title: Mediation analysis to estimate direct and indirect milk losses associated with bacterial load in bovine subclinical mammary infections.
abstract: Milk losses associated with mastitis can be attributed to either effects of pathogens per se (i.e. direct losses) or to effects of the immune response triggered by the presence of mammary pathogens (i.e. indirect losses). Test-day milk somatic cell counts (SCC) and number of bacterial colony forming units (CFU) found in milk samples are putative measures of the level of immune response and of the bacterial load, respectively. Mediation models, in which one independent variable affects a second variable which, in turn, affects a third one, are conceivable models to estimate direct and indirect losses. Here, we evaluated the feasibility of a mediation model in which test-day SCC and milk were regressed toward bacterial CFU measured at three selected sampling dates, 1 week apart. We applied this method on cows free of clinical signs and with records on up to 3 test-days before and after the date of the first bacteriological samples. Most bacteriological cultures were negative (52.38%), others contained either staphylococci (23.08%), streptococci (9.16%), mixed bacteria (8.79%) or were contaminated (6.59%). Only losses mediated by an increase in SCC were significantly different from null. In cows with three consecutive bacteriological positive results, we estimated a decreased milk yield of 0.28 kg per day for each unit increase in log2-transformed CFU that elicited one unit increase in log2-transformed SCC. In cows with one or two bacteriological positive results, indirect milk loss was not significantly different from null although test-day milk decreased by 0.74 kg per day for each unit increase of log2-transformed SCC. These results highlight the importance of milk losses that are mediated by an increase in SCC during mammary infection and the feasibility of decomposing total milk loss into its direct and indirect components.
authors.affiliation: 1Department of Animal Production, Farah Research Centre from the Faculty of Veterinary Medicine,University of Liège,Quartier Vallée 2,4000 Liège,Belgium.
authors.forename: J
authors.initials: J
authors.lastname: Detilleux
authors.affiliation: 2Large Animal Clinic, Farah Research Centre from the Faculty of Veterinary Medicine,University of Liège,Quartier Vallée 2,4000 Liège,Belgium.
authors.forename: L
authors.initials: L
authors.lastname: Theron
authors.affiliation: 3Department of Parasitic and Infectious diseases, Farah Research Centre from the Faculty of Veterinary Medicine,University of Liège,Quartier Vallée 2,4000 Liège,Belgium.
authors.forename: J-N
authors.initials: JN
authors.lastname: Duprez
authors.affiliation: 4Association Wallonne de l'Elevage,4 rue de Champs Elysées,5590 Ciney,Belgium.
authors.forename: E
authors.initials: E
authors.lastname: Reding
authors.affiliation: 1Department of Animal Production, Farah Research Centre from the Faculty of Veterinary Medicine,University of Liège,Quartier Vallée 2,4000 Liège,Belgium.
authors.forename: N
authors.initials: N
authors.lastname: Moula
authors.affiliation: 1Department of Animal Production, Farah Research Centre from the Faculty of Veterinary Medicine,University of Liège,Quartier Vallée 2,4000 Liège,Belgium.
authors.forename: M
authors.initials: M
authors.lastname: Detilleux
authors.affiliation: 4Association Wallonne de l'Elevage,4 rue de Champs Elysées,5590 Ciney,Belgium.
authors.forename: C
authors.initials: C
authors.lastname: Bertozzi
authors.affiliation: 2Large Animal Clinic, Farah Research Centre from the Faculty of Veterinary Medicine,University of Liège,Quartier Vallée 2,4000 Liège,Belgium.
authors.forename: C
authors.initials: C
authors.lastname: Hanzen
authors.affiliation: 3Department of Parasitic and Infectious diseases, Farah Research Centre from the Faculty of Veterinary Medicine,University of Liège,Quartier Vallée 2,4000 Liège,Belgium.
authors.forename: J
authors.initials: J
authors.lastname: Mainil
mesh_terms: D000822:Animal Husbandry / Q000379:methods; D000818:Animals; D058345:Asymptomatic Infections; D058491:Bacterial Load / Q000662:veterinary; D001530:Belgium; D002417:Cattle; D002452:Cell Count / Q000662:veterinary; D005260:Female; D007774:Lactation; D008414:Mastitis, Bovine / Q000276:immunology* / Q000382:microbiology*; D008892:Milk / Q000166:cytology* / Q000378:metabolism / Q000382:microbiology*; D008954:Models, Biological
keywords: bovine; mastitis; mediation analysis; milk loss; tolerance