Comparative preventive efficacy of ProHeart[R] 12, Heartgard[R] Plus and Interceptor[R] Plus against a macrocyclic lactone-resistant strain in dogs

The current studies compared ProHeart.sup.[R] 12, Heartgard.sup.[R] Plus and Interceptor.sup.[R] Plus for preventive efficacy against JYD-34, a macrocyclic lactone (ML)-resistant strain of Dirofilaria immitis in dogs. In two studies, each using 24 adult beagles, dogs were allocated to four treatment groups (n = 6): placebo-treated control; ProHeart 12 as per label (0.5 mg/kg moxidectin); Heartgard Plus (HGP) as per label (minimum 6 [micro]g/kg ivermectin); and Interceptor Plus (INP) as per label (minimum 0.5 mg/kg milbemycin oxime). In both studies, ProHeart 12 was administered as a single subcutaneous dose on day 0, and HGP and INP were administered orally on days 0, 30, 60, 90, 120 and 150. In Studies 1 and 2, dogs were inoculated with 50 third-stage heartworm larvae (JYD-34 strain) on days -30 and 165, respectively. In Study 2, treatment for both HGP and INP was continued on days 180, 210, 240, 270, 300 and 330. Adult heartworm recoveries were performed on day 185 in Study 1 and on day 360 in Study 2. In Studies 1 and 2, all placebo-treated dogs developed adult heartworm infections (geometric mean, 29.9 and 34.9 worms/dog, respectively). A single dose of ProHeart 12 was 100% effective in preventing the development of adult JYD-34 heartworms when treatment was initiated 30 days after heartworm inoculation, while six consecutive monthly doses of HGP and INP were only 10.5% and 14.6% effective, respectively. The mean worm count for the ProHeart 12-treated group was significantly lower (P < 0.0001) than that for the placebo control, HGP- and INP-treated groups. In Study 2, the dogs treated with ProHeart 12 had an efficacy of 98.3%. All dogs treated with HGP and INP for 12 consecutive months had adult heartworms with efficacies of 37.7% and 34.9%, respectively. The mean worm count for the ProHeart 12-treated dogs was significantly lower (P < 0.0001) than those for the control group, HGP- and INP-treated groups. A single administration of ProHeart 12 was 98-100% effective in preventing the development of the ML-resistant JYD-34 heartworm strain and was significantly better than multiple consecutive monthly doses of either Heartgard Plus or Interceptor Plus in both studies.