Neonatal isoerythrolysis, or NI, is a rare immune-mediated disease that is caused when newborn kittens with type A blood drink colostrum from a mother with type B blood. The antibodies contained in the colostrum are absorbed. They then bind to the kitten's erythrocytes (red blood cells) and destroy them (erythrolysis, hemolysis). Blood type A is dominant over blood type B, therefore type A cats can either be homozygous (genotype AA) or heterozygous (genotype AB). Tomcats that are homozygous type A will always produce type A kittens when bred to queens with type A or B blood. Approximately 95% of type B queens have strong isoantibodies against type A blood.

Affected kittens are born healthy and start to nurse vigorously. Once they drink the colostrum, they start to show clinical signs within hours to days, stop nursing, and fail to thrive. They develop hemolytic anemia within one to two days after birth and some or all of the litter will die. Early clinical signs of the disease are pigmenturia (dark red-brown urine). Affected kittens will be anemic, jaundiced, and depressed. Immune mediated hemolysis, disseminated intravascular coagulation, acute renal failure, and anemia are the probable causes of death.

Blood tests, urinalysis, and blood typing can diagnosis the condition. Pigmenturia in fading kittens is highly diagnostic for NI.

Affected or at-risk kittens should not be permitted to nurse from their mother for the first three days. A blood type A queen can be substituted for nursing, or a milk replacer can be used for the first three days after birth. The kittens can be returned to their mother after three days, because the colostrum will no longer be present at that point.

Even if the kittens are removed from their mother as soon as the first clinical signs develop, the mortality rate will be high. NI is best prevented by avoiding incompatible matings between type B mothers and type A fathers. Cord blood can be used to determine the kitten's blood type or to perform a crossmatch with serum from the mother. Kittens with blood type B can be immediately placed back with their queen. Type A (or AB) kittens may safely receive milk or colostrum from a type A mother.

Prevention is the preferred "treatment" for NI. Affected kittens need supportive care, blood transfusions, etc.; however the prognosis is guarded in spite of any treatment, so prevention should be the main key of defense.

Blood type A and B frequency and risk for neonatal isoerythrolysis in the United States

Table from THE FADING KITTEN SYNDROME AND NEONATAL ISOERYTHROLYSIS by Urs Giger, DVM, DACVIM, Margret L. Casal, DVM, and Andrea Niggemeier, DVM; research supported in part by grants from the Winn Foundation and the National Institutes of Health (HL02355, RR02512).