Epidemiological features of BLV natural infection

Abstract

Dairy farms are heavily infected with Bovine Leukemia Virus (BLV) in Argentina and many other countries, where a control strategy should be design based on the behavior of natural. We conducted a series of studies with the aim to better understand the epidemiology of BLV. Infected new born calves were present in 3 studied farms (8.3%-11%). Proviral load (PVL) was very low in 9/10 new born analyzed calves but rapidly augmented during the first months of age. Cross-sectional studies showed that the rate of high PVL between seroreactors raised together with the prevalence, from 20.2% at 8 months of age to 44.4% in 26 months heifers, with similar levels to adult lactating cows of the same farm. Low fluctuation of blood PVL was observed on animals with naturally-acquired infections. We also observed 10 seroconversions between young heifers with an initial elevation of PVl. The presence of provirus in colostrum was significantly correlated with blood PVL (p<0.0001). Provirus in milk was detectable in bulk tank (17.2%) and individual samples (40.4%). Colostrum of individual cows showed different provirus/antibodies dual profiles that permit to speculate about different infective/protective potential among infected animals. These findings suggest animals would be exposed to the infective challenge since a very young age. Consequently, it must be control as soon as possible after birth. The main focus should be put on the newborn infected calves that could play the role of main propagators together with the putative oral exposition through colostrum and milk.