Neonatal isoerythrolysis (NI) is a blood disorder that occurs in some foals less than one week of age, writes Dr. Christina S. Cable in the February edition of The Horse. The foals are not born with this disease. The problem begins when the foal nurses from the mare for the first time. As the foal ingests and absorbs the mare's colostrum, the antibodies that help protect the foal from other diseases begin to cause a disease of their own.Mares, unlike humans, don't pass antibodies across the placenta. Therefore, if the foal does not nurse the colostrum, it will not receive protection against common "bugs" in its environment.However, certain mares produce an antibody against a red blood cell group or factor that is transferred to the foal via the colostrum. If the foal is unfortunate enough to have that particular red blood cell group, then the antibody transferred from the mare will attack and destroy the foal's red blood cells.In a susceptible foal, the antibodies' attack on the red blood cells results in the foal becoming anemic. The anemia can develop slowly over several days, or happen very rapidly, with the foal becoming weak, lethargic, or recumbent (unable to rise) in a matter of hours. How quickly the signs develop in the foal often is dependent on how much colostrum it absorbed and how destructive the antibodies are.What are the signs of NI? Over the course of a few hours to a few days, the affected foal will begin to weaken. Then it will become lethargic, have a decreased suckle response, play less, and develop pale or jaundiced (yellow-colored) mucous membranes. As the disease progresses, the foal will develop rapid breathing (tachypnea) and rapid heart rates (tachycardia), which are caused by the decreasing number of red blood cells in its body. As the number of red blood cells decreases, the foal eventually will collapse, unable to provide enough oxygen for its brain and body to survive. But if caught early, NI is treatable and the foal should make a complete recovery.Your veterinarian should evaluate any foal that is acting quiet, is weak, or appears to be sick. Other diseases, such as sepsis, can have similar signs and also require immediate attention. NI is relatively uncommon, occurring in less than 1% of Thoroughbred foals.