Evaluation of the University of Minnesota Easy Culture System II and the 3M Petrifihn for diagnosis of mastitis causing organisms

Despite extensive research in the field of bovine mastitis, it is still widely considered the most costly infectious disease of dairy cattle. In Canada, mastitis treatment accounts for more than half of all antibiotics used by dairy producers. Public concern over the use of such antibiotics in animal agriculture requires the prudent use of these products on dairy farms. In the mid-1990s, researchers began to call for a more targeted approach to mastitis therapy. Several studies have been conducted to find appropriate diagnostic tools that could be used in such a manner. However, further research is required to validate the tests that are currently available.;The objective of the first study was to determine test characteristics (versus standard culture techniques) of 2 potential on-farm culture systems for clinical mastitis, the Minnesota Easy Culture System II Bi-plate (University of Minnesota, Minneapolis, MN) and the Petrifilm (3M, Minneapolis, MN). The tests were evaluated using 282 clinically positive mastitic milk samples (not frozen) from 21 commercial dairy farms to determine their ability to differentiate appropriate treatment groups. All cases that had gram positive growth were considered treatment candidates, while cases that grew gram negative organisms only, or yielded no bacterial growth, were classified as no treatment. Both the Bi-plate and 3M Petrifilm systems were highly sensitive, 97.9% (95% CI: 94.0 - 99.6) and 93.8% (95% CI: 87.7 - 97.5), respectively. The very high negative predictive values (96.4% (95% CI: 89.9 - 99.3) for Bi-plate and 89.4% (95% CI: 79.4 - 95.6) for Petrifilm) were important attributes of each of these tests, which will minimize the number of cases requiring therapy that go untreated. The results indicate that both tests have the potential to appropriately categorize clinically mastitic cases.;The primary objective of the second study was to compare microbiological results of the University of Minnesota Tri-plate and the 3M Petrifilm Staph Express (STX) Count Plate to standard culture techniques for identification of clinical mastitis caused by S. aureus. The secondary objective of this study was to evaluate the Tri-plates ability to differentiate Streptococcus spp. from other gram positive organisms. Both the University of Minnesota Tri-plate and the 3M STX Petrifilm were able to successfully detect S. aureus in 271 clinically positive mastitic milk samples (not frozen) from 21 commercial dairy farms when the tests were used in a laboratory setting. The presence of beta hemolysis on the Tri-plate was a highly sensitive method (97.9%, 95% CI: 88.7 -- 99.0) to diagnose S. aureus when read by a trained laboratory technician. Yet, when read by individuals with limited microbiology experience, the Se of the Tri-plate was much lower, ranging from 43.2% to 59.1%. Specificity of the Tri-plate was higher when read by the inexperienced readers (93.8% - 95.9%) than by the laboratory technician (81.8%). The Tri-plate was able to successfully differentiate Strep. spp. from other gram positive organisms in clinically mastitic cases.;Finally, the objective of the third study was to evaluate the use of two on-farm tests as part of a selective dry cow therapy program. The 3M Petrifilm and the University of Minnesota Tri-plate were evaluated on their ability to classify a sample as either infected or uninfected at the time of dry-off. Each test was evaluated individually as well as in combination with somatic cell count data obtained from monthly Dairy Herd Improvement records. Using the 3M Petrifilm alone to classify a sample as infected resulted in a sensitivity of 100% (95% CI: 94.7 -- 100) and a specificity of 61.2% (95% CI: 53.8 -- 68.1). Using the Tri-plate alone to classify a sample as infected resulted in a sensitivity of 96.4% (95% CI: 90.2 -- 98.8) and a specificity of 44.6% (95% CI: 35.9 -- 53.6). Both the 3M Petrifilm and University of Minnesota Tri-plate are rapid and reliable tests that could be used successfully in a selective dry cow therapy program.