Research Phd Theses

Selective antimicrobial treatment at dry off in dairy cows

Selective dry cow therapy (SDCT), in which only infected quarters/cows are treated with antimicrobials, constitutes an alternative to blanket dry cow therapy (BDCT) where all quarters of all cows at dry off receive antimicrobials, regardless of their infection status, for a more judicious use of antimicrobials. The objective of this thesis was to shed more light on targeted antimicrobial treatment decisions of infected quarters of cows at dry-off.

A randomized controlled trial was designed and a total of 569 cows (2,251 quarters) from 9 dairy herds in Québec, Canada with a bulk tank somatic cell count (SCC) <250,000 cells/mL were systematically enrolled and randomly allocated to 4 groups: 1) antimicrobial treatment alone to all quarters; 2) antimicrobial treatment combined with a teat sealant to all quarters; 3) selective antimicrobial treatment alone based on milk culture results on Petrifilm®; and 4) selective antimicrobial treatment combined with a teat sealant based on milk culture results on Petrifilm®. In the selective antimicrobial treatment groups, uninfected quarters received only a teat sealant. A reduction in antimicrobials use of 51.7% (95% CI: 39.2 – 64.3) was obtained and no significant differences were detected between quarter-based selective and blanket dry cow therapies, in terms of elimination of intramammary infections (IMI) and prevention of new IMI during the dry period, risk of a first case of clinical mastitis, daily mean milk yield and somatic cell count in the first 120 days of the subsequent lactation.

In addition to this randomized controlled trial, quarter milk culture using Petrifilm® was compared with SCC history through a Bayesian estimation of diagnostic accuracy to identify quarters or cows that should possibly be treated with antimicrobials in selective treatment protocols at dry off. Considering the availability of SCC data, the easiness of using just the last DHI test before dry off, and the high NPV that could be achieved, producers may consider using just the last DHI test as a potential tool to identify cows that should be treated with antimicrobials at dry off. Adding quarter-level on-farm milk culture using Petrifilm® to cows identified as unhealthy using cow-level SCC data could improve the test accuracy (mainly the PPV) and further reduced the use of antimicrobials.

A follow up on the use of teat sealant was performed to determine the proportion of quarters that had retained the sealant plug until the first milking after calving and the persistence of internal teat sealant residues in milk after calving. A sealant plug was present at first milking after calving for 83% quarters, quarters without a plug at first milking appeared to have been protected from new IMI during dry period, and the sealant residues could be observed in milk up to 12 days in milk, although 75% of the quarters had expelled the last residues by 5 days in milk.

A systematic review and a series of meta-analyses were conducted to investigate the efficacy of SDCT compared with BDCT, to guide decision-makers and users to engage in a more effective and judicious use of antimicrobials at dry-off. Thirteen articles representing 12 controlled trials, whether randomized or not, were available for analyses. Evidences strongly support that SDCT would reduce the use of antimicrobials at dry off by 66% (95% CI: 49 – 80), without any detrimental effect on udder health or milk production during the first months of the subsequent lactation, if, and only if, internal teat sealants are used for healthy, untreated quarters/cows.

Fidèle Kabera received his Doctorate in Veterinary Medicine (2007) and a post-graduate diploma in animal production (2008) at the Ecole Inter-Etats des Sciences et Médecine Vétérinaires de Dakar, Sénégal. He contributed to the training of technicians in animal production and health at the University of Rwanda (2008 – 2015). He completed his PhD in veterinary sciences in 2021 at the Université de Montréal.

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