Immunity to Babesia bigemina in experimentally infected cattle

SYNOPSIS Two groups of 5 and 6 Babesia bigemina“vaccine donor” animals of which 8 had been splenectomized were challenged 6 and 12 months respectively after they had lost their carrier state. All animals of the former, and 3 of the latter group survived; the remaining 3 animals succumbed to the challenge and died. It was concluded that premunity to B. bigemina is followed by sterile immunity which lasts for at least 6 months. Thereafter it fades gradually with time, depending on the immune response of the host, but can last for at least 12 months. Six splenectomized animals, which had lost their infectivity after treatment of their initial B. bigemina parasitemia at the rapidly rising phase with 1 mg/kg Berenil, died on challenge. It was concluded that a minimum period of contact between host and parasite is required for the acquisition of immunity to B. bigemina. Capillary tube agglutination titers were generally higher in the protected than in the unprotected animals. They remained fairly high for a long period after animals had lost their carrier state, which indicated the sensitivity of the CA test but rendered it unreliable for the detection of carrier animals.