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Background: Newborn calves are agammaglobulinemic, immunosuppressed and immunologically immature at birth. The passive immune transfer is fundamental to protect calves against pathogens. The decay of maternal antibodies precedes the immune maturation at puberty enhancing the susceptibility of calves to infections caused by BVDV. Then, the objective of this research was to evaluate the interface between passive and active immunity for Bovine Viral Diarrhea Virus (BVDV) in Holstein dairy heifers in the first 13 months of age to detect susceptibility periods and establish prophylactic measures on prevention of Bovine Viral Diarrhea. Materials, Methods & Results: Sera were analyzed from 585 heifers by serum neutralization (SN) and enzyme linked immunosorbent assay (ELISA) for the p80 protein of BVDV. Heifers were categorized according to their age by the month of life. Heifers were seropositive (100%) from 1st to 13th. Median of neutralizing antibodies (Ab) titers obtained from 1st up to 13th month were 316.2; 125.9; 63.1; 50.1; 50.1; 39.8; 63.1; 63.1; 39.8; 79.4, 100.0; 74.4; and 79.4, respectively. The neutralizing Ab titers obtained in 1st month were different of the values observed from 2nd until 13th (P < 0.001), furthermore the Ab titers from 2nd month was statistical different of 4th (P = 0.01) and 6th (P = 0.05). The frequencies (%) of positive heifers for p80 from 1st up to 13th were 24.7; 18.2; 10.4; 11.8; 73.3; 73.8; 72.4; 58.1; 45.9; 48.4; 46.2; 43.8 and 61.5, respectively. The correlation observed for neutralizing Ab titers and age was negative and weak (ρ= -0.299; P < 0.001). On the other hand, the correlation between positive heifers for p80 and age was positive and moderate (ρ= 0.319; P < 0.001). Discussion: The newborn calves had higher titers of neutralizing antibodies than other age groups and some calves were seropositive for the p80 protein. This profile points to the transfer of maternal antibodies produced by vaccination and/or natural exposure to BVDV. The exposure of the cows to the inactivated and live virus stimulates the production of neutralizing antibodies to the structural proteins of the virus, particularly the glycoprotein E2, detected by the serum neutralization test. The titers of serum neutralizing antibodies and the frequencies of seropositive for p80 protein decreased gradually from the first to the 4-6th month of life due to the metabolization of maternal immunoglobulins acquired by ingestion of colostrum. Frequencies of seropositive animals for protein p80 increased from the fifth month of life, which is the same moment that was observed declined of neutralizing maternal antibody titers. The phase of higher frequency of p80 positive animals coincides with greater rates of Babesiosis and Anaplasmosis. This history could justify the importance of the BVDV immunodepression as a risk factor for concomitant diseases. In general, the neutralizing antibody titers increased after the peaks of positive reactions to the p80 protein, but this moment coincides with the primo-vaccination in calves. Therefore, it is not possible to state the origin of these antibodies. Correlations between ages and serologic tests are consistent with previous data reporting the decrease in antibody titers and increase of seropositive animals for p80 protein, from the first month of life to puberty. In conclusion, maternal neutralizing Ab titers had gradual decreased whereas the frequency of positive heifers for p80 had increased values. The inversion observed between the maternal antibody titers and the increase in antibody for p80 indicates the moment of greatest risk for natural infections caused by BVDV.

Interface between Maternal Antibodies and Natural Challenge for Bovine Viral Diarrhea Virus (BVDV) in Holstein Heifers