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Clinical and subclinical mastitis incidence in pasture-based dairy cows
S McDougall1,2, J Williamson3, J Lacy-Hulbert3
1Tāwharau Ora - School of Veterinary Science, Massey University, Palmerston North, New Zealand.
Aims:
To estimate prevalence and incidence of intramammary infections (IMI) using herd test somatic cell counts (SCC) or quarter-level microbiology in a single pasture-based, seasonal calving dairy herd.
Methods:
Over four seasons (2010/11-2013/14) milk samples were collected for microbiology from quarters of all cows at four times; calving, MID1 (mean 116 (SD 21) days in milk (DIM)), MID2 (mean 204 (SD 21) DIM), and at drying off (DO; mean 266 (SD 30) DIM). SCC was determined every 1-4 (median = 2) weeks. Quarters were defined as having a new IMI if a bacterium was isolated that was not present at the preceding milk sampling and a cow was defined as having a new IMI where one or more quarters had a new IMI. Cows were defined as having a new high SCC where SCC increased to ≥ 200,000 cells/mL. Binary logistic regression and Poisson models were used to estimate prevalence and incidence rate (IR) of new infections. Cow-level IR estimates from bacteriology and SCC were compared using χ2 analyses, and the sensitivity and specificity were estimated for SCC to estimate IR, assuming bacteriology was the gold standard.
Results:
The prevalence of cows with a high SCC was 12.7 (95% CI = 12.3-13.1)% and quarter-level prevalence of IMI was 10.7 (95% CI = 10.2-11.1)%. The unadjusted IR for a new cow-level high SCC was 2.2 (95% CI = 2.0-2.4)/1,000 cow-days and for a new quarter-level IMI was 0.82 (95% CI = 0.71-0.95)/1,000 quarter-days at risk. Prevalence and IR were associated with DIM (p < 0.001), age (p < 0.001), and season (p < 0.001). The agreement between the IR based on herd test SCC and microbiology at cow-level was 77.1% (p < 0.001), with the level of agreement highest in mid-lactation. The sensitivity and specificity of an increase in SCC were 29.8% and 88.9%, respectively.
Conclusions:
Prevalence and IR varied by season, age and DIM. While the IR estimates differed between estimates based on a high SCC at the cow-level, and quarter-level microbiology, the level of agreement was 77% suggesting utility of cow-composite SCC data as a mastitis monitoring tool. However, the sensitivity of incidence based on elevated SCC was only approximately 30% relative to quarter-level microbiology as the gold standard.
Clinical Relevance:
This study provides the first estimates of prevalence and incidence of IMI in pasture-based dairy cows.Abbreviations: DIM: Days in milk; DO: Dry Off; EMM: Estimated marginal mean; IMI: Intramammary infection; IR: Incidence rate; NAS: Non-aureus staphylococci; SCC: Somatic cell count; TAR: Time at risk.
