Clinical and subclinical mastitis incidence in pasture-based dairy cows.

Abstract

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.