Today's Veterinary Practice

JUL-AUG 2017

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26 AHS HEARTWORM HOTLINE PEER REVIEWED Regardless of whether HPT is used in post-treatment testing, dogs should be tested for both antigen and microfilariae 6 to 12 months after completion of adulticidal therapy to assess treatment efficacy. Many factors can affect detection of heartworm antigen in samples, leading to NAD results in truly infected dogs. They include the stage of infection, concurrent medications (including doxycycline and heartworm preventives), and microfilarial status of the animal ( Table 2 ). Stage of Infection Although detection of D immitis is typically expected 7 months after infection (with earliest detection 5 months after infection), 7 some evidence suggests that HPT may allow earlier antigen detection. • In one study of experimentally infected dogs, 100% of infections were detected after HPT of test samples obtained 4 months after infection. 8 When samples were obtained 5 months after infection, only 42.6% of infections were detected in samples without HPT, while 100% of those infections were identified after HPT. 8 • In another study of experimental infection in dogs, positive antigen test results were obtained 31 to 36 days sooner in heated (days 127 to 132 after infection) versus unheated (day 163 after infection) serum samples. 9 • In a study of experimental infection in cats, detection of heartworm antigen was possible as early as 5.5 months after infection when samples underwent HPT. 5 The potential for earlier detection of heartworm antigen after HPT of samples from recently infected animals is theorized to be the result of a more robust immune response early after infection, along with the lower level of antigen produced by nongravid female worms, which both lead to greater immune complex formation and subsequent antigen blocking. Heartworm Treatments Several studies have suggested that the administration of macrocyclic lactones and/or antibiotic therapy (eg, doxycycline) can affect immune complex formation and subsequently interfere with antigen testing. • A study of 19 naturally infected dogs being managed with monthly topical 10% imidacloprid plus 2.5% moxidectin along with oral doxycycline (10 mg/kg q12h for 30 days, every 6 months) demonstrated a substantial variation in timing of antigen detection between HPT and non-HPT samples. 10 Among dogs that initially tested negative with non-HPT samples, antigen was detected in 50%, 95%, and 100% of dogs tested with HPT after 6, 12, and 18 months of therapy, respectively. • Another report of 29 shelter dogs with antigen detection after HPT demonstrated 3.8 times greater odds of immune complex interference with test results when a history of macrocyclic lactone administration was reported. 2 • Fifty-three percent of a cohort of privately owned dogs that had a negative antigen test result and received monthly macrocyclic lactones and doxycycline had detectable antigen after HPT of serum samples. 3 There are a few possible explanations for the influence of macrocyclic lactone and doxycycline administration on immune complex interference with diagnostic test results. Although doxycycline has some anti- inflammatory activity, administration of macrocyclic lactones and doxycycline in dogs with active heartworm infection is not adulticidal in the short term. The persistence of live heartworms allows for continued antigenic stimulation, subsequent inflammatory response, and antibody production in the face of a decreasing antigen load. Secondarily, use of these medications often results in sterilization of the female worms and subsequent decrease in antigen release. These factors could contribute to a relative antibody TABLE 2 Effect of Heat Pretreatment on Clinical Factors CLINICAL FACTOR IMPACT OF HEAT PRETREATMENT RATIONALE Stage of infection May allow earlier antigen detection Immune response may be more robust immediately after infection, resulting in increased antibody production Concurrent treatments (macrocyclic lactones, doxycycline) May reduce frequency of negative test results in infected animals Prolonged infection promotes continued inflammatory response in face of decreased antigen load, disrupting antigen-to-antibody ratio Microfilariae May reduce frequency of negative test results in infected animals Circulating microfilariae promote continued inflammatory response and continued antibody formation

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