Abstract
Mastitis due to intramammary infection is one of the most economically relevant diseases in dairy cows, causing reductions in milk quality and quantity. Currently, mastitis monitoring is based on somatic cell count (SCC) and bacteriologic culture (BC) of milk. Nevertheless, inflammation-specific protein markers might provide more sensitive and reliable assays, enabling immunoassay-based screening strategies. Cathelicidin is an inflammatory protein released in milk that has recently demonstrated fair reliability and diagnostic potential for ewe mastitis. To assess its performance in cows, 531 quarter milk samples from 2 herds were tested using cathelicidin ELISA, SCC, and BC. We found that 29.0% of samples were positive for cathelicidin, 18.8% had SCC >200,000 cells/mL, and 13.7% were BC-positive. Cathelicidin showed a strong positive correlation with SCC as demonstrated by receiver operating characteristics curve analysis and by the clustering of cathelicidin-negative and cathelicidin-positive samples in association with low and high SCC values, respectively. For evaluating the diagnostic performance of a novel test, BC cannot be considered a reliable gold standard for true disease status because of its known limitations. Therefore, we assessed the sensitivity (Se) and specificity (Sp) of the milk cathelicidin ELISA using a latent class analysis approach together with BC and SCC by considering different diagnostic thresholds to identify the preferred Se/Sp combination. We modeled conditional dependence of cathelicidin and SCC to account for their close association. The cathelicidin ELISA showed higher Se than SCC and BC for almost all threshold combinations. In fact, at the best-performing threshold combination, the Se of cathelicidin was 80.6%, 6.2 percentage points higher than that of SCC >200,000 cells/mL (74.4%) and similar to that of SCC >100,000 cells/mL (80.2%). Most importantly, this Se was obtained with a loss in Sp of only 1.4 percentage points compared with SCC >200,000 cells/mL (94.9% Sp for cathelicidin vs. 96.3% for SCC >200,000). The limited Se of BC (38.8%) was also confirmed in this study, and BC showed a slightly lower Sp than both cathelicidin and SCC for most of threshold combinations. This study confirmed that cathelicidin is released in the milk of cows with mastitis and that its presence is highly correlated with SCC. The measurement of cathelicidin by ELISA may hold significant potential for improving the sensitivity of mastitis detection in dairy cows while maintaining high specificity.
Highlights
Mastitis is a major cause of economic loss in the dairy industry worldwide
29.0% of the samples were positive for cathelicidin, 18.8% had somatic cell count (SCC) >200,000 cells/mL, and 13.7% were positive for bacteriologic culture (BC)
Measurement of SCC in milk has long been established as the standard for monitoring mastitis in cows, and the presence of SCC >200,000 cells/mL is typically considered a strong indicator of mastitis (Schukken et al, 2003)
Summary
Mastitis is a major cause of economic loss in the dairy industry worldwide. It remains one of the most relevant diseases in dairy cows, responsible for changes in milk quality and reduced milk production (Bradley, 2002; Le Roux et al, 2003; De Vliegher et al, 2012). Despite its relevance, mastitis can be difficult to detect before the onset of clinical symptoms, before the appearance of milk quality alterations, or when the case is subclinical. Increased SCC indicates only that an IMI may have occurred (Dohoo and Leslie, 1991; Dohoo, 2001; Fox, 2013), and the presence of a low number of cells in milk is only very likely associated with the absence of mastitis. The threshold of 200,000 cells/mL is currently believed to provide the least diagnostic error when determining whether an IMI is present (Dohoo and Leslie, 1991; Schukken et al, 2003; Bradley and Green, 2005)
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