Neonatal Isoerythrolysis in Kittens

What is neonatal isoerythrolysis?

Neonatal isoerythrolysis (NI) or hemolytic icterus is a serious and life-threatening condition caused when kittens who have type A blood nurse from their mother who has type B blood during the first 24 hours of life. The queen’s first milk is known as colostrum contains naturally occurring alloantibodies against the kitten’s type A blood group. This leads to the destruction of the kitten’s red blood cells. It is thought to be a major cause of fading kitten syndrome due to the destruction of the red blood cells (hemolytic anemia).

Unlike other animals who can develop alloantibodies as a result of previous pregnancies to offspring with different blood groups, cats with B type blood have naturally occurring alloantibodies to A-type blood and do not need to have had prior exposure. Cats with type A blood also have naturally occurring alloantibodies to type B blood, however in much lower numbers and this generally doesn’t pose a problem to kittens. 99% of cats in the United States are type A, type B is more common among purebred cats.

Type B mother Type A Kitten Unsafe for the first 24 hours
Type B mother Type B kitten Safe
Type A mother Type A or B kitten Safe

The queens first milk is known as colostrum. Colostrum is produced in the first 24 – 72 hours after birth. It provides newborns with essential nutrients and antibodies which protect them from infection.

Cat blood groups

Cats have one of three blood groups, A, B and AB (AB is very uncommon).

A is dominant over b (dominant genes are always listed in UPPER CASE and recessive genes in lower case). Genes come in pairs (one from the mother, one from the father) and their blood group may be the following:

  • A/A (homozygous)
  • A/b (heterozygous)
  • b/b (homozygous)
  • AB (heterozygous)*

*Note: Ab (heterozygous) is completely different from the AB blood group.

Type A blood group is the most common blood group in cats. Type B can occur between 10-60% in Devon Rex, Cornish Rex, Exotic Shorthair, Abyssinian, Turkish Van, Turkish Angora and British Shorthair cats. AB is extremely rare.

The mortality rate of neonatal isoerythrolysis is high, urgent and aggressive medical treatment is vital. The condition is seen more often in purebred cats than domestic/mixed breed types.

Clinical signs

Kittens are healthy at birth, but within the first few days of life, their health deteriorates rapidly due to the destruction of the red blood cells and the resulting anemia.

  • Lethargy
  • Rapid pulse
  • Increased respiration
  • Jaundice (yellowing of the mucous membranes) in the mouth and eyes
  • Brownish-red urine
  • Tail tip necrosis
  • Respiratory difficulty
  • Some kittens will stop nursing from the mother
  • Failure to thrive
  • Sudden death

The severity of symptoms can vary from kitten to kitten, this may be down to the amount of colostrum each kitten has had.


A presumptive diagnosis of neonatal isoerythrolysis can be made from presenting symptoms. Confirmation will be via blood testing of the queen and the affected kitten(s).

What is the treatment for neonatal isoerythrolysis in kittens?

Remove type A kittens immediately, type B kittens can continue to nurse.

The veterinarian may perform a blood transfusion, preferably washed blood which removes the serum component containing alloantibodies from the kitten’s mother. This sounds counter-intuitive, why would you transfuse a different type of blood into the kitten, however, the queen’s blood is not going to respond to her own type B antibodies in the kitten’s blood.

If the queen can not make a blood donation, then another type B cat should be used, again once the blood has been washed. After 3 days the kitten should be producing antibodies against type B blood, therefore if a second transfusion is required down the line, type A blood should be used in the Type A kitten.

Prognosis is poor, even with treatment.


Prevention is the best way to address blood type incompatibilities. Avoid mating a queen with a B blood type with a tom who has an A blood type.

If such a mating does occur, preventing the kittens from drinking the colostrum will be necessary. It is always important to know the blood group of the queen prior to mating. With this knowledge, if you know the queen has a type B blood group, and are able to determine the blood type of the kitten(s) prior to her nursing any kittens with the same blood type as the queen (ie: B type), are fine to nurse.

Type A kittens with a type B mother must avoid nursing from her for the first 24 hours. They can either go to a queen who has A blood type or given a kitten milk replacer (KMR). After 24 hours the kittens can return to their mother and safely nurse.

Why is it safe to return the kitten to its mother after 24 hours?

When kittens are born, their gut is extremely permeable, this means that antibodies can easily travel through the intestinal wall and into the rest of the body. Normally this is good as it allows antibodies from the mother’s colostrum to easily get into her kittens and give them passive immunity while their immune system develops. However after 24 hours, permeability has declined significantly, and the queen’s colostrum (which contains high numbers of antibodies) changes to milk, with lower numbers. So, decreased gut permeability PLUS lower numbers of antibodies in the milk.

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  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio