title: Challenges for bovine viral diarrhoea virus antibody detection in bulk milk by antibody enzyme-linked immunosorbent assays due to changes in milk production levels.
abstract: BACKGROUND
  Bovine viral diarrhoea (BVD) is considered eradicated from Denmark. Currently, very few (if any) Danish cattle herds could be infected with BVD virus (BVDV). The Danish antibody blocking enzyme-linked immunosorbent assay (ELISA) has been successfully used during the Danish BVD eradication program, initiated in 1994. During the last decade, the cattle herd size has increased while the prevalence of BVDV has decreased. In this study, we investigated how these changes could affect the performance of the Danish blocking ELISA and of the SVANOVIR® BVDV-Ab indirect ELISA. The latter has successfully been used to eradicate BVD in Sweden. Data (2003-2010) on changes in median herd size and milk production levels, occurrence of viremic animals and bulk milk surveillance were analysed. Additionally, the Danish blocking ELISA and the SVANOVIR ELISA were compared analyzing milk and serum samples. The prevalence of antibody positive milking cows that could be detected by each test was estimated, by diluting positive individual milk samples and making artificial milk pools.
  
  
  RESULTS
  During the study period, the median herd size increased from 74 (2003) to 127 cows (2010), while the prevalence of BVDV infected herds decreased from 0.51 to 0.02 %. The daily milk yield contribution of a single seropositive cow to the entire daily bulk milk was reduced from 1.61 % in 2003 to 0.95 % in 2010 due to the increased herd size. It was observed that antibody levels in bulk milk decreased at national level. Moreover, we found that when testing bulk milk, the SVANOVIR® BVDV-Ab can detect a lower prevalence of seropositive lactating cows, compared to the Danish blocking ELISA (0.78 % vs. 50 %). Values in the SVANOVIR® BVDV-Ab better relate to low concentrations of antibody positive milk (R(2) = 94-98 %), than values in the blocking ELISA (R(2) = 23-75 %). For sera, the two ELISAs performed equally well.
  
  
  CONCLUSIONS
  The SVANOVIR ELISA is recommended for analysis of bulk milk samples in the current Danish situation, since infected dairy herds e.g. due to import of infected cattle can be detected shortly after BVDV introduction, when only few lactating cows have seroconverted. In sera, the two ELISAs can be used interchangeably.
authors.affiliation: Section of Epidemiology, National Veterinary Institute, Technical University of Denmark, Bülowsvej 27, DK-1870, Frederiksberg C, Denmark. alefo@vet.dtu.dk.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Foddai
authors.affiliation: Section of Epidemiology, National Veterinary Institute, Technical University of Denmark, Bülowsvej 27, DK-1870, Frederiksberg C, Denmark. claes_private@hotmail.com.
authors.forename: Claes
authors.initials: C
authors.lastname: Enøe
authors.affiliation: Statistics and Data Analysis Section, Department of Applied Mathematics and Computer Science, Technical University of Denmark, Matematiktorvet, DK-2800, Lyngby, Denmark. anst@imm.dtu.dk.
authors.forename: Anders
authors.initials: A
authors.lastname: Stockmarr
authors.affiliation: Knowledge Centre for Agriculture, Cattle, Danish Cattle Federation, Agro Food Park 15, Skejby, 8200, Aarhus N, Denmark. kaspar.krogh.dk@gmail.com.
authors.forename: Kaspar
authors.initials: K
authors.lastname: Krogh
authors.affiliation: Section for Virology, National Veterinary Institute, Technical University of Denmark, Lindholm, DK-4771, Kalvehave, Denmark. aase.uttenthal@gmail.com.
authors.forename: Åse
authors.initials: Å
authors.lastname: Uttenthal
mesh_terms: D000818:Animals; D000914:Antibodies, Viral / Q000032:analysis*; D001912:Bovine Virus Diarrhea-Mucosal Disease / Q000175:diagnosis* / Q000453:epidemiology / Q000821:virology; D002417:Cattle; D003612:Dairying / Q000379:methods; D003718:Denmark / Q000453:epidemiology; D001908:Diarrhea Viruses, Bovine Viral / Q000302:isolation & purification*; D004797:Enzyme-Linked Immunosorbent Assay / Q000662:veterinary*; D005260:Female; D007774:Lactation; D008892:Milk / Q000378:metabolism / Q000821:virology*; D015995:Prevalence
chemical_list: D000914:Antibodies, Viral