![]() This will reduce new infections, eliminate existing susceptible larvae, and allow older heartworms (between 2 and 4 months of age) to mature to a point where they would be more susceptible to melarsomine.” There are published data, however, that argue against this practice. The current AHS recommendations are that upon diagnosis one should wait “2 to 3 months prior to administering melarsomine. The “susceptibility gap” has been defined by the 2014 American Heartworm Society (AHS) Guidelines as being a period of about 2 to 3 months based upon the efficacies of the products “as per product labels”. This is because of the rapid growth of heartworms that occurs within a dog from 3 to 9 months post infection. Overall, this paper shows that beginning concomitant treatment of a dog with a heartworm infection with a macrocyclic lactone and melarsomine dihydrochloride nearest to the time of diagnosis removes heartworms while they are most susceptible to treatment with both chemical classes and while they are likely smaller in size relative to starting treatment after a 1- to 3-month delay. This paper examines the research publications related to the “gap” and suggests based on this review of the data that starting the treatment of dogs with both macrocyclic lactones and melarsomine dihydrochloride very near the time of diagnosis will cover all stages better than waiting 1 to 3 months to initiate melarsomine dihydrochloride administration. The “susceptibility gap” has been defined as the time between when a diagnosis of an adult heartworm infection is made and when any heartworms in the dogs are too old or too young to be susceptible to treatment with either a macrocyclic lactone or melarsomine dihydrochloride in the form of RM 340 (Rhône Mérieux) or Immiticide® (Merial). This paper examines the “susceptibility gap” in heartworm therapy. The collective data that has been reviewed indicates that continued macrocyclic lactone administration with two additional injections of melarsomine dihydrochloride a month later will protect dogs against all heartworm stages, including those heartworms 2 months of age or younger at diagnosis, when both treatments are started upon diagnosis. Delaying treatment has the disadvantage of allowing disease progression and continued heartworm growth. ![]() Starting treatment at diagnosis appears to be acceptable for maximal heartworm clearance based on published data. Thus, no improvement in efficacy is expected with a delay in initiating therapy with both melarsomine dihydrochloride and macrocyclic lactones, even with the presence of younger heartworms. With the development and US Food and Drug Administration (FDA) approval of spinosad + milbemycin oxime (Trifexis®, Elanco), milbemycin oxime + praziquantel (Interceptor® Plus, Novartis, now Elanco), and milbemycin oxime + lufenuron + praziquantel (Sentinel® Spectrum®, Novartis, now Virbac), it was shown that repeated treatments of dogs with milbemycin oxime also has efficacy against 3-month-old heartworms. ![]() Thiacetarsamide has been shown to be 99.7% effective against 2-month-old heartworms and other work has shown that melarsomine dihydrochloride is 100% efficacious against these younger forms. ![]() ![]() When it was given to dogs with older heartworms, 7 and 12 months of age, a single injection was only 55.6% and 51.7% effective, respectively. A single intramuscular injection of 2.5 mg melarsomine dihydrochloride/kg has an efficacy of 82.1% against 4-month-old worms. Melarsomine dihydrochloride was originally also investigated as a “preventive” as well as a treatment for adult heartworm infection where it would be given to dogs by intramuscular injection every 4 months therefore, there was early interest in its ability to kill younger worms. This was previously defined within the American Heartworm Society guidelines as a period of about 3 months “as per product labels.” It can be postulated, however, that a susceptibility gap does not exist with the combination of continued macrocyclic lactone therapy coupled with a three-dose melarsomine dihydrochloride protocol where the first intramuscular treatment is near the time of first diagnosis. The “susceptibility gap” in a dog diagnosed with adult heartworms has been defined as the period of time in which some Dirofilaria immitis stages are not susceptible to treatment with either macrocyclic lactones or melarsomine dihydrochloride. ![]()
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