doxycycline treatment also appears to have a slow-kill effect on adult heartworms, but more work on this isneeded to determine the level of efficacy and length of time needed for all the worms to die. moreover, such treatment should greatly reduce further selection of filarial strains that may carry genes that confer resistance to the macrocyclic lactone (avermectin/milbemycins) heartworm preventives, improve prophylactic efficacy, and subsequently lead to a reduction in cases of lack of efficacy