Treating a heartworm infestation is difficult and dangerous. It is far easier and more effective to prevent the problem in the first place. In theory, the best way to prevent heartworms is to keep your dog from being bitten by a mosquito. Unfortunately, preventing mosquito bites can never be 100 percent effective. Dogs can be reasonably protected if they remain indoors in the late afternoon and evening, when mosquitoes are feeding.

Areas of most frequent heartworm infestation are along coastal regions, where swamps or other brackish water provide ideal conditions for mosquitoes to breed. Since mosquitoes have a flight range of one quarter mile, spraying around the yard and kennel and removing standing water can be partially effective, but they will never eliminate the threat.

If you live or travel with your dog in an area where heartworm is endemic, your dog should be on a heartworm prevention program. Ask your veterinarian about local prevalence and follow their recommendations for prevention. Most dogs should be on a heartworm preventive program.

A prevention program should be started at 6 to 8 weeks of age in endemic areas, or as soon thereafter as climate conditions dictate. In the Deep South, where mosquitoes are a year-round problem, dogs should be kept on preventive drugs all year long. In areas where it is not necessary to administer the drug year round, start one month before the mosquito season and continue one month beyond the first frost (generally from May or June to November or December). Heartworm prevention is important for the dog’s whole life. Some owners may elect to give heartworm preventives year round for zoonotic parasite protection and to reduce the risk of breakthrough heartworm disease in case they miss a monthly dose. All dogs 7 months and older should have an antigen test for heartworms before starting a prevention program. If the test is positive, a microfilaria concentration test should be performed. The antigen test should be repeated annually or as frequently as your veterinarian recommends-even if the dog is on a heartworm prevention program. Many heartworm preventives can cause illness if given to a dog with circulating microfilaria.

There are a number of drugs currently in use as heartworm preventives. They include ivermectin (Ivomec, Heartgard), milbemycin oxime (Interceptor), and selamectin (Revolution).

Heartgard is an effective preventive that is given once a month. This drug acts on the L₄ larvae. It has the advantage that dogs do not have to be heartworm-free to initiate therapy; dogs infected for as long as two months before treatment will not develop heartworms. If a monthly dose is missed, restart the drug and obtain a heartworm antigen test seven months later. Heartgard is marketed in chewable tablets of different sizes, depending on the weight of the dog. The recommended dose is generally considered to be safe to use on Collies and other herding breeds. However, with safer alternatives available, most owners avoid this for the breeds with the genetic defect that causes sensitivity to ivermectin.

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WebMD Veterinary Reference from "Dog Owner's Home Veterinary Handbook"