Protection against and establishment of Dictyocaulus viviparus following primary infection at different dose levels.

In two experiments, calves were primary infected with 1 of 12 (Experiment 1) or 6 (Experiment 2) different dose levels of Dictyocaulus viviparus infective larvae (L3), ranging from 5 to 2000 L3. To study the level of protection induced by the primary infection a challenge infection with 2000 L3 was given on day 10 (Experiment 1) or day 35 (Experiment 2). In both experiments, challenge control calves were included. Eleven days later, the challenge calves were necropsied for worm counts. Results were compared with predictions from a simulation model. Establishment of the primary infection was dose independent, lying on average in the range of 20-30%. The ratio female:male worms in the counts from the primary or from the challenge infection was consistently close to 1:1 irrespective of primary infection dose level or protection having developed in some of the calves. Level of early protection (10 days after a primary infection-Experiment 1) against establishment of the challenge infection depended significantly on primary infection dose level (P<0.01). At 10 days, after a primary infection, low dose levels did not result in protection against a challenge infection. In contrast, similarly low dose levels did result in partial protection, 35 days after the primary infection. Results confirmed that our provisional simulation model satisfactorily predicts primary infection outcomes, but that it does not accurately predict levels of protection and immunity against re-infections.