There can be few events as distressing for a cat breeder as watching a litter of kittens fade and die within the first 48 hours of life. Yet this has been the experience of many British Shorthair breeders until it was recognized that the reason was that the queen had a different blood type from her kittens and was killing them, unintentionally, through her milk. It is particularly a problem in the British breeds, as almost 60% are blood group B, but it can affect almost any breed of cat, including our dear old domestic moggies.

Knowing your cat's blood group can save their and their kittens' lives.

What are feline blood groups or blood types?
Our blood consists of red blood cells, white blood cells and plasma. The red cells carry oxygen and their well-being is essential for life. Red blood cells, like most other components of the body, are made of protein, and proteins can be recognised and destroyed by the body's immune system. The cat has essentially two blood groups: A and B (although there is a third, very rare, group, AB, which is a combination of the two). The red blood cell proteins are A in one group, B in the other (see figure 1). In fact, the proteins on the surface of type A and B red blood cells are only slightly different, but of course the immune system can detect the difference.

As you know, if you have a blood transfusion, it is essential to get the right type of blood, a transfusion of the wrong blood type could kill you - it is no different for cats. The immune system of the cat or person receiving the blood transfusion reacts to the blood which it sees as alien, and the consequences can be fatal.

What is neonatal isoerythrolysis?
A recently recognised cause of fading kittens in pedigree cats is neonatal isoerythrolysis, where kittens have a different blood type from their queen, and, on suckling her milk, receive antibodies against their own red blood cells. The antibodies cause destruction of the kittens red blood cells which leads to jaundice (see figures 2 and 3), brown urine, and rapid death. In less severe cases, the tail tip drops off.

Samples of blood from the kittens and queen can be blood typed to confirm the diagnosis.

Prevention of neonatal isoerythrolysis

1. Typing the queen and tom before mating
Prevention involves blood typing the queen and any prospective tom that she may be mated with. Cats get one gene for blood type from their mother (in the egg) and one from their father (in the sperm). The genes for blood type A are dominant to the genes for blood type B, which means that a cat with a mixture of genes - Ab (note that this is different from the third, rare, blood group AB) - will have blood type A A blood group B queen can give birth to a blood type A kitten if mated to a blood type A tom carrying the genes for blood type B, (as in figure 4 and table 1). This is the situation in which neonatal isoerythrolysis is most likely to occur.

Table 1 - What happens when you mate a blood group B queen to a blood group A tom cat depends on whether his genes are purely A or a mixture of A and B. The kitten's genes are represented by the shaded boxes. The biggest risk of neonatal isoerythrolysis is when kittens with blood type A are born to blood type B queens.

Tom's genes            Queen's genes b b  A Ab Ab  A Ab Ab		Tom's genes Queen's genes b b  A Ab Ab  b b b
In this case all the kittens will be blood group A, but will be carriers of blood group B.		In this case, half the kittens will be blood group B (blue shaded boxes) and half will be blood group A, but carriers of blood group B genes.

Cats with blood type B are recessive (i.e. they all have two b genes, genotype bb) and can be safely mated to other blood type B cats - all the kittens will be blood type B (table 2.) and neonatal isoerythrolysis will not occur.

KEY

Gene Type	Blood Group
AA	A
bb	B
Ab	A

Table 2. - What happens when you mate two blood group B cats together.

Tom's      genes	Queen's  genes
	b	b
b	bb	bb
b	bb	bb

All of the kittens will be blood group B (shaded area represents kittens of such a mating) and there will be no risk of neonatal isoerythrolysis.

Table 3. - What happens when you mate two cats with blood type A together.

Tom's genes	Queen's genes
	A	A
A	AA	AA
A	AA	AA

In the simplest scenario, all the genes of the tom and queen code for blood type A and all the kittens will be blood group A. However, since blood group A is dominant to blood group B, one or both cats could be carrying the gene for blood group B.

Tom's     genes       Queen's  genes         A     b                  A         AA     Ab         A         AA     A

Table 3a     Tom's     genes       Queen's  genes         A     b        A         AA     Ab         b         Ab     bb

In the first example above, the tom has both versions of his gene (i.e. the one from his mother and the one from his father) coding for blood group A, therefore all his kittens will be blood type A. Half of his kittens will carry the gene for blood group B, which they have gotten from their queen. None of the kittens will be at risk of neonatal isoerythrolysis as they all have the same blood group as their queen.