Evaluation of a real-time polymerase chain reaction assay for rapid identification of methicillin-resistant Staphylococcus aureus directly from nasal swabs in horses.

This research assessed the effectiveness and accuracy of a real-time polymerase chain reaction (RT-PCR) assay to rapidly identify methicillin-resistant Staphylococcus aureus (MRSA) in horses, and compared it to traditional culture techniques. The study found that the RT-PCR test had poor agreement with culture tests and a high rate of unresolved results, indicating it may not be a practical screening method for MRSA in horses.

Study Purpose and Methology

• The research aimed to evaluate the usefulness and efficiency of an RT-PCR test for timely identification of MRSA from nasal swabs in horses. The intent was to assess if this test could serve as an effective and quicker alternative to standard culture methods.
• The assay under assessment, named IDI-MRSA, has previously been validated for diagnostic use in humans. However, this was the first instance of evaluating its performance in horses.
• The investigators utilized nasal swabs from 293 horses. These samples were analyzed both by the PCR testing and standard culture methods, a step undertaken to compare the results generated by both approaches.

Results and Findings

• Initially, only 176 out of the 293 samples provided valid PCR results. Among these valid samples, the results for two samples were positive while 167 were negative, as determined by both PCR and culture methods.
• There were seven samples that were recognized as positive by PCR but were negative according to culture testing. However, no samples were found negative by PCR and positive by culture. This discrepancy led to a kappa statistic of 0.35, indicating poor agreement between the two testing methods.
• Among the remaining, unresolved 117 samples, 105 were labelled as “unresolved” initially. After one freeze-thaw cycle of the lysates – a suggested solution for “unresolved” samples – more than half (61/110, or 55%) remained unresolved.

Conclusion

• The study concluded that the IDI-MRSA assay may not be an effective screening tool for detecting MRSA in horses. The significant disagreements with the culture methods and the high unresolved rate (37%) significantly reduce the clinical utility of the test.
• Thus, while the RT-PCR method might provide quicker results, its lack of accuracy and reliability compared to traditional culture techniques limits its practical use in MRSA screening for horses.