The effects of continuous
oxytetracycline administration on the development of parasitaemia of Babesia divergens during both natural and artificial
infections were studied. During natural exposure on grazing heavily infested with Ixodes ricinus, seven out of 42 cattle with no previous exposure to
tick-borne diseases were injected every four days with a long acting preparation of
oxytetracycline at a dose rate of 20 mg/kg. During the six week grazing period 21 untreated cattle developed a patent parasitaemia of B divergens and all became seropositive by the fluorescent antibody test. In contrast, no parasites were observed in treated cattle and antibody titres remained low. Artificial
infections were studied with different dose levels of
oxytetracycline and their effects on antibody stimulation noted. First, four groups of cows were infected with 10(8) erythrocytes infected with B divergens, three groups being injected every four days with the long acting
oxytetracycline formulation at dose levels of 20, 10 and 5 mg/kg, respectively. The highest level completely inhibited parasite replication and antibody formation; the same was observed in one animal dosed
at 10 mg/kg but the remainder, plus those treated at 5 mg/kg, developed both low parasitaemia and high antibody titres. The untreated cows developed severe
babesiosis. A further untreated control group was added and three weeks after cessation of
oxytetracycline treatment all were infected with 10(9) erythrocytes infected with a homologous isolate of B divergens. The controls, plus those in which the previous
infection had been completely inhibited, developed severe clinical
babesiosis but the remainder were refractory to parasite development.