Clinical assessment of heartworm-infected Beagles treated with a combination of imidacloprid/moxidectin and doxycycline, or untreated.

BACKGROUND: Administration of moxidectin topically and doxycycline PO has been utilized experimentally as an alternative treatment for heartworm disease. However, clinical effects of this protocol remain poorly characterized. OBJECTIVE: To evaluate the clinical and postmortem findings associated with administration of doxycycline and monthly 10% imidacloprid + 2.5% moxidectin (IMD + MOX, Advantage Multi/Advocate) to Dirofilaria immitis-experimentally infected as compared to nontreated control dogs. ANIMALS: Sixteen purpose-bred, female, Beagle dogs. METHODS: Prospective, blinded, experimental study. Animals with surgically transplanted adult heartworms were randomized into 2 study groups of equal size: a nontreated control group (n = 8) and an IMD + MOX and doxycycline-treated group (n = 8). Randomization was performed using a complete block design according to circulating microfilarial concentrations, measured before treatment. Serum biochemical profiles, CBCs, thoracic radiographs and echocardiograms were performed prior to and 3 weeks after transplantation, and monthly for 10 months. Postmortem gross and histopathologic evaluations were performed. RESULTS: Compared to control animals, mean ± SD serum alanine aminotransferase (181 ± 203 U/L vs 33 ± 7 U/L; P < .0001) and alkaline phosphatase (246 ± 258 U/L vs 58 ± 19 U/L; P < .0001) activities were significantly higher in the treated group on day 28. Radiographic and echocardiographic evidence of heartworm disease was observed in both groups; however, no significant differences in these variables were noted between groups. Mean ± SD pulmonary arterial thrombus score was significantly higher in the treated vs nontreated group (3.9 ± 0.4 and 1.5 ± 2.1, respectively; P = .01). CONCLUSIONS AND CLINICAL IMPORTANCE: The treatment protocol was well-tolerated with no clinically relevant adverse effects for any variable evaluated during the observational period.

Evaluation of heat-treating heartworm-positive canine serum samples during treatment with Advantage Multi® for Dogs and doxycycline.

BACKGROUND: The use of heat-treatment in canine and feline serum has been hypothesized to break the formation of antigen-antibody complexes, thereby freeing the heartworm antigen allowing for detection by commercially available heartworm antigen kits. While studies have analyzed the effect of heat-treating serum and plasma samples in the detection of heartworm antigen, these studies have not utilized necropsy verified results for validation. This study evaluated the use of heat-treating serum samples in experimentally infected dogs during adulticidal treatment in comparison with necropsy adult heartworm recovery. METHODS: As part of a primary study, a total of 16 dogs were experimentally infected with 16 sexually mature adult heartworms using surgical transplantation, allocating 8 dogs in both the control and treated group. Treated dogs received 10 months of topical administration of Advantage Multi® for Dogs (10% Imidacloprid + 2.5% Moxidectin) every 4 weeks and 30 days of 10 mg/kg doxycycline BID. Blood samples were collected from all study animals prior to surgical transplantation of adult heartworms, on study days 0, 1, 3, 7, 14, 21, 28, and every 4 weeks thereafter for the duration of this study. Concentration of heartworm antigen was tested using the DiroCHEK® heartworm antigen test kit using serum samples both pre- and post-heat-treatment. Serum samples were heat-treated at 103 °C in a dry heat block for 10 min and centrifuging at 1818× g for 20 min. RESULTS: There were a total of 4 instances (days 56, 140, 224 and 252) in 3 treated dogs in which a serum sample converted from negative for the detection of heartworm antigen prior to heat-treatment to positive for the detection of heartworm antigen post-heat-treatment. At necropsy, these dogs had no adult heartworms recovered and were all negative on antigen testing prior to and after heat treatment. There was 100% accuracy in the detection of either no infection, or 1-2 adult heartworm infections using the DiroCHEK in serum samples with and without heat-treatment at the time of necropsy. CONCLUSIONS: The DiroCHEK accurately diagnosed all dogs with live adults recovered at necropsy as heartworm antigen positive and all those dogs with no live adults recovered at necropsy as heartworm antigen negative without the use of heat-treatment for samples taken on the day of necropsy. Therefore, these results indicate that the use of heat-treating serum samples did not provide data of any additional value in the diagnosis of heartworm-positive dogs receiving treatment in this study. Additionally, these results may indicate that the conversion of serum samples from negative to positive for the presence of heartworm antigen with heat-treatment may not always accurately diagnose live adult heartworm infections since no adult heartworms were recovered at necropsy for those dogs in which a conversion event occurred. These conversion events may be detecting residual antigen leftover after all adult worms have died or may even be detecting off- target antigens, which have been denatured during heat-treatment. While a necropsy was not performed at the time of the conversion events, no live adult worms were recovered from any of the dogs in which a conversion event occurred earlier in treatment.

Assessment of parasitological findings in heartworm-infected beagles treated with Advantage Multi® for dogs (10% imidacloprid + 2.5% moxidectin) and doxycycline.

