Feline Blood Type

Three blood types have been identified in cats - type A (most common), type B, and type AB (very rare). In the United States, nearly 95% of all domestic mixbreed cats are blood type A. AH Siamese, Burmese, Tonkinese, American Shorthair, and Oriental Shorthair cats tested so far have type A blood. However, the following pure breeds have a 10-50% frequency of blood type B cats:

- Abyssinian
- Birman
- British Shorthair
- Cornish Rex
- Devon Rex
- Exotic
- Japanese Bobtail
- Persian
- Scottish Fold
- Somali
- Sphynx

The frequency of blood type B in other pure breeds has not been determined. Similar to humans, cats have naturally occurring antibodies to blood types other than their own. Blood type A cats have low amounts of weak antibodies to red blood cells (RBCs) from type B cats. Blood type B cats have high amounts of strong antibodies to type A RBCs. Because of these strong anti-A antibodies, incompatibility between type A and type B cats can cause potentially fatal reactions under two circumstances:

- transfusion of type B cats with type A blood
- breeding of type A toms to type B queens to produce type A kittens

Transfusion of blood from a donor cat to a recipient cat of the same blood type rarely causes problems, and the transfused RBCs usually last for 30 days in the recipient. However, transfusion of blood from a type A donor to a type B recipient results in rapid destruction of the transfusion within minutes to hours due to the strong anti-A antibodies in the type B recipient. This transfusion reaction causes anaphylactic shock and even death, and can be prevented by blood typing all cats prior to transfusion. A simple, rapid card test (DMS Laboratories) is now available for determining blood types in cats.

The transfer of maternal antibodies via colostrum from the queen to the newborn kitten not only provides protection from infection, but also may cause a life-threatening crisis called neonatal isoerythrolysis. Neonatal isoerythrolysis (NI) is a major cause of the Fading Kitten Syndrome in purebred cats. NI is caused by a blood type incompatibility between the queen and her kittens. Since blood type A is dominant over type B, the mating of a type A tom to a type B queen produces type A kittens. When the type A kittens nurse the type B queen, they ingest colostrum containing antibodies to their blood type. These anti-A antibodies in the colostrum are absorbed from the intestine into the blood during the first day of life. The antibodies rapidly destroy the kittens' RBCs similar to a transfusion reaction.

The kittens appear healthy and vigorous at birth, but some kittens suddenly die after nursing while others 'Fade" and die 3-7 days later. The fading kittens stop nursing, become pale or jaundiced due to RBC destruction, and have red urine. The red urine distinguishes NI from all other causes of the Fading Kitten Syndrome. Breeders should check all kittens for red urine during the first 2 days of life. The diagnosis of Nl is confirmed by blood typing the tom and queen, or the queen and kittens.

Once the symptoms of NI are noticed, some kittens may be saved by a transfusion of RBCs from the queen. Since the queen's blood contains anti-A antibodies, the RBCs must be thoroughly washed with saline to remove these antibodies. The washed RBCs are given intraperitoneally (5 ml per kitten. Surviving kittens usually recover to full health but may develop necrosis of the tail tip at 2 weeks of age.

In the Devon Rex, British Shorthair, and Cornish Rex, nearly 25% of all matings are incompatible producing kittens at high risk for NI. If an incompatible mating is known to occur, NI can be prevented by removing the kittens at birth prior to nursing, and fostering them on a type A queen or hand-rearing on artificial milk for 2 days. These kittens should receive serum from a type A cat to provide immune protection (5 ml per kitten intraperitoneally). After 2 days, the kittens may be safely returned to the queen since anti-A antibodies in the milk can no longer be absorbed from the intestines.

Knowledge of the feline blood type is important when transfusing blood, diagnosing and treating NI in newborn kittens, and counseling breeders on selection of breeding cats. Only blood type-compatible breedings should be performed in breeds with a high frequency of blood type B. Breeders may even elect to eliminate type B queens from the breeding pool.