Neonatal isoerythrolysis involving the Qc and Db antigens in a foal.

In 1992, a multiparous 13-year-old Thoroughbred mare and her 48-hour-old colt were examined because of possible neonatal isoerythrolysis (NI). Supportive treatment was administered, and the foal recovered without requiring a transfusion. According to the owners, the mare had delivered foals without incident during 1987 and 1991. The mare was barren during 1993, but in 1994, delivered a filly that developed severe NI. The foal was given 3 transfusions and eventually recovered without complications. Blood typing analysis of the mare and its foals indicated that all 4 foals were positive for the Qc, Db, and Dq antigens, and the 3 most recently born foals were positive for the Ua antigen; however, the marc was negative for the Qc, Db, Dq, and Ua antigens. The mare did not have alloantibodies against Ua and did not react to the Dq antigen. However, in 1994, the mare reacted against the Db (the reaction was characterized by strong agglutination and an increase in titer at the time of parturition and a subsequent decrease) and Qc (the reaction was characterized by weak lysis and an increase in titer at the time of parturition and a subsequent decrease) antigens. Results of testing in this mare and foals suggested that although approximately 90% of all cases of NI in horses are attributable to the Aa or Qa antigen, other antigens may be involved.