BACKGROUND: Anecdotal reports support the position that the adulticidal heartworm treatment utilizing doxycycline and Advantage Multi®/Advocate® for Dogs (10% imidacloprid + 2.5% moxidectin) has successfully converted antigen-positive dogs to antigen-negative. To date, no controlled experimental studies have demonstrated the adulticidal efficacy of this treatment regimen. The aim of this study was to evaluate the parasitological and clinical efficacy of Advantage Multi® for Dogs (IMD + MOX) and doxycycline in heartworm-infected beagles. METHODS: This study utilized 16 dogs, 8 dogs in each of non-treated control and treated groups. A total of 16 adult Dirofilaria immitis (Missouri strain) were surgically transplanted into the jugular vein of each study dog. The treatment regimen of monthly IMD + MOX topically (per labeled dosage and administration) for 10 months and 10 mg/kg doxycycline BID orally for 30 days was initiated 30 days post-surgical transplant. Echocardiograms, radiographs, complete blood counts, clinical chemistry profiles, heartworm antigenemia and microfilaremia were evaluated every 4 weeks. Serum samples were assayed for heartworm antigen using the DiroCHEK® heartworm antigen test. The DiroCHEK® was performed according to the manufacturer's recommendations and read using a spectrophotometer at 490 nm. RESULTS: All dogs tested positive for the presence of heartworm antigen post-surgical transplant and prior to treatment. Heartworm antigen levels began declining in treated dogs 3 months post-treatment. Non-treated control dogs remained antigen-positive. No microfilariae were detected in treated dogs after 21 days post-treatment. At necropsy, adult heartworms were recovered from all non-treated control dogs with a range of 10-12 adult worms/dog for an average recovery of 10.6 adult heartworms/dog. In the IMD + MOX- and doxycycline-treated dogs, the range of adult heartworms recovered was 0-2 adult worms/dog, with five dogs having no adult heartworms present. The average adult heartworm recovery was 0.6/dog in the treated group. This treatment regimen demonstrated a 95.9% efficacy in eliminating adult heartworms (P < 0.0001). CONCLUSIONS: This study demonstrated that this treatment regimen successfully eliminated D. immitis microfilariae by 21 days post-treatment, reduced heartworm antigen concentration over time, and had a 95.9% efficacy in the elimination of mature adult heartworms. Based on this study, we conclude that this treatment regimen is a relatively quick, reliable and safe option to treat canine heartworm infection as compared to other treatment regimens involving macrocyclic lactones, when the approved drug melarsomine dihydrochloride is unavailable, contraindicated or declined by an owner unable to afford the more costly treatment or concerned about the potential side effects.

Moxidectin steady state prior to inoculation protects cats from subsequent, repeated infection with Dirofilaria immitis.

BACKGROUND: Infection of cats with Dirofilaria immitis causes seroconversion on antibody tests and pulmonary pathology, often without subsequent development of adult heartworms. Consistent administration of topical 10% imidacloprid-1% moxidectin has been shown to result in sustained plasma levels of moxidectin in cats after three to five treatments, a pharmacokinetic behavior known as "steady state". METHODS: To evaluate the ability of moxidectin at "steady state" to protect cats from subsequent infection with D. immitis, cats (n = 10) were treated with the labeled dose of topical 10% imidacloprid-1% moxidectin for four monthly treatments. Each cat was inoculated with 25 third-stage larvae of D. immitis 7, 14, 21, and 28 days after the last treatment; non-treated cats (n = 9) were inoculated on the same days, serving as infection controls. Blood samples were collected from each cat from 1 month prior to treatment until 7 months after the final inoculation and tested for antibody to, and antigen and microfilaria of, D. immitis. RESULTS: Measurement of serum levels of moxidectin confirmed steady state in treated cats. Cats treated with topical 10% imidacloprid-1% moxidectin prior to trickle inoculation of D. immitis L3 larvae throughout the 28 day post-treatment period remained negative on antibody and antigen tests throughout the study and did not develop gross or histologic lesions characteristic of heartworm infection. A majority of non-treated cats tested antibody positive by 3-4 months post infection (6/9) and, after heat treatment, tested antigen positive by 6-7 months post-infection (5/9). Histologic lesions characteristic of D. immitis infection, including intimal and medial thickening of the pulmonary artery, were present in every cat with D. immitis antibodies (6/6), although adult D. immitis were confirmed in only 5/6 antibody-positive cats at necropsy. Microfilariae were not detected at any time. CONCLUSIONS: Taken together, these data indicate that prior treatment with 10% imidacloprid-1% moxidectin protected cats from subsequent infection with D. immitis for 28 days, preventing both formation of a detectable antibody response and development of pulmonary lesions by either immature stages of D. immitis or young adult heartworms.

Heat treatment prior to testing allows detection of antigen of Dirofilaria immitis in feline serum.

BACKGROUND: Diagnosis of Dirofilaria immitis infection in cats is complicated by the difficulty associated with reliable detection of antigen in feline blood and serum samples. METHODS: To determine if antigen-antibody complex formation may interfere with detection of antigen in feline samples, we evaluated the performance of four different commercially available heartworm tests using serum samples from six cats experimentally infected with D. immitis and confirmed to harbor a low number of adult worms (mean = 2.0). Sera collected 168 (n = 6), 196 (n = 6), and 224 (n = 6) days post infection were tested both directly and following heat treatment. RESULTS: Antigen was detected in serum samples from 0 or 1 of 6 infected cats using the assays according to manufacturer's directions, but after heat treatment of serum samples, as many as 5 of 6 cats had detectable antigen 6-8 months post infection. Antibodies to D. immitis were detected in all six infected cats by commercial in-clinic assay and at a reference laboratory. CONCLUSIONS: These results indicate that heat treatment of samples prior to testing can improve the sensitivity of antigen assays in feline patients, supporting more accurate diagnosis of this infection in cats. Surveys conducted by antigen testing without prior heat treatment of samples likely underestimate the true prevalence of infection in cats.