Facteurs de risque modifiables associés à l’incidence, l’élimination et la prévalence d’infections intra-mammaires chez la vache laitière en lactation

Subclinical mastitis is a very frequent and costly health issue that can be detected using somatic cell count (SCC) measurements, but requires the use of milk bacteriological culture for identification of the causal pathogen.  Because of this latter difficulty, nearly all subclinical mastitis research has focused on prevalent intramammary infections (IMI) and less is known on risk factors for IMI incidence or elimination.  The main objective of this thesis was to identify the manageable risk factors associated with the incidence, elimination, and prevalence of IMI of importance in Canadian dairy herds. First, a systematic review of the literature on the associations between management practices used on farms and SCC was carried out.  Management practices consistently associated with SCC were identified and differentiated from other practices for which anecdotic reports were available. Then, a bilingual questionnaire was developed, validated, and employed to measure the practices used on a sample of 90 Canadian dairy herds.  To validate this tool, measures of repeatability and of validity of the questionnaire’s items were analyzed and an evaluation of the equivalence of the English and French versions was conducted.  These analyses indicated that the questionnaire was, in general, acceptable, but also pinpointed some problematic items.  These items were recategorized when possible or otherwise excluded from subsequent analyses to ensure good data quality.  Most of the herds studied were already using post-milking teat disinfection and blanket dry cow therapy.  Many other frequently recommended practices were not widely adopted. Next, manageable risk factors associated with Staphylococcus aureus IMI incidence, elimination, and prevalence were investigated on the 90 selected herds in a longitudinal fashion.  The S. aureus IMI incidence appeared to be a stronger determinant of the herd prevalence than the elimination rate.  Among other practices, wearing gloves during milking, using pre-milking teat disinfection, and having an adequate teat end condition showed desirable associations with the outcomes.  These results highlight the importance of good milking practices to achieve a long-term reduction of S. aureus IMI prevalence. Finally, manageable risk factors associated with coagulase-negative staphylococci (CNS) IMI incidence, elimination, and prevalence were investigated in a similar manner.  Because of the greater limitations of milk bacteriological culture to identify IMI caused by this group of pathogens, a semi-Bayesian latent class model approach was used.  Sensitivity and specificity estimates from an internal validation study were used to link the observed IMI milk culture result to the latent true quarter IMI status.  Non-adjusted estimates of IMI incidence, elimination, prevalence, and of associations with expositions appeared to be considerably biased by the diagnostic procedure’s imperfections.  Most often, estimates were biased toward the null value.  Again, IMI incidence was the main determinant of the herd IMI prevalence.  Sand and wood products bedding were associated with lower CNS incidence and prevalence.  Sending cows to pasture was also associated with lower CNS IMI incidence and prevalence.

Keyphrases: Mastitis, Dairy cows, Canada, Management practices, Somatic cell count, Staphylococcus aureus, Coagulase-negative staphylococci, Questionnaire, Validation, Translation