| Popis: |
One hundred and sixty-one calves that had received colostrum from bovine leukemia virus (BLV)-infected cows were followed to evaluate factors associated with the decay of colostral antibodies to BLV. Three survival parametric models (Weibull, log-normal and log-logistic) were fitted to data on the age at which non-infected calves converted from a seropositive to a seronegative status (retroconversion). For 97 non-infected calves, the fit of the Weibull model for the age at retroconversion was better than those of the log-logistic and log-normal models; values for λ and α for the Weibull model were 0.014 and 1.59. Non-infected calves with high initial levels of BLV colostral antibodies became sero-negative at an older age than calves with low initial 3evels ( P P = 0.328) after adjusting for the initial level of colostral antibodies. The rate of antibody decay in most calves that seroconverted later because of active infection was within the range of that in non-infected calves. Six calves that did not exhibit any decay could have been identified as infected as early as 80 days of age. Results indicate that the use of initial BLV colostral antibody concentration in models depicting decay of colostral antibodies will improve the detectability of early BLV calfhood infection. For initial colostral antibody concentration scored from 1 to 4 (1 lowest, 4 highest), no detectable antibodies were predicted in non-infected calves after 39, 67, 115 and 196 days of age for scores 1, 2, 3 and 4 respectively. |
