When to see a vet urgently
Deterioration in a newborn is urgent, particularly if:
- the kitten does not suckle, becomes limp or stays apart from the others;
- the mucous membranes are pale or yellowish;
- urine becomes reddish or very dark;
- breathing is laboured, the kitten becomes cold or sudden deaths occur in the same litter.
Do not wait for every sign or the next weighing. Contact a vet immediately; you can find a vet near you for help accessing care.
Overview
Cats have three main blood groups: A, B and AB. If a group B female has kittens with a group A or AB male, some kittens may inherit group A. The mother's colostrum then contains antibodies against the A antigen that destroy the newborns' red blood cells.
The disease appears within the first hours or days of life. Kittens initially seem healthy, but weaken, become lethargic and may die after suckling colostrum.
Colostrum is the first milk produced after birth. Its antibodies normally protect kittens against infection. In isoerythrolysis, some of these antibodies recognise the kitten's own red blood cells as foreign. The problem develops when antibodies are absorbed from the intestine into the bloodstream. A kitten can therefore be born in normal condition even though the risk can already be predicted from the parents' blood groups.
The mother's group B blood is not itself a disease, nor is her milk generally “spoilt”. The risk comes from a particular combination of antibodies and the kitten's red blood cell antigens. Blood groups can differ within a litter, and not every kitten is necessarily in the same situation. Every deteriorating kitten needs examination even if their siblings seem healthy.
Causes
- Incompatible maternal and kitten blood groups:
- a group B mother × a group A or AB father means group A or AB kittens are at risk.
- Antibodies in the mother's colostrum against foreign red blood cell antigens, particularly anti-A.
- Blood group inheritance: blood groups are inherited, making the choice of mating partners crucial.
Blood group is an inherited trait, but receiving antibodies triggers the kitten's disease, rather than genetic differences alone. A group A parent may also carry genetic information associated with group B. The parents' expressed blood groups and their genetic makeup are therefore different information, and every blood group in a litter cannot always be predicted from one group label.
Severity depends partly on the mother's antibodies and how much colostrum the kitten receives. Not every group A or AB kitten from an at-risk pairing develops equally severe symptoms. This variation does not justify trying to let an at-risk kitten suckle without a plan made in advance, however.
Risk factors
- Breeds where group B is common, including British Shorthair, Devon Rex, Cornish Rex, Persian and Exotic Shorthair cats.
- Parents whose blood groups have not been tested.
- Unplanned matings without checking blood group compatibility.
Blood group proportions within breeds vary between countries and study populations. Group B cannot be recognised by appearance, and cats from the same breed or family must not be assumed to have the same group. Domestic cats can also have an incompatible combination, so a risk list focused on pedigree cats does not exclude other kittens.
If the mother's blood group or the father's identity is unknown, previous healthy litters do not guarantee the next litter's safety. Keeping breeding and health records helps anticipate risk, but each litter's plan is based on its own parents and, when necessary, testing the kittens.
Typical symptoms
Kittens with isoerythrolysis may show:
- weakness and refusal to suckle;
- pale or yellow mucous membranes;
- dark urine (haemoglobinuria);
- increased breathing and heart rate;
- deaths during the first day or first few days of life.
Some kittens develop a delayed form, with tissue death at the tail tip at 1–2 weeks of age.
A tail-tip change does not mean acute weakness should be monitored at home for weeks. It may be a later finding in a milder case or a kitten that survived. In the severe form, deterioration can be rapid, and jaundice or visibly dark urine may not develop before collapse.
Reduced suckling may go unnoticed if the whole litter is monitored as one group. Watch whether every kitten attaches to a teat and keeps up with the others. Record each kitten's weight changes and behaviour separately. Weight loss or failure to grow needs assessment even if isoerythrolysis is not the cause.
Complications and consequences
- severe anaemia;
- oxygen deprivation in tissues;
- acute kidney failure caused by haemoglobinuria;
- high neonatal mortality;
- loss of an entire litter for the breeder.
In anaemia, too few red blood cells are available to carry the oxygen tissues need. During haemolysis, substances released from the cells may enter the urine, and haemoglobin breakdown products can cause jaundice. Similar colour changes have other causes too, so they cannot confirm the diagnosis alone.
A weak newborn easily becomes cold and dehydrated. Low body temperature in turn impairs suckling and digestion, so the problems reinforce each other. Separating a kitten from the mother alone does not solve the situation: warmth, nutrition and other care must be arranged immediately.
Diagnosis at the vet
- history, including parental blood groups and previous kitten deaths;
- clinical signs of haemolysis in newborns;
- laboratory tests, including haematocrit, a blood smear and bilirubin;
- blood typing of the mother and father before mating;
- newborn testing for compatibility between the kitten's and mother's blood.
Interpreting blood groups alongside symptoms
Your vet needs the time of birth, first suckling and symptom onset, and details of every death in the litter. Bring the parents' blood typing reports if available. Also mention whether kittens have already been hand-reared or moved to another nursing queen.
Parental blood typing predicts risk, but an affected kitten's condition is assessed separately. If needed, the kitten's blood group is determined and their red blood cells compared with a sample containing the mother's antibodies. A newborn's blood group can also be tested using umbilical cord blood collected by an agreed method; plan sampling with your vet.
