Staphylococcus aureus (SA) is the most prevalent contagious mastitis pathogen on US dairies and contributes significantly to cow mortality and loss of revenue. Despite its prevalence, our understanding of the epidemiology of naturally occurring SA intramammary infections (IMI) is limited. The research described herein attempts to provide a basic understanding of the epidemiology of naturally occurring SA IMI.
Using samples collected during two studies, we found that the sensitivity of microbiologic culture for detecting SA IMI using a 0.01mL inoculum was 91% and the specificity was 99.4%. Using a larger (0.1mL) inoculum the sensitivity increased to 96.8% and the specificity was 99.3%. This study demonstrated a clear benefit of using a larger inoculum volume (0.1mL) for culture in the microbiologic detection of SA IMI.
The daily shedding patterns of SA over three 21 day sample periods was evaluated. Although the amount of SA recovered from infected quarters varied tremendously, 97.7% of the samples were culture positive. There were no consistent patterns of shedding or a relationship between shedding of SA and SCC. Using pulse field gel electrophoresis (PFGE) to evaluate the association of strain type and shedding of SA, quarters grouped into PFGE pulsotype 1 shed at consistently higher levels and with comparatively little variability, (median cfu/0.01mL=100) while quarters placed in PFGE pulsotype 2 shed SA at consistently lower levels (median cfu/0.01mL=59, p< 0.0001).
The second study examined naturally occurring SA IMI on 2 different dairies over an entire lactation further demonstrating a consistent recovery of SA from naturally infected quarters provided a large enough (0.1mL) inoculm was used. The sensitivity of culture was 95%, compared to 85% when using a smaller volume (0.01mL). There were no consistent shedding patterns, other than that of consistently higher versus consistently lower. Quarters shedding consistently higher levels of SA were again grouped into PFGE pulsotype 1 further supporting the influence of strain type on either persistence or pathogenicity. Strains clustered into PFGE pulsotype 1 had a median cfu/0.01mL of 83 while strains grouped into PFGE pulsotype 2 had a median cfu/0.01mL of 17.
The third study examined the effect of strain type on SA IMI cure rates. Evaluating cure by parity, 42% of the first lactation cows cured while only 10% of the multiparous cows cured.
We found no significant difference (p=0.64) in cure rates in quarters with low quarter level SCC (200,000 cells/mL) and high SCC. Nor was there any significant difference (p = 0.35) when evaluating cure using composite test day information.
In conclusion, when using a larger inoculum volume, the influence of bacterial shedding on the microbiological diagnosis of SA IMI is minimal. While there were no shedding pattern or cycle identified in shedding, quarters infected with bacteria in pulsotype 1 shed at significantly higher levels and experience fewer cures over the dry period. This highlights the potential for pre-screening heifers prior to parturition for SA IMI and possible dry cow therapy providing a cure for SA IMI before the heifer enters the milking parlor.