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. ino culation of 10(10), 10(9) or 10(6) Anaplasma marginale-infected erythr ocytes. The mean time taken to reach a 1% parasitaemia was 7.3, 13.9 a nd 19.9 days in the 10(10), 10(8) and 10(6) infection dose groups, res pectively. The rates of increase in parasitaemias during the exponenti al phase of parasite multiplication were similar for the 3 groups (dou bling 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 ma ximum parasitaemia attained in the 10(10), 10(8) and 10(6) infection d ose groups was 23.7, 14.7 and 8.7%, respectively. The time taken to re ach the treatment criterion (packed cell volume decrease to 15% or low er) from a 1% parasitaemia was similar for the 3 groups. These results showed that the pathological outcome (anaemia) of anaplasmosis were s imilar over the 10 000-fold infective dose range tested. Copyright (C) 1996 Australian Society for Parasitology.