pathogen infections in the SCC range 50,000 to 200,000 cells/ mL nearly doubled from 14.3% at DSCC of <50% to 26.1% at DSCC of >70%.
A combination of SCC and DSCC The actual test characteristics, specifically sensitivity, specificity, negative and positive predictive values, were evaluated for de- tection of mastitis caused by major pathogens (because treat- ment of infections caused by minor pathogens is unwanted). Test characteristics were determined separately for DHI samples and hand-stripped samples. In both cases, the test characteris- tics were established for using SCC alone, DSCC alone, and the combination of SCC and DSCC each as indicator to identify in- fections caused by major pathogens. All results of this analysis are summarised in Table 2. Most interestingly, classifying cows as infected based on a cut-off of 200,000 cells/mL for SCC alone and in terms of using SCC combined with DSCC based on either >60% and/or >200,000 cells/mL, the sensitivity changed from 47 to 66% and the specificity from 74 to 54%. More specifically, the number of correctly identified infections could be increased by 40% from 30 to 42 when adding DSCC as an indicator in this case. At the same time, the negative predictive value changed from 84 to 86% and the positive predictive value from 32 to 27%. It is worth noting that DHI samples were collected in the period up to 42 days before dry-off. Our study confirmed that DHI samples are offering a valuable indication of the in- tramammary infection status by the time cows are dried off. Sensitivity could generally be improved by changing from a higher to a lower SCC cut-off. However, the findings from our study show that sensitivity can be improved when adding DSCC as an additional indicator, even if the SCC cut-off remains unchanged. Such an improvement in sensitivity is of practical importance because the identification of as many infections by major pathogens is clearly desired in terms of dry cow therapy. Without any doubt such cases present a risk for udder health issues at the beginning of the subsequent lactation when they are undetected and left untreated at dry-off. At the same time, truly healthy cows should not be treated to reduce the consumption of antimicrobials. Thus, parameters and cut-offs with the highest sensitivity and negative predictive value would be the best choice for basing dry cow therapy decisions on.
Results from hand-stripped samples Another interesting finding was that the test characteristics of DHI samples and hand-stripped samples are at the same levels (Table 2). This highlights that the additional effort con- nected with collection of hand-stripped samples in the time period 5 days before dry off would not be justified in cows that are enrolled for DHI testing anyway. Nevertheless, it could be recommendable and worthwhile to perform a California masti- tis test on the day of dry off to confirm the validity of the results obtained through the last DHI testing (e.g. confirm absence of mastitis) and include the results in the decision making process
100
60 80
10 20 30 40 50
0 0 200 400 600 SCC (x 1,000 cells/mL)
regarding dry cow therapy. The findings mentioned above further highlight that it would be recommendable to collect a hand-stripped sample in the period 5 days before dry off for cows that are not enrolled to DHI programmes and work with the results in terms of the dry cow therapy decision. It should be noted that we used bacteriological culture for identifica- tion of mastitis pathogens and thus the definition of the mas- titis status of cows at the end of lactation. However, it can come to misclassification (in particular false-negatives) of the mastitis status, e.g. because of intermittent shedding of mas- titis pathogens, shedding of too-low small numbers of the pathogens and ceased growth of pathogens. On the other hand, major mastitis pathogens were identified in the low SCC area in previous studies and interpreted as mastitis in its early stage based on elevated proportions of PMN/DSCC.
Conclusions Our study provides first insights on utilising the new DSCC parameter as a supplementary tool to SCC in connection with dry cow therapy decisions. The new parameter allows to improve sensitivity for identification of major pathogen infections at the end of lactation and thus helps to lower the risk that cows with such infections are dried off untreated. Yet, more work is needed to refine the practical application of this tool. Moreover, other practical applications of the new DSCC-based udder health management tool are identification of mastitis in its early stage and chronically infected cows during the lactation period.
This is a condensed version of the original research paper: Inves- tigation of differential somatic cell count as a potential new sup- plementary indicator to somatic cell count for identification of intramammary infection in dairy cows at the end of the lacta- tion period.
https://doi.org/10.1016/j.prevetmed.2019.104803.
▶ COW HEALTH | AUGUST 2020 31 800 1000
DSCC (%)
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