title: Structural equation models to estimate risk of infection and tolerance to bovine mastitis.
abstract: BACKGROUND
  One method to improve durably animal welfare is to select, as reproducers, animals with the highest ability to resist or tolerate infection. To do so, it is necessary to distinguish direct and indirect mechanisms of resistance and tolerance because selection on these traits is believed to have different epidemiological and evolutionary consequences.
  
  
  METHODS
  We propose structural equation models with latent variables (1) to quantify the latent risk of infection and to identify, among the many potential mediators of infection, the few ones that influence it significantly and (2) to estimate direct and indirect levels of tolerance of animals infected naturally with pathogens. We applied the method to two surveys of bovine mastitis in the Walloon region of Belgium, in which we recorded herd management practices, mastitis frequency, and results of bacteriological analyses of milk samples.
  
  
  CONCLUSIONS
  Structural equation models suggested that, among more than 35 surveyed herd characteristics, only nine (age, addition of urea in the rations, treatment of subclinical mastitis, presence of dirty liner, cows with hyperkeratotic teats, machine stripping, pre- and post-milking teat disinfection, and housing of milking cows in cubicles) were directly and significantly related to a latent measure of bovine mastitis, and that treatment of subclinical mastitis was involved in the pathway between post-milking teat disinfection and latent mastitis. These models also allowed the separation of direct and indirect effects of bacterial infection on milk productivity. Results suggested that infected cows were tolerant but not resistant to mastitis pathogens.
  
  
  CONCLUSIONS
  We revealed the advantages of structural equation models, compared to classical models, for dissecting measurements of resistance and tolerance to infectious diseases, here bovine mastitis. Using our method, we identified nine major risk factors that were directly associated with an increased risk of mastitis and suggested that cows were tolerant but not resistant to mastitis. Selection should aim at improved resistance to infection by mastitis pathogens, although further investigations are needed due to the limitations of the data used in this study.
authors.affiliation: Department of animal production, University of Liège, Liège, 4000, Belgium. jdetilleux@ulg.ac.be
authors.forename: Johann
authors.initials: J
authors.lastname: Detilleux
authors.forename: Léonard
authors.initials: L
authors.lastname: Theron
authors.forename: Jean-Noël
authors.initials: JN
authors.lastname: Duprez
authors.forename: Edouard
authors.initials: E
authors.lastname: Reding
authors.forename: Marie-France
authors.initials: MF
authors.lastname: Humblet
authors.forename: Viviane
authors.initials: V
authors.lastname: Planchon
authors.forename: Camille
authors.initials: C
authors.lastname: Delfosse
authors.forename: Carlo
authors.initials: C
authors.lastname: Bertozzi
authors.forename: Jacques
authors.initials: J
authors.lastname: Mainil
authors.forename: Christian
authors.initials: C
authors.lastname: Hanzen
mesh_terms: D000822:Animal Husbandry; D000818:Animals; D001419:Bacteria / Q000302:isolation & purification; D002417:Cattle; D003627:Data Interpretation, Statistical; D060467:Disease Resistance; D005260:Female; D008414:Mastitis, Bovine / Q000453:epidemiology* / Q000209:etiology / Q000382:microbiology; D008892:Milk / Q000382:microbiology; D015233:Models, Statistical; D012307:Risk Factors