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Equine September 2008 (Vol 3, No 7)

Letters — PCR

by Carin Ponder, RVT


    I'm writing in response to "Evaluating Polymerase Chain Reaction-Based Tests for Infectious Pathogens" (July/August). This review and its companion article, "Polymerase Chain Reaction Test Interpretation" (May), provide a good overview of the basics of polymerase chain reaction (PCR) and its potential uses for infectious disease testing. Nevertheless, I would like to add a few clarifying comments to the article in the July/August issue.

    First, although the author states that the information presented in Table 1 is a "sampling," readers should be aware that it is a very small representation of what is currently available from the many state and commercial laboratories that offer PCR testing. It should also be noted that Table 1 summarizes the results of numerous studies, some of which were not conducted to validate a diagnostic PCR test and, therefore, may not accurately reflect the complete picture of PCR testing for a particular pathogen. The reader needs to understand that these studies draw conclusions based on the samples the authors chose to study/test, which can influence the interpretations. For example, Table 1 does not mention guttural pouch lavage for Streptococcus equi testing, although this sample is highly recommended. In addition, the table does not discuss some of the test protocols currently in widespread use, such as differential testing for the "wild-type" versus "mutant" equine herpesvirus (EHV) 1 and its associated ramifications for neurologic disease.

    Second, I would like to correct the information given for Equine Biodiagnostics, which is now IDEXX Equine Biodiagnostics. While the laboratory originated as a business startup at the University of Kentucky in 1995, it was never a university laboratory and, since 2003, has been a part of IDEXX Laboratories. Furthermore, the PCR test menu at IDEXX Equine Biodiagnostics is much broader than the one test (Sarcocystis neurona) listed in Table 1 . We currently offer seven of the 13 tests listed in the table (Salmonella spp, S. equi, Rhodococcus equi, EHV-1 wild-type and mutation, EHV-4, influenza A, and S. neurona), with others to be added soon. Readers can find the complete test menu, which is updated frequently, at www.idexx.com/ebi.

    Finally, I not only agree with the author's recommendation that a given laboratory should be questioned about its assays, I would also suggest that the requesting veterinarian is responsible for asking how a given test has been validated, how it compares to the relevant gold standard (although PCR is often "better"), and what quality control measures are used by the laboratory. In addition, veterinarians need to understand a test's appropriate uses, interpretations, and limitations. Review articles, such as these published in Compendium Equine, are important sources of information for veterinarians and are, therefore, most useful if they are as complete, accurate, and up-to-date as possible.

    Jennifer Morrow, PhD

    Medical Director

    IDEXX Equine Biodiagnostics

    The Author's Response

    Thank you for the updated information. My goal in Table 1 was to provide practitioners with a starting point for some of the tests. However, I recognize that in the rapidly expanding field of PCR testing, test availability and, indeed, the types of testing are changing rapidly. Therefore, I highly recommend that each practitioner develop a relationship with his or her preferred laboratory to allow discussion of the tests of interest and the optimum samples to provide.

    Julia Paxson, PhD, DVM

    I'm writing to ask for clarification regarding information in "Evaluating Polymerase Chain Reaction-Based Tests for Infectious Pathogens" (July/August). Table 1 indicates that to diagnose influenza and EHV-1 and EHV-4 infections, the "Best results are from guttural pouch swabs." Since this is not the diagnostic sample type recommended in the AAEP infection control guidelines1 for these diseases, I'm hoping the author can shed some light on her experience with testing this sample to diagnose influenza and EHV infection. In addition, I hope the author can describe the method used to collect a guttural pouch swab for testing purposes. A recent proceedings article2 from the AAEP suggests that the optimal sample for diagnosing EHV-1 infection is nasal or nasopharyngeal swabs, along with whole blood. To my knowledge, nasal or nasopharyngeal swabs are recommended for testing for influenza. In summary, I hope the author can (1) explain why her article indicates that guttural pouch swabs yield the best results for diagnosing influenza and EHV-1 and EHV-4 infections and (2) clarify for practitioners why her article does not compare her recommendations for diagnostic methods to those detailed in the AAEP infection control guidelines.

    Josie Traub-Dargatz, DVM, MS, DACVIM

    Colorado State University

    The Author's Response

    Thank you so much for your letter. In Table 1 , the conclusions regarding sample collection for EHV-1, EHV-4, and influenza A should read "Best results are from nasopharyngeal swabs." Since these three viruses are predominantly found in the upper airway and are not usually found in the guttural pouches, sampling the pouches would not be optimal. Thank you for bringing the error to my attention.

    Julia Paxson, PhD, DVM

    1. Equine Infectious Disease Outbreak: AAEP Control Guidelines. Accessed August 2008 at www.aaep.org/control_guidelines_nonmember.htm.

    2. Pusterla N, Mapes S, Wilson D.Comparison of the diagnostic sensitivity of nasopharyngeal and nasal swabs and use of viral loads for the molecular diagnosis of equine herpesvirus-1 infection. Proc AAEP 2007:220-224.

    References »

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