Haematocrit measures the proportion of red blood cells in blood. A blood smear, bilirubin and other findings help assess haemolysis and its consequences. The extent of testing is adapted to the kitten's small size and condition. Stabilising treatment is not delayed simply to obtain a complete set of tests.
Not every fading kitten has isoerythrolysis
Weakness in a newborn can also involve infection, consequences of a difficult birth, a congenital abnormality or inadequate nutrition. A group B mother alone does not automatically explain every kitten death. The mother's health, the kittens' temperature and successful feeding are also assessed.
Treatment
In practice, there is no effective treatment for newborns. The main measures are:
- immediately separating kittens from the mother and switching to artificial feeding or milk from a compatible nursing queen;
- transfusing the mother's washed group B red blood cells, which are not destroyed by antibodies received through colostrum; this is performed only at specialist clinics;
- supportive care, including warmth and fluids.
Treatment is available, although its effectiveness may be limited. It may include a blood transfusion as well as supportive care, but treating a newborn is demanding and the effects of antibodies already absorbed are difficult to stop. Early assessment and preventing further exposure are therefore essential.
The prognosis depends on severity: early recognition and separation from colostrum improve the chance of survival, but many kittens die in the first few days.
The limits of transfusion and separation
The washed red blood cells mentioned are a specially prepared blood product from which antibody-containing plasma is removed. This does not mean that a kitten can simply receive unprocessed maternal blood. Your vet selects the product and donor and monitors the transfusion. Transfusion can support oxygen transport, but cannot replace other care.
Separation aims to prevent further antibody absorption, rather than leave the kitten without food or warmth. Safe alternative feeding must be arranged at the same time. Kitten milk replacer is different from cow's milk or an improvised homemade mixture. Choosing a suitable nursing queen also involves her blood group and stage of milk production.
Temporary care and monitoring
A cold or very weak kitten needs veterinary assessment before ordinary bottle feeding. If suckling is unsuccessful, do not force food into the mouth, as it may enter the airways. Your vet can arrange or teach assisted feeding suited to the situation. Tube feeding should not be attempted without practical instruction.
Provide warmth in a controlled way to avoid overheating or burns. Record feeding, weight, activity and elimination in a care diary. Identify each kitten reliably so that one kitten's deterioration is not hidden by the others' good growth.
Avoiding colostrum also reduces intake of normally beneficial antibodies. Milk replacer provides nutrition but does not fully reproduce colostrum's immune protection. Your vet assesses any additional support and hygiene needs and monitors both the separated kittens and their mother.
Prevention
- Always blood type cats used for breeding.
- Do not mate group B females with group A or AB males.
- If the pairing cannot be avoided, keep kittens apart from the mother for the first 24 hours, while their intestines still absorb colostral antibodies, and give milk replacer before returning them to her.
- Keep records of inherited traits in breeding programmes.
This first-day arrangement concerns an anticipated risk; it is not general advice to separate every newborn from the mother. A kitten confirmed as group B is in a different situation from an at-risk group A or AB kitten. The duration of separation and return to suckling are agreed according to birth time, blood typing and condition; an ill kitten is not returned simply because a set time has passed.
Make the plan before birth. It should include test results, necessary care supplies, alternative feeding and rapid access to veterinary assessment. Decisions in the first hours then do not have to be made only when faced with a deteriorating kitten.
Important for cat owners
Neonatal isoerythrolysis is a serious immune conflict between blood groups: antibodies in a group B mother's colostrum destroy the red blood cells of group A or AB kittens. If you breed cats, remember that blood typing breeding cats before mating is an essential way to prevent kitten deaths. Appropriate pairing and prompt action help avoid tragic consequences.
This article provides general information and does not replace veterinary care. Contact a vet immediately if you notice signs of illness in kittens.
Frequently asked questions
Can a group B mother have healthy kittens?
Yes. Group B is not a disease, and group B kittens, for example, do not have the same risk associated with anti-A antibodies. The breeding combination and possible kitten blood groups should be assessed in advance.
Is every kitten in an at-risk litter equally ill?
Not necessarily. Blood groups, maternal antibodies and the amount of colostrum received can vary. Monitor each kitten's suckling, weight changes and condition separately; a healthy sibling does not rule out illness in the others.
Is switching an affected kitten to milk replacer enough?
Not necessarily. Antibodies already absorbed can continue damaging red blood cells, and the kitten may need hospital care or a blood transfusion. Alternative feeding, warmth and urgent veterinary assessment must be arranged together.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
- Feline blood groups, Governing Council of the Cat Fancy, accessed 2026-11-05.
- A questionnaire on survival of kittens depending on the blood groups of the parents, Journal of Feline Medicine and Surgery / PubMed Central, accessed 2026-11-05.
- Regenerative Anemias in Animals, Merck Veterinary Manual, accessed 2026-11-05.
- Management of the Neonate in Dogs and Cats, Merck Veterinary Manual, accessed 2026-11-05.
- Feeding Orphaned Kittens, VCA Animal Hospitals, accessed 2026-11-05.
- Blood Transfusions in Dogs and Cats, Merck Veterinary Manual, accessed 2026-11-05.
- Anaemia, Cornell Feline Health Centre, accessed 2026-11-05.
- Blood Groups in Dogs and Cats, Merck Veterinary Manual, accessed 2026-11-05.