believed to be type C. Additionally, the presence of the
Mik antigen was discovered in 2007, and information on this antigen is still being gathered [ 4 ].
Blood types differ due to genetically determined antigens present on the surface of red blood cells( RBCs). An alloantigen is a type of antigen that varies among members of the same species. In other words, different cats can have slightly different versions of the same antigen.
The alloantigen present on type A RBCs is N-glycolylneuraminic acid. On type B RBCs, it is N-acetylneuraminic acid. On type AB, also now referred to as type C, both alloantigens are present and the plasma does not contain alloantibodies to type A or B blood [ 5, 6 ]. Mik is an erythrocyte antigen that has been discovered on the RBCs of some domestic shorthair cats, which can cause alloantibodies to form. Cats with the Mik antigen are classed as Mik-positive and those without it are classed as Mik-negative. The presence or even the absence of this antigen can trigger an immune response resulting in a transfusion reaction when a cat receives a blood transfusion from an incompatible donor [ 4 ].
Incompatible blood groups
As described above, feline blood types are genetically determined and characterised by the presence of specific surface antigens on erythrocytes, with naturally occurring alloantibodies formed against antigens that are not present on the individual ' s own RBCs. Transfusion of blood of incompatible types may result in an immune-mediated haemolytic transfusion reaction [ 6 ].
For example, cats with blood type B possess naturally occurring anti-A antibodies and therefore must not receive type A blood. Similarly, type A cats produce anti-B antibodies and are incompatible with type B blood donors. Cats with type C blood lack antibodies to both A and B antigens. Ideally, these patients should receive blood from a direct type match [ 6 ]. In the absence of compatible donors for this blood group, type A packed red blood cells( pRBCs) are preferred; however, if supplies are limited, blood of either type A or B may be used following satisfactory cross-matching.
Cats expressing the Mik antigen should, where feasible, be transfused with blood from donors matched for Mik status, whether Mik-positive or Mik-negative [ 4 ].
These basic principles of blood typing and transfusion are also applicable to the breeding of cats. Breeding of cats with incompatible blood types can cause fatalities in litters, known as neonatal isoerythrolysis or kitten erythrolysis, where neonates may present with‘ fading kitten syndrome’ [ 6 ]. In essence, when type A or type C males are bred with type B queens, the type A allele will be dominant and will produce a litter of kittens with blood type A. If this litter
Volume 41( 3) • June 2026
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