As a result of the advances in veterinary medicine the use and availability of blood products has dramatically increased. Understanding the genetics and role of blood types in dogs and cats is important for veterinarians, breeders, and pet owners to appreciate, because of the impact blood type incompatibilities can have. Blood typing, in any species, is not only necessary to improve the safety and efficacy of transfusions, but the knowledge it provides can prevent fatal neonatal isoerythrolysis during the first days of life in cats. Methods for blood typing and crossmatching have become available for use in practice and in the laboratory.

BLOOD TYPES

Blood types refer to genetic markers on the surface of a red blood cell (erythrocyte) that are both species-specific and antigenic. Antigenicity refers to the likelihood the immune system will react and make antibodies, known as alloantibodies or isoantibodies, against the foreign substance; these antibodies can be detected in the plasma. A set of allelic blood types (two to several markers at the same gene locus) make up a blood group system. Species-specific antisera or chemical reagents, directed against either canine or feline red blood cell antigens, are used in in-vitro blood-typing methods. A positive result may be in the form of agglutinating or lysing red cells referred to as hemagglutination and/or hemolysis, respectively. Individuals lacking a particular red cell antigen may develop antibodies against that blood type. These alloantibodies are responsible for incompatibility reactions.

CANINE BLOOD TYPES

Dog blood types are commonly referred to as Dog Erythrocyte Antigens (DEA), followed by a number. There are at least a dozen DEA types known. A dog can be positive for a specific DEA, meaning the antigen of that blood type is present on the red cell surface, or can be negative, indicating the antigen is missing. The DEA 1 system appears to have more than one type, but DEA 1.1 in this blood group system has garnered the most clinical attention, as it is the most antigenic and is responsible for serious transfusion reactions. Approximately 50% of all dogs are positive for the DEA 1.1 antigen.

No clinically significant alloantibodies, or preformed antibodies against the blood type the dog is lacking, have been recognized in the dog prior to sensitization with cells bearing the foreign antigen. Sensitization would occur if blood positive for DEA 1.1 was transfused into a DEA 1.1 negative dog; the result would be the formation of strong alloantibodies against the DEA 1.1 antigen. A delayed transfusion reaction can be seen in as little as a week following the original mismatched transfusion. Subsequent transfusions with DEA 1.1 positive blood to a DEA 1.1 negative dog would be much more deleterious and result in life-threatening hemolytic reactions because of the preformed alloantibodies against the DEA 1.1 antigen. The role of DEA 1.1 blood typing is, therefore, crucial, especially for potential canine donors. If the blood type of the recipient is unknown, a DEA 1.1 negative dog is mandated. In-house blood-typing cards are available (DMS Laboratories, Flemington, NJ) and other laboratory methods are also being standardized.

Following a transfusion alloantibodies also may develop against other known or unknown blood types; these alloantibodies may be responsible for incompatibility reactions with subsequent transfusions. Blood compatibility testing, known as crossmatching, is directed at identifying possible incompatibilities against any blood type. More specifically, a crossmatch indicates the serologic compatibility or lack thereof between the intended donor and recipient.

FELINE BLOOD TYPES

Only one blood group system has been identified in the cat. Three blood types make up the AB blood group system: type A, type B, and type AB. The inheritance pattern, natural occurrence of alloantibodies, and varied breed distribution are all of considerable importance to breeders as well as veterinarians. The A allele is dominant over the B allele. AB is a rare blood type and appears to be recessive to A, but codominant to B. Type A cats are by far the most prevalent world wide, but among certain purebred cats, the frequency of type B tends to be much higher. Some feline breeds with higher type B frequencies include the Devon Rex, British Shorthair, Exotic Shorthair, Turkish Van and Turkish Angora, although the percentages vary between zero to 60% depending on the breed. Among domestic shorthair cats, higher percentages of type B cats can be seen in certain geographic regions relative to others.

In contrast to dogs, cats do possess naturally occurring alloantibodies against the blood type antigen they are lacking. Type B cats have strong anti-A antibodies, while type A cats have generally weak anti-B alloantibodies. Given the potentially fatal hemolytic transfusion reaction that can result from a mismatched transfusion, all feline blood donors as well as recipients should be typed or, if typing is not available, crossmatched.

In addition, neonatal isoerythrolysis is caused by the naturally occurring anti-A alloantibodies present in a type B queen's colostrum. If type A or even type AB kittens are born to a type B queen that was mated to a type A or AB tom, these kittens would likely succumb due to the passive acquisition of strong anti-A antibodies present in the colostrum. These anti-A alloantibodies are only absorbed during the first 16 hours and cause lysis of the kittens' red blood cells. Anemia, jaundice, darkly-pigmented urine, anorexia, or sudden death are typical features of neonatal isoerythrolysis. Therefore, blood typing of all cats used for breeding and selection of compatible mates is necessary to avoid the loss of kittens within the first week of life. Blood typing cards are available for use in practice (DMS Laboratories, New Jersey) and others are being developed. Confirmatory typing and crossmatching procedures are currently being developed. The Transfusion Laboratory at the Ryan Veterinary Hospital of the University of Pennsylvania accepts samples for the assessment of feline and canine blood types and resolving difficult compatibility issues (www.vet.upenn.edu/penngen).

Blood type A and B frequency in cats (*breeds with rare AB cats)