Advances in veterinary medicine in recent years have dramatically increased the availability and use of canine and feline blood products. Understanding the role of blood types in dogs and cats and the inheritance of blood types is therefore important for veterinarians, breeders, and pet owners. Blood typing, in any species, is necessary to improve the safety and efficacy of transfusions. In cats, the knowledge which blood typing provides can also prevent fatal neonatal isoerythrolysis during the first days of life. Methods for blood typing and crossmatching have become available for use in the laboratory and in clinical practice.

Blood Types

Blood types represent genetically determined markers on the surface of red blood cells (erythrocytes) and are both species-specific and antigenic. Antigenicity refers to the likelihood that the immune system will react and make antibodies, known as alloantibodies or isoantibodies, against the foreign substance. These antibodies can be detected in the animal's plasma.

A blood group system is made up of a set of allelic blood types (i.e. two or more markers at the same gene locus). Both species-specific antisera and chemical reagents, directed against specific red blood cell antigens, are used in in-vitro blood-typing methods. A positive result occurs if there is clumping (hemagglutination) or rupture (hemolysis) of red blood cells exposed to the antiserum or typing reagent. Individuals who lack a particular red cell antigen may develop antibodies against the blood type containing that antigen if sensitized to it via a mismatched transfusion. These alloantibodies are responsible for incompatibility reactions.

Canine Blood Types

Canine blood types are commonly referred to as Dog Erythrocyte Antigens (DEA), followed by a number. There are at least a dozen DEA types known, although currently we can only test for a few. In addition to the known DEA systems, the University of Pennsylvania has recently identified a novel canine blood group system known as Dal. Dal may be missing in an unknown proportion of Dalmatians.

A dog can either be positive or negative for each specific DEA. If positive, the antigen of that blood type is present on the red cell surface and if negative, the antigen is missing. The DEA 1 system appears to have several subtypes, namely DEA1.1, DEA1.2 and possibly DEA1.3. DEA 1.1 has garnered the most clinical attention as it is the most antigenic and is responsible for serious clinical transfusion reactions. Approximately 50% of all dogs are positive for the DEA 1.1 antigen.

In dogs without prior sensitization to cells bearing a foreign antigen, no clinically significant alloantibodies have been recognized against blood types different to the individual's own type. Sensitization will occur if DEA 1.1 positive blood is transfused into a DEA 1.1 negative dog. This results in the formation of strong alloantibodies against the DEA 1.1 antigen. A delayed transfusion reaction occurs as the body builds up these alloantibodies over time and destroys the transfused red blood cells still in circulation. Delayed transfusion reactions can be seen in as little as a week following the original mismatched transfusion. Subsequent transfusions with DEA 1.1 positive blood to an already sensitized DEA 1.1 negative dog are much more serious. In the time since the mismatched transfusion was administered, the body has formed alloantibodies. These antibodies are now available to bind to the foreign antigen on the DEA1.1 positive cells and destroy the red cells resulting in life-threatening hemolytic reactions. The role of DEA 1.1 blood typing is therefore crucial to ensure that donor and recipient have the same blood type. If the blood type of the recipient is unknown a DEA 1.1 negative donor is essential to avoid sensitizing the recipient in case they are DEA1.1 positive. Blood-typing cards are available for in-house use (DMS Laboratories, Flemington, NJ) and a novel gel test for laboratory use has recently been standardized.

Correctly typed and matched blood only takes into account the DEA 1.1 antigen. Thus, following a DEA 1.1 matched transfusion, alloantibodies may still develop against other known or unknown blood types. These alloantibodies may become responsible for incompatibility reactions with subsequent transfusions. Blood compatibility testing, known as crossmatching, is used to identify possible incompatibilities against any blood type. More specifically, a crossmatch indicates the serologic compatibility, or lack thereof, between the recipient and the intended donor.

Feline Blood Types

Only one blood group system, the AB system, has been well defined in the cat. Three blood types make up the AB blood group system: type A, type B, and type AB. Cats must have one of these blood types, there is no type "O". The "a" allele is dominant over the "b" allele. Type AB is a rare third allele which appears to be recessive to "a", but codominant to "b". Type A cats are by far the most prevalent world wide in domestic cats, but in some pure breeds the frequency of type B cats can be greater than 50%. Although the percentage of type B cats does vary depending on the breed, some breeds are known to have higher type B frequencies. These breeds include the Devon and Cornish Rex, British and Exotic Shorthair, the Turkish Van and the Angora. The majority of domestic shorthair cats have blood type A, but higher percentages of blood type B can be seen in certain geographic regions relative to others. Although the clinical relevance is not yet known, our laboratory recently identified a new feline blood group called Mik.

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 will result from a mismatched transfusion, all feline blood donors, as well as recipients, should be blood typed. If blood typing is not available, the recipient and intended donor should be crossmatched to ensure blood type compatibility.

In addition, neonatal isoerythrolysis is caused by the naturally occurring anti-A alloantibodies present in a type B queen's colostrum. Since type B is a recessive blood type, if a type B queen is mated to a type A or AB tom, we expect from half to all of her kittens to be blood type A (or AB). These kittens will passively acquire strong anti-A antibodies via the colostrum and will likely succumb to neonatal isoerythrolysis. Although these anti-A alloantibodies are only absorbed during the first 16 hours of life, they cause lysis of the kittens' red blood cells resulting in anemia, jaundice, darkly-pigmented urine, anorexia or sudden death. Therefore, all cats used for breeding should be blood typed and compatible mates must be selected based on these results to avoid the loss of kittens from neonatal isoerythrolysis.

The inheritance pattern, natural occurrence of alloantibodies, and varied breed distribution are all of considerable importance to cat breeders as well as veterinarians. Blood typing cards for use in practice (DMS Laboratories, New Jersey,) are available. Furthermore, a novel gel test typing method has been introduced for feline typing.

The Transfusion Laboratory at the Ryan Veterinary Hospital of the University of Pennsylvania accepts samples for the assessment of feline and canine blood typing and resolving difficult compatibility issues (penngen@vet.upenn.edu; www.vet.upenn.edu/penngen).

Blood type A and B frequency in cats

*breeds in which the rare type AB has been found