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Hi there, I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with dogs and cats. I'm sorry that you're having such a stressful time with your critters! I'll do my best to help. When you said "The vet said he has a heart murmur" - to which pet are you referring? Map or Paco? I wasn't sure the way it's reading which pet you mean there.
Doxycycline is typically treated initially with doxycyline and continuation of a heartworm preventive. However, the American Heartworm Society (https://www.heartwormsociety.org/) does also strongly recommend the use of immiticide injections to kill the adult worms as well. Otherwise, the worms live out their lives inside Map's pulmonary arteries and can continue to cause significant damage while we wait for them to die on their own. Some heartworm preventives have a "slow kill" type of action on the adult worms, so it may shorten their life, but they still live plenty long enough to cause significant damage to the heart, lungs, and kidneys. If your vet hasn't discussed adulticide therapy (immiticide injections) and/or you're just not feeling like you click with this vet, it's definitely work seeking a second opinion. As vets, we want you to be completely comfortable with the care you're receiving, even if it means going somewhere else. There are all sorts of clients and all sorts of veterinarian personalities, so we don't all click 100% of the time.
I know that there's probably more info you are looking for here - please reply with questions for me.