how do I compare macrocyclic lactone-based heartworm preventives in terms of efficacy and safety?

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Macrocyclic lactone (ML) heartworm preventives differ meaningfully in efficacy against resistant Dirofilaria immitis isolates, while sharing a broadly favorable safety profile at label doses in dogs and cats without MDR1 mutations. Ivermectin has the most potent safety-net and adulticidal activity among the MLs, milbemycin oxime has the least, and selamectin and injectable moxidectin fall in between. Drug effects with ivermectin are not enhanced by increasing the dose or shortening the dosing interval; continuous monthly treatment is required to produce the full effect, and the earlier treatment is initiated relative to larval age, the more stunted the worms and the shorter their survival time.1 Against susceptible isolates, all approved MLs provide high efficacy. A single oral dose of moxidectin (NexGard® PLUS, targeting the minimum label dose of 12–24 µg/kg) achieved 100% efficacy against the susceptible SC-20 isolate in dogs.2 The four label-approved feline MLs — ivermectin, selamectin, moxidectin (topical, 1% in combination with imidacloprid), and milbemycin oxime — were each approved based on L3 challenge models.7 Against ML-resistant isolates, moxidectin at higher doses or with more monthly administrations substantially outperforms ivermectin and milbemycin oxime. Against the ZoeLA resistant strain, oral moxidectin at 24 µg/kg for four or six monthly doses achieved ≥96.1% efficacy, while Heartgard® Plus (ivermectin/pyrantel) and Interceptor® Plus (milbemycin oxime/praziquantel) achieved only 18.7% and 21.2%, respectively — neither significantly different from untreated controls.5 Against the JYD-34 resistant isolate, the same moxidectin regimen achieved ≥95.9% efficacy versus 63.9% for ivermectin/pyrantel and 54.6% for milbemycin oxime/praziquantel.5 Dose and duration of moxidectin exposure are the key variables against resistant isolates. Five monthly doses of moxidectin at 6 µg/kg provided 83.3–90.2% efficacy against JYD-34, while five monthly doses at 9 µg/