Anaplasma marginale: effect of challenge of cattle with varying doses of infected erythrocytes

Three groups, each of 6 Hereford cattle, were infected by the i.v. inoculation of 10(10), 10(8) or 10(6) Anaplasma marginale-infected erythrocytes. The mean time taken to reach a 1% parasitaemia was 7.3, 13.9 and 19.9 days in the 10(10), 10(8)and 10(6) infection dose groups, respectively. The rates of increase in parasitaemias during the exponential phase of parasite multiplication were similar for the 3 groups (doubling time 0.9 days). The exponential increase of the parasitaemia in the 10(10) dose group extended to a higher level or 10(6) dose groups (to approximately 10% compared with 3%). The mean maximum parasitaemia attained in the 10(10), 10(8), and 10(6) infection dose groups was 23.7, 14.7 and 8.7%, respectively> The time taken to reach the treatment criterion (packed cell volume decrease to 15% or lower) from a 1% parasitaemia was similar for the 3 groups. These results showed that the pathological outcome (anaemia) of anaplasmosis were similar over the 10,000-fold infective dose range tested.