Fig. 1

This illustration is predicated upon dogs beginning treatment with a macrocyclic lactone (ML) at the time of diagnosis and continuing the treatment for the next 6 months (or getting a second injection with the slow-release injectable product). The numbers along the horizontal line at the bottom indicate the age of worms present in a dog at the time of treatment (ie, diagnosis). The vertical line at the left represents the day the dog is diagnosed and treated. The bars and arrows to the left of the line represent the following: the blue bar represents the 100% efficacy as per the original CVM-FDA product approval; the blue arrow represents the 100% efficacy based upon published literature and so recognized by the American Heartworm Society; the red arrows represent continued treatments with MLs; the faint arrows represent that a next treatment will be administered. The small green arrowhead represents a single injection of melarsomine dihydrochloride at the time of diagnosis, and the larger green arrowhead represents the two injections that will be administered 24 h apart 30 days after the first injection. The red checkered boxes on the right of the vertical line show the ages of worms killed by repeated monthly treatment with macrocyclic lactones (MLs) where the callouts present the percent efficacies published for the MLs. The vertical green bars represent the treatment of dogs with melarsomine dihydrochloride (Immiticide®) against worms of given ages (see text for references). Beginning ML treatment on Day 0, the checkered box shows that if a dog is treated with an ML when the diagnosis is made that the treatment with a ML will be 100% efficacious against any larvae that are 30 days old or less. The second red checkered box shows that the repeated ML treatments will have 100% efficacy against worms less than 2 months old; thus, efficacy against worms less than 60 days of age with the two repeated ML treatments will be 100%. This red checkered box shows that efficacy against worms 90 days old or less with the tested products having efficacies of 95.1% to 98.5%. The red checkered box shows that the treatment with MLs at 120 days after dogs receive heartworm larvae will have efficacies of 41.4% to 96.8% against these worms that are 4 months old or less. A single injection of melarsomine dihydrochloride (Immiticide®) is 82.1% effective against 4-month-old adults (indicated by the green vertical bar at 120 days on the left end of the red box at the level of the smaller green arrowhead). The green vertical bar at 60 days represents what occurs if a dog is treated with two IM injections of melarsomine dihydrochloride when heartworms in the dogs are 2 months old, where efficacy was shown to be 100% (see text for reference), and the green checkered box between 60 and 120 days is suggesting that a single melarsomine dihydrochloride injection given to dogs with 120-day-old worms is likely to have efficacy against heartworms that are less than 4 months old as suggested by the 100% efficacy of the two IM treatments against 60-day-old heartworms. The last two melarsomine dihydrochloride injections a month after the singe injection should have an efficacy of 99% against adult worms that are 5 months old or older (indicated by the green vertical bar at 150 days)