This is a serious disease that primarily affects dogs. It is rare in cats, despite claims by the preventive pharmaceutical manufacturers. It can be treated with homeopathy and herbs, but this should be under the care of an experienced veterinarian.
That being said, pharmaceutical heartworm preventives are generally very effective at protecting dogs against the disease. However, I would not recommend their use in cats, as the incidence does not warrant the drug use in most holistic veterinary opinions. In dogs, the "monthly" preventives are effective if given at six-week intervals, and possibly even at seven-or eight-week intervals. These drugs kill any larvae that have been injected within the previous six to eight weeks, so the drugs protect for the prior period, not the future period. Thus, it is correct to wait until about six weeks after the first mosquitoes appear and continue until you see no more mosquitoes, giving one dose after the end of mosquito season. The "daily" preventives are almost a thing of the past, but these are usually effective if given every other day.
Although the prescription preventive drugs are considered to be generally safe, they can initiate an autoimmune disease in susceptible animals. This includes thyroid diseases.
Natural Remedies to Prevent Heartworm
There is a homeopathic nosode that is made from heartworm larvae that is employed commonly by holistic veterinarians as a preventive to avoid the drug side effects. However, many question its effectiveness, though I have several clients who use the nosode (apparently successfully) with animals in heartworm endemic areas. Though most dogs given the nosode have no trouble with heartworms, I am aware of some cases where the nosode did not protect. I believe it does offer some protection, though it may be incomplete. It is likely that we simply just don't know yet how to give the nosode. As of this writing, I suggest using a 30C potency and dosing at two-week intervals. I have seen no problems with this schedule, and though I use it myself with success, I cannot say for sure that it is fool-proof protection for your dog. If you decide to try the nosode, you must understand that its effectiveness is currently unknown and it is not FDA or Veterinary approved.
I generally do not recommend vaccines and drugs for most animals, and when possible I try to avoid the conventional approach of using heartworm drugs because they are especially risky for animals with thyroid disease. Monthly heartworm preventives as well as "sulfa" drugs are known to occasionally trigger autoimmune thyroid disease.