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 than in the 10 8 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.