Blood Type Incompatibility: The First 72 Hours

Neonatal isoerythrolysis kills kittens in the first three days, and it runs in one direction only: a type B queen bred to a type A tom. Type B cats carry naturally occurring antibodies against type A blood. Colostrum concentrates those antibodies, and a type A kitten that nurses in its first 16–24 hours absorbs them straight through the gut wall and haemolyses its own red cells. A type A queen bred to a type B tom is not a risk, because type A cats do not carry anti-B antibodies of consequence.

Three types, and where B is common

Type A is the majority in most of the cat population, often above 90%. Type B is rare overall but clusters hard by breed: British Shorthairs, Devon Rex, Cornish Rex, Turkish Van and Turkish Angora, and some Persian and Scottish Fold lines run 20–40% type B, with regional pockets higher. Type AB is uncommon, usually under 1%, and is best confirmed by serology rather than a DNA panel alone.

The practical consequence is that a breeder in a B-heavy breed cannot assume anything. A British Shorthair cattery with a third of its cats type B will produce incompatible matings by accident unless every cat is typed before the first litter, not after the first dead one.

Why only the queen's type decides the outcome

Antibody transfer in cats happens through colostrum, and the kitten gut absorbs large molecules only for roughly the first 16–24 hours of life. That window is the entire mechanism. A type B queen has circulating anti-A titres even if she has never been bred, so her first litter is at risk, not just her second. A type A queen has no meaningful anti-B, so her type B kittens are safe. Type AB queens have no antibodies against either, and type AB kittens are not at risk from either parent.

Risk by pairing

Queen	Tom	Kitten risk
B	A	Highest. All type A kittens at risk; type B kittens safe. Expect losses at 12–72 hours if colostrum is not blocked.
B	B	None. All kittens type B.
B	AB	Moderate. Type A kittens at risk, type AB kittens generally safe.
A	B	None for the kittens. The queen has no anti-B, so mismatched kittens are not attacked.
A	A	None. All kittens type A.
AB	Any	None. No anti-A or anti-B antibodies to transfer.

The timeline you are racing

Birth, then nursing within the first hour, then absorption over 16–24 hours. Signs usually appear 12–24 hours after the first feed and peak between 48 and 72 hours. A kitten that was vigorous at birth and is limp at 30 hours has not caught an infection; that is too fast. The first 72 hours are the danger zone, and after about 4 days the gut has closed and fresh red cells are being made, so a kitten that survives the window usually recovers, sometimes with a necrotic tail tip that drops later.

What it looks like

• Dark red-brown urine, or urine that stains the bedding rust-coloured.
• Gums and tongue pale, then yellow within a day.
• Weakness, refusal to nurse, low body temperature under 35.5 °C.
• Rapid breathing and a heart rate that climbs as the anaemia worsens.
• Tail-tip necrosis at 5–10 days in kittens that survive the acute phase.

Jaundice in a 2-day-old kitten is an emergency. Call the vet the same hour, and bring the queen's and tom's blood types with you if you have them.

What to do in the first two hours

1. If the queen is type B and the tom type A, do not let the kittens nurse. Separate them at birth, before the first feed, and keep the queen calm nearby.
2. Hand-rear with a commercial queen milk replacer for 24–48 hours, feeding every 2 hours in the first 48 hours, 3–5 ml per feed for a 100 g kitten.
3. Warm first. A cold kitten cannot digest; bring it to 36.5–37.5 °C before feeding.
4. Call the vet for a packed cell volume and, if the kitten is already anaemic, a plasma or washed-cell transfusion.
5. After 24–48 hours the gut has closed and the kittens can return to the queen, provided she is still producing milk.

Testing, and testing again

Type both parents before the first mating, not after the first loss. In-clinic card tests give an answer in minutes and are adequate for screening; a laboratory panel confirms AB. Kittens cannot be typed reliably at birth from maternal antibody-contaminated samples, so retest at 6–8 weeks if the type matters for a future breeding programme.

Keep the pairing records attached to the same file as your neonatal monitoring sheet, because the first 24-hour weight loss is often the earliest sign, before the urine changes. If the litter was delivered surgically, the recovery protocol in C-section recovery determines how fast the queen can be reunited with kittens you have separated. Queen condition in the first week is covered in postpartum queen care.

Five habits that prevent it

• Type every cat at intake, including pet-quality cats kept as companions.
• Never plan a B queen to an A tom; if it happens by accident, write the protocol on the nursery wall.
• Keep two bottles, a warmer and milk replacer in stock before the due date.
• Weigh newborns at birth, 12 hours and 24 hours; a 10% drop triggers action.
• Record blood types on the pedigree paperwork you hand to buyers.

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