Resistance of Calves to Reinfection with Eimeria bovis

SYNOPSIS. An immunity to reinfection with E. bovis was demonstrated in 3 experiments involving 60 calves. This immunity develops rapidly, as indicated by resistance to a challenge given 14 days after the immunizing inoculation. In 3 groups of 3 to 6 young calves each, immunity was still present to a moderate degree 2 to 3 months after inoculation; in one group of 5 animals about a year old there was apparently a high degree of immunity about 7 months after the last inoculation. In one experiment an immunizing inoculum of 10,000 oöcysts did not produce as much immunity as 50,000 oöcysts. In 2 experiments there appeared to be little difference in the immunity produced by a single inoculation of 50,000 as compared with 100,000 oöcysts, but inoculation with 100,000 oöcysts, resulted in substantially longer and more severe illness than 50,000 oöcysts. There appeared to be no appreciable difference in clinical symptoms or development of immunity between calves given a single immunizing inoculum and those given the same number of oöcysts in 5 equal inocula on successive days. Treatment with sulfamethazine and sulfamerazine (Merameth) 13 to 15 days after inoculation alleviated the clinical symptoms of coccidiosis without interfering appreciably with the development of immunity. In one experiment with 7 calves, no beneficial effect was noted from 1 or 2 transfusions of 500 ml. of plasma and leucocytes from immune calves into 4 calves 1 and 12 days or 11 days after a challenge inoculation.