Predicting intramammary infection status at drying off using indirect testing of milk samples

Abstract

AIMS To evaluate the Rapid Mastitis test (RMT, or California Mastitis test) and electrical conductivity (EC) at drying off when used alone or in combination with herd test data (maximum or last herd test somatic cell counts (SCC) before drying off), to define cows or quarters with intramammary infection, using microbiological culture as the gold standard. METHODS Quarter-level milk samples (n=609) from clinically healthy cows (n=153), in three herds in the Waikato region of New Zealand, were tested at drying off using the RMT and EC, and were collected for microbiological culture. The maximum SCC and the SCC at the last herd test of the preceding lactation were determined for each cow. The sensitivity, specificity, and positive and negative predictive values for each test were calculated for different cut-points, using microbiological culture as the gold standard. Receiver operating characteristic curves were generated and the area under the curve (AUC) was calculated for each test. The same parameters were calculated for combinations of two tests in parallel or in series. RESULTS Infection with any pathogen was detected in 62/153 (40.5%) cows and 99/609 (16.3%) quarters, and with major pathogens in 7/153 (4.6%) cows and 8/609 (1.3%) quarters. When predicting infection with any pathogen at the cow-level, the coefficient of agreement was highest for SCC (<0.32) and RMT (<0.28) and lowest for EC (<0.12); the AUC for RMT and EC when used singly ranged from 0.57-0.69, and in combination with SCC ranged from 0.68-0.75. AUC were similar for tests that used either the last or the maximum SCC. CONCLUSIONS AND CLINICAL RELEVANCE When evaluated singly, RMT and EC had only low to moderate diagnostic utility compared to bacteriological culture. When they were combined with SCC and interpreted in parallel, the results were improved, but only moderately. For herds that conduct herd testing, a single herd test late in lactation was as predictive of intramammary infection at drying off as multiple herd tests through the lactation. For herds that do not conduct herd testing, RMT has greater utility than EC.

Evaluation of 4 different teat disinfection methods prior to collection of milk samples for bacterial culture in dairy cattle

“…Additionally, however, freezing of milk samples is commonplace in many studies on IMI. Milk sample contamination or false-positive results can occur easily on farm, with contamination rates ranging from 2.3 to 6.6% in other studies (Gohary and McDougall, 2018;Naqvi et al, 2018). In the present study, the contamination rate in group 4, the group that followed NMC teat preparation guidelines, was slightly higher than in previous reports, with a contamination rate of 10%.…”

Randomized controlled non-inferiority trial investigating the effect of 2 selective dry-cow therapy protocols on antibiotic use at dry-off and dry period intramammary infection dynamics

“…Another potential source of variability in responses to algorithm-guided SDCT is the algorithm itself. Many algorithms have been recommended by mastitis experts (Bradley et al, 2018;Gohary and McDougall, 2018;Vasquez et al, 2018), which use as few data points as possible (typically using only SCC and clinical mastitis data), to facilitate implementation under farm conditions. In the study by Scherpenzeel et al (2014), the SCC component of Non-inferiority analysis of dry period IMI cure risk for 2 selective dry cow therapy programs (SDCT), as compared with blanket dry cow therapy.…”

“…To our knowledge, no algorithm-guided SDCT trials have compared effects of different SCC cut points on measures of udder health. However, many algorithms with different SCC thresholds have been evaluated in their ability to predict IMI at dry-off (Pantoja et al, 2009;Gohary and McDougall, 2018;Lipkens et al, 2019). A recent multi-herd study in Belgium concluded that a single algorithm was unlikely to be highly accurate for all cows, and that other variables, such as herd-level estimates of subclinical mastitis prevalence, cow milk yield, and parity, should be incorporated into more sophisticated algorithms.…”

“…Among the disadvantages of algorithm-guided SDCT are that it requires the herd to be participating in a DHIA testing program and to be accurately detecting and recording clinical mastitis events into electronic herd records. Furthermore, several studies have shown the algorithms are not sensitive predictors of IMI caused by "minor" pathogens such as NAS and Corynebacterium spp., because such IMI often fail to increase SCC above commonly used SCC thresholds (e.g., 200,000 cells/mL; Gohary and McDougall, 2018). However, this potential limitation may actually be an advantage, as it is unclear whether quarters infected with minor pathogens benefit from antibiotic treatment.…”

Effect of using internal teat sealant with or without antibiotic therapy at dry-off on subsequent somatic cell count and milk production

“…It has been suggested to use at least one record within 90 d of drying off to identify cows more likely to contract mastitis in the periparturient period (Green et al, 2007). Gohary and McDougall (2018) found that a single herd test in late lactation was as useful as multiple tests across the lactation as a predictor of IMI at drying-off.…”