Association between the level of antibodies in bulk tank milk and bovine respiratory syncytial virus exposure in the herd

Klem, T. B.; Tollersrud, T.; Østerås, O.; Stokstad, M.

The prevalence of antibodies against BRSV in BTM in the study area was similar for each of the four screenings, and thus from season to season. The time points were chosen in order to cover two winter seasons, as the peak incidence of clinical cases of respiratory disease by experience is expected to occur in the winter months. Some herds remained negative during the study period, but the majority were positive on both occasions. In a few herds, the results indicated elimination and new introduction of BRSV infection. Measurement of antibodies in BTM is a very slow changing tool to monitor BRSV infection in a herd. Klem and others (2013) found the elimination rate and number of new introductions of virus on a herd level to be high. In that study, the herds were classified based on serological findings in a group of young animals, which gives a more updated classification. Testing using BTM has lower power with regard to the correct classification of a herd's virus status than testing of individual milk samples and testing of serum from young animals (Klem and others 2013, Ohlson and others 2013) . Some negative herds were located in close proximity to positive herds. This may be due to better biosecurity measures in the negative herds. Another possible explanation is that the nearby positive herds are negative for virus, and thus do not represent a risk. Since the The herds were selected based on a high increase in level of antibodies (PP) against BRSV in two consecutive BTM samplings prior to the sampling occasion shown *The time since infection is based on analyses of antibodies in serum from the lactating cows and younger animals. The lower limit is defined by the age of the oldest animal negative for BRSV antibodies and the upper limit by the youngest positive animal. When all animals tested positive, the time since infection is indicated as less than the age of the youngest sampled animal PP, per cent positivity present study also shows that herds can be BTM-antibody-positive for more than four years without reinfection, the latter is likely. It would be interesting to see if herds can keep a negative status in proximity to herds that go through a new infection. In this study, both the number of negative herds and herds with new infection were too low to allow such investigations. Relatively large fluctuations in the PP values in samples taken only two weeks apart were found, and thus care should be taken when interpreting the results of a single BTM sample. A high value does not necessarily mean that BRSV has been present in the herd recently. Even a relatively large increase in paired samples taken two weeks apart can be observed in herds without viral introduction, based on the negative antibody status of younger animals. Detection of antibodies against BRSV in young cattle to reveal new introduction into a herd should be an adequate measure as the virus is reported to spread rapidly in herds during outbreaks (Rossi and Kiesel 1974 , Stott and others 1980 , Verhoeff and van Nieuwstadt 1984 , Bidokhti and others 2009 . The fluctuation cannot be explained by methodological errors as analyses of reproducibility show stable values for the positive controls. The level of antibodies in BTM is influenced by the composition of the milking herd. The immune status (antibody levels against BRSV) of the individual cows, the milk yield and which animals contribute to the bulk tank milk from day to day are the main reasons for fluctuating lev