This is interesting (wonder if it has anything to do with delay of tx): From:
American Heartworm Society | Canine Guidelines ADULTICIDE THERAPY Melarsomine Dihydrochloride
Melarsomine, administered via deep intramuscular injection into the belly of the epaxial lumbar muscles, is the only adulticidal drug approved by the FDA for heartworm treatment. Mild swelling and some soreness at the injection site may be present for a few days, but this can be minimized by ensuring that the injection is deposited deeply with a needle of appropriate length and gauge for the size of dog and body condition. Strictly adhering to the manufacturer’s instructions for administration is imperative. Exercise restriction during the recovery period is essential for minimizing cardiopulmonary complications (see Pulmonary Thromboembolism).
Melarsomine has not been shown to have any activity against worms less than 4 months old. The two-injection protocol (i.e., two injections of 2.5 mg/kg body weight 24 hours apart) listed on the product insert for treating class 1 & 2 heartworm disease kills only about 90% of the adult worms. The three-dose alternate protocol (one injection of 2.5 mg/kg b. wt. followed at least one month later by two injections of the same dose 24 hours apart) listed for treating class 3 heartworm disease kills 98% of the worms.
These overall efficacy values reflect the percentage of worms killed in groups of dogs and not the percentage of dogs cleared of worms, which are considerably lower than these overall efficacy values.
Staging of the disease and use of the two-injection protocol has failed to adequately ensure treatment success. Therefore, regardless of the stage of the disease, the three-injection alternative protocol is the treatment of choice of the American Heartworm Society and several university teaching hospitals, due to the increased safety and efficacy benefits and decreased possibility that further treatment with melarsomine would be necessary. Furthermore, by initially killing fewer worms and completing the treatment in two stages, the cumulative impact of worm emboli on severely diseased pulmonary arteries and lungs is reduced.