DYNAMIC OF NATURAL BOVINE LEUKEMIA-VIRUS INFECTION IN A DAIRY-HERD INCHILE

This study examines BLV infection over a 3 year-period, in a closed Ho lstein herd of the Region Metropolitana (central Chile) as an informat ion model of the epidemiological pattern of this disease under special management and seasonal conditions. The herd, annually, had an averag e population of 400 cows and BLV infected animals were not preferentia lly culled. From 1988 to 1991, BLV antibodies were detected twice a ye ar (winter and summer) in the cows over 6 months old by the agar-gel i mmunodiffusion test. The animals were classified in 6 and 4 cohorts by age and date of birth respectively. The proportion of prevalence and incidence rate was calculated for the entire herd and for each cohort for each time-period. During the study, the total sequential prevalenc es of BLV infection fluctuated between 35.9% and 28.5% (p > 0.05) Ther e were no differences in the prevalence rates between cohorts accordin g to the date of birth. Analysis of cohorts by age showed age related trends of prevalence of BLV infection (p < 0.05) in heifers and young cows. Relatively few old cows developed new infections. The six month percentage incidence rate in the five inter-test periods were 7.5, 3.5 , 6.1 and 6.5 with a significant difference (p < 0.05) only between th e first and the second period. Incidence rate of BLV infection increas ed in cows over 24 and up to 48 months of age, which is the age that h eifers became exposed to adult infected cows. In cows over 60 months o f age the incidence of BLV seroconversion decreased. The results of th is investigation and the evaluation of incidence rate of BLV infection and the proportion of prevalence will allow control programs in herds with similar management to develop.