Mastitis insights gained from different udder health groups

Mastitis, mainly caused by bacterial intramammary infection (IMI), is still the costliest disease in milk production. The disease causes an elevated somatic cell count (SCC) in the milk. A commonly used cut-off to distinguish between cows likely to be infected and normal is a SCC at 200,000 cells/mL. However, a major problem of the spread and persistence of mastitis within dairy herds is still the subclinical form, a condition where the udder and the milk appear normal although the mammary gland is inflamed, infected, or both. Detecting these cows early is very important because they act as a reservoir for bacteria, resulting in an unnoticeable spread of mastitis to healthy herd mates (Halasa et al, 2007). Even when the SCC is under the 200,000 cells/mL cut-off level, it does not rule out that the cow is fighting an infection and is in an early stage of mastitis already.

Increased sensitivity to detect IMI

Since recently, differential somatic cell count (DSCC) is available in addition to SCC and the availability of new technology allows the simultaneous determination of both in milk-testing laboratories. The DSCC represents the combined proportion of polymorphonuclear neutrophils (PMN) and lymphocytes as a percentage of total SCC. Using a combination of both SCC and DSCC gives a better understanding of the udder health status and leads to an increase of sensitivity to detect IMI in udder health monitoring programmes. Earlier studies by Wall et al. (2018) showed that DSCC increased significantly following IMI even when SCC did not. Recent studies (Schwarz et al., 2019 and 2020) described test characteristics and predictive values for the combination of DSCC and SCC in connection with IMI at the end of the lactation period or during lactation. In both studies, sensitivity for detection of IMI by major pathogens increased with the combination of SCC and DSCC compared to SCC alone.

Newly defined udder health groups

Due to the novelty of the DSCC parameter, its associations with the performance of dairy cows (e.g. milk production) have yet to be described. A new study delved into this more. The study is based on data originating from routinely performed DHI testing in Austria (federal state Styria only), China (province Henan), Estonia, Germany (federal state Thuringia), and Spain (province Lugo, Galicia). In total, 961,835 test-day results generated in these countries between January 2019 and March 2020 were available for data analyses. Cows were categorised into four newly defined udder health groups (UHG) based on their test-day DSCC and SCC results as described in Table 1. The SCC and DSCC cut-offs used for categorisation were identified as ideal cut-offs in a previous study (Schwarz et al., 2020). Moreover, the SCC cut-off of 200,000 cells/mL is also recommended by IDF (2013). The main part of the study was focused on analysing the performance of dairy cows in these four different groups.

Productivity difference between UHG A and B

The results of the study clearly showed that the performance of dairy cows differed significantly between the four UHG across all countries/regions investigated. Table 2 shows the milk production per udder health group for the data from Germany.

When looking at the results for Austria, China, Estonia, Germany, and Spain (n = 961,835), as anticipated, healthy cows (UHG A) indicated the highest productivity. Cows in UHG B indicated significantly lower productivity compared to cows in UHG A (despite similar SCC results), which might be explained by the fact that these cows are in the early stage of mastitis (Schwarz et al., 2011a, b, 2020; Pilla et al., 2012) as discussed above. Inflammatory reactions (i.e. elevated DSCC) require high amounts of energy (Sordillo, 2016) and thus it can be assumed that this energy, in turn, is not available for milk synthesis. Besides major pathogens, more harmful minor pathogens such as S. chromogenes, S. epidermidis, S. haemolyticus and S. simulans might also trigger inflammatory reactions in UHG B (Schwarz et al., 2020). The small but significant differences in performance between cows in UHG A and B might not be noticeable as such on a day-to-day basis in dairy farms. Nevertheless, they are relevant because cows in group B have an increased risk for IMI caused by major pathogens (Schwarz et al., 2020) and the lower performance observed in this study indirectly confirms on-going inflammatory processes as discussed in detail above. Hence, there is a certain risk that such cows may develop high SCC and/or transmit mastitis pathogens to herd mates.

Productivity difference between UHG C and D

While it was expected to find low dairy cow performance in the SCC area >200, 000 cells/mL, the evident difference in performance depending on DSCC above (UHG C) or below cut-off (UHG D) was intriguing. The immune system of cows in UHG C is actively combating mastitis pathogens and thus consuming energy that is not available for milk synthesis. Cows in UHG D indicated generally the lowest performance. It is known that mastitis pathogens and the immune response may damage udder tissue and that this worsens with the duration of mastitis (Zhao and Lacasse, 2008). This might explain the lower performance of cows suffering chronic/persistent mastitis. The differences between cows in groups C and D would likely be noticeable in day-to-day management on dairy farms, and particularly group D would be of high interest given the low productivity of such cows.

Older cows in groups C and D

Comparing the proportion of cows in the different UHG by parity and days in milk revealed that cows with multiple parities, as well as late lactating cows, occur more frequently in groups C and D than younger and fresh lactating cows. This is in agreement with previous research (Olde Riekerink et al., 2007) demonstrating that older and late-lactating cows develop and maintain high SCC more often than young cows. The seasonal changes in distribution of cows among UHG observed in the study are in line with the fact that the incidence of mastitis is higher during summer months compared to winter months (Olde Riekerink et al., 2007).

Effect on milk revenue per cow

The estimated milk value (EMV) provides information on the revenue generated per cow and day and thus essentially summarises the dairy cow performance (milk yield, fat and protein production) in one number. In this study, the EMV was calculated per cow and test-day according to local milk prices from June 2020. Interestingly, the results revealed that EMV was significantly different in cows with the same SCC (e.g. either ≤ or >200,000 cells/mL) but different DSCC results (either > or ≤65 %) each. This, in turn, opens up the possibility to further optimise profitability of a dairy herd (e.g. through increasing the proportion of cows in UHG A).

Conclusion

This study demonstrated changes in performance of dairy cows depending on their udder health status as defined based on the combination of SCC and DSCC (i.e. udder health groups). The distribution of cows among the four UHG also varied hugely between farms. This study also showed that the combined use of DSCC and SCC opens up the possibility to categorise cows into four UHG and provides more detailed insights (e.g. cows in early stage of mastitis, cows with chronic mastitis) compared to working with SCC alone. In particular cows in UHG B and D are of interest as they cannot be identified working with SCC only. However, actual measures in terms of changes of day-to-day udder health management on dairy farms require further investigation.

Read the article describing the practical value of DSCC for dairy farmers and the full length insightful interview with Qnetics’ General Manager & Head of DHI operations, who have successfully implemented DSCC as a valuable tool to improve udder health management.