Heartworms Resistant to Preventive Medication?

Heartworm prevention: Major factors of resistance First, heartworm preventives were designed and marketed at doses intended to prevent infection by killing third-stage and young fourth-stage larvae, not microfilariae. Thus, microfilariae may persist despite exposure to a macrocyclic lactone (ML); these drug-selected microfilariae can be transmitted between dogs by mosquitoes. This seems an unwise course of action if one wants to prevent selection of resistance through the effects of MLs on microfilariae.