Today's Veterinary Practice provides comprehensive information to keep every small animal practitioner up to date on companion animal medicine and surgery as well as practice building and management.
Issue link: http://todaysveterinarypractice.epubxp.com/i/38074
| DIAGNOSIS OF CANINE HEARTWORM INFECTION Potential Study Bias Some potential for bias existed in the performance of diagnostic tests in this study. UÊ i>ÀÌÜoÀmÊinviVÌi`Ê>n`ÊnoninviVÌi`Ê>nim>lÃÊÜiÀiÊ selected from distinctly different populations rather than from a spectrum of diseased and nondiseased subjects.14 UÊ 7ÊinviVÌi`Ê>nim>lÃÊÜiÀiÊ>llÊ>ÀÌiviVi>llÞÊinviVÌi`°Ê Of these 47 dogs, we received information on the infection dose for 25 dogs, with necropsy data avail- able on 6 dogs. UÊ/hiÊÜoÀmÊLÕÀ`inÃÊinÊÌhiÊ>ÀÌiviVi>llÞÊinviVÌi`Ê`o}ÃÊ were similar to the mean number of worms report- ed for naturally infected dogs in Michigan, higher than those reported for Maine, and lower than those reported for Florida.8,15 However, margin- ally infected animals (worm burden of < 2 adult females), which can represent the largest numbers of false negative animals, were underrepresented and, therefore, our reported sensitivity values may be artificially elevated. On the other hand, reported specificity values may be artificially elevated due to incorporation bias within the CHW negative group (see Standards for Heartworm Testing).14 The accepted gold standard is absence of adult worms at necropsy, but our disease- free population was defined by alternative criteria that included previous negative CHW antigen test results. This can lead to significant bias and artificial elevation of test specificity. STANDARDS FOR HEARTWORM TESTING The accepted gold standard for CHW testing is presence or absence of adult worms in the pulmonary arteries and/or heart on necropsy examination.7-9 However, necropsy as a gold standard for diagnostic assays presents difficult procedural and ethical challenges. Therefore, our evaluation of positive samples for gold standard status was based on partial verification. We included a limited number of necropsy- confirmed samples obtained from a laboratory involved in CHW research. In addition, we included blood from dogs that were experimentally infected with CHW. Samples from healthy dogs that were highly likely to be CHW-negative due to geographic location and previous negative CHW antigen test results and/or history of preventive medication were considered to be negative for the purposes of this study (Table 2). 36 Today’s Veterinary Practice July/August 2011 Disclosure Statement Partial funding for this study was provided by SafePath Laboratories (safepath.com). Minimizing Bias /hiÊlikilihoo`ÊovÊLi>ÃÊÜ>ÃÊminimiâi`ÊÌhÀoÕ}hÊÃiliV- ÌionÊovÊÌÀ>iÌÃÊÌh>ÌÊÀi`ÕVi`ÊÌhiÊlikilihoo`ÊovÊÌhiÃiÊÃÕL- jects to be infected including: UÊ iÃi>Ãi-vÀiiÊVliniV>lÊ«ÀiÃinÌ>ÌionÊ>llÊ`o}î UÊ,iÃi`inViÊinÊ>ÊloÜ-ÀiÃkÊ>Ài>Ê>llÊ`o}î UÊ iÃÌoÀÞÊ ovÊ >`miniÃÌÀ>ÌionÊ ovÊ 7Ê «ÀiÛinÌ>ÌiÛiÊ (most dogs) UÊ*ÀiÛioÕÃÊ ni}>ÌiÛiÊ ÌiÃÌÊ ÀiÃÕlÌÃÊ mÕlÌi«liÊ ÌimiÃÊ inÊ some dogs). However, because the previous negative test results ÜiÀiÊ`iÌiÀmini`ÊLÞÊ£ÊovÊÌhiÊ{ÊÌiÃÌÊkiÌÃÊ/iÃÌÊ in this study, the potential for bias, although small, is still present. CONCLUSION nÊmoÃÌÊV>ÃiÃ]Ê`i>}noÃiÃÊovÊ 7ÊinviVÌionÊiÃÊÀil>ÌiÛilÞÊ straightforward and easily accomplished through in-clinic testing. However, an understanding of the heartworm life cycle and recognition of the limita- tions of current diagnostic procedures are critical to successful diagnosis. Two major limitations of most CHW tests are: UÊ n>LiliÌÞÊÌoÊ`iÌiVÌÊinviVÌionÊ`ÕÀin}ÊÌhiÊlon}Ê«Ài«>- tent period UÊ iVÀi>Ãi`ÊÃinÃiÌiÛiÌÞÊovÊ`iÌiVÌionÊÜhinÊÛiÀÞÊviÜÊoÀÊ no female worms are present. Commercial CHW antigen tests have high sensitivity and specificity but performance may be compromised when these limitations are encountered. Acknowledgements We gratefully acknowledge veterinary technicians Kathryn Gresco, Eva Jaeger, and Kimberly Holt for their help with the CHW screening clinic. We also thank Dr. Brenda Knowlton and student representative Bridget Morton of Pfizer Animal Health for helping plan and organize the clinic. CHW = canine heartworm; EDTA = ethylenediaminetetraacetic acid; ELISA = enzyme-linked immunosorbent assay; ICT = immunochromatographic test; PCR = polymerase chain reaction References 1. Bowman D, Little SE, Lorentzen L, et al. Prevalence and geographic distribution of Dirofilaria immitis, Borrelia burgdorferi, Ehrlichia canis, and Anaplasma phagocytophilum in dogs in the United States: Results of a national clinic-based serologic survey. Vet Parasitol 2009; 160(1- 2):138-148. 2. Bowman DD, Atkins CE. Heartworm biology, treatment, and control. Vet Clin North Am Small Anim Pract 2009; 39:1127-1158. 3. Lee ACY, Montgomery SP, Theis JH, et al. Public health issues con- cerning the widespread distribution of canine heartworm disease. Trends Parasitol 2010; 26(4):168-173. ®ÊÕÌiliâi`Ê