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PEER-REVIEWED Diagnosis of Canine HEARTWORM nviVÌion Margaret C. Barr, DVM, PhD; Elizabeth P. Boynton, DVM; Peggy L. Schmidt, DVM, MS, Diplomate ACVPM; Frank Bossong, DVM; and Gary R. Johnston, DVM, MS, Diplomate ACVR Dirofilaria immitis, the causative agent of canine heartworm disease, induces significant cardiopulmonary pathology in dogs in the United States and worldwide. A lthough highly effective methods are available to prevent infection in domestic dogs, D immitis has continued to spread throughout the U.S. Recent estimates of preva- lence in dogs presented for veterinary care, based on a national clinic-based survey, range from 0% in a few Western and Mid- western U.S. areas to almost 7% in some regions of the South- eastern states along the Mississippi River and the Gulf Coast.1 Factors influencing the spread of canine heartworm (CHW) include: UÊ nVÀi>Ãi`ÊmoÛiminÌÊovÊ`omiÃÌiVÊ`o}ÃÊ>VÀoÃÃÊÌhiÊ1°-° UÊ iÜiÀÊmoõÕiÌoÊ>n`ÊÛiVÌoÀÊVonÌÀolÊ«Ào}À>mÃÊinÊÀiëonÃiÊÌoÊ concerns about pesticide safety UÊ >VkÊovÊVom«li>nViÊÜiÌhÊ«ÀiÛinÌionÊ«Ào}À>mà UÊ nVÀi>Ãi`Ê inviVÌionÊ ovÊ non`omiÃÌiVÊ V>ni`Ê ÀiÃiÀÛoiÀÊ hoÃÌÃ]Ê such as the American coyote.2 DIAGNOSIS OF INFECTION n-VliniVÊ`i>}noÃiÃÊovÊ 7ÊinviVÌionÊiÃÊÌÞ«iV>llÞÊ>VVom«liÃhi`Ê through detection of circulating microfilaria or soluble CHW >nÌi}in]Ê>n`ÊiÃÊ`i«in`inÌÊÕ«onÊ>lloÜin}Ê>`iµÕ>ÌiÊÌimiÊvÀomÊ potential infection date to test date to account for the prepatent period (see Life Cycle of Heartworms, page 32). Because of this lag time, a general rule of thumb is to wait until a dog is 7 months of age before testing for the first time.4 30 Today’s Veterinary Practice July/August 2011