Effects of seropositivity for bovine leukemia virus, bovine viral diarrhoea virus, Mycobacterium avium subspecies paratuberculosis, and Neospora caninum on culling in dairy cattle in four Canadian provinces - Tiwari, A; VanLeeuwen, J A; Dohoo, I R; Stryhn, H; Keefe, G P; Haddad, J P (Vet Microbiol, 2005)