Medication to Treat Canine Heartworm

Heartworms are transmitted by mosquitoes.

Heartworms are a potentially deadly parasitic infection transmitted to dogs via mosquito bites. The immature heartworms (microfilariae) travel through the bloodstream, where they lodge in the heart muscle and grow into large adult worms. They are easily detected via a blood test, and easily prevented with one of several different monthly medications. If you have to treat a heartworm positive canine, you can choose from two effective methods--and some alternative methods that may be ineffective.

Prevention

Preventing heartworms is the easiest way to keep a dog heartworm negative. Preventatives do not stop mosquitoes from biting your dog. They work by killing the microfilariae--visible only through a microscope--while they are still tiny and harmless. Heartworm preventatives include ivermectin, milbemycin oxime, selamectin and moxidectin and are available by prescription through veterinarians. Most are given monthly, although daily and six-month injectables are also available. Depending on your region and whether or not mosquitoes are present year round or only during warm months, heartworm preventatives are given either all year or only during the spring, summer and fall. The American Heartworm Society recommends year-round prevention to ensure compliance.

Dogs at Risk

Any dog can be at risk, even if they rarely go outside, and newborn puppies are also susceptible. The University of Georgia College of Veterinary Medicine recommends starting puppies on heartworm preventatives by 8 weeks of age, and doing annual blood (antigen) tests. All dogs should be tested for heartworms before starting preventatives because the preventative medication can cause life-threatening complications in heartworm-positive dogs.

Candidates for Treatment

Most dogs can be successfully treated for heartworms, although the risk of complications rise depending on the severity of the infection, according to the American Heartworm Society. There may only be one or two adult heartworms and no clinical symptoms in the early stages. In the most severe stage (stage 4) a dog may lose weight, be exercise intolerant and have heart damage with hundreds of worms lodged in the heart.

Types of Treatment

Only one treatment is approved for a dog infected with adult heartworms. Melarsomine dihydrochloride (immiticide) is a modified arsenic and very effective. It's administered in a series of three intramuscular injections in the back muscles. The first injection starts killing the adult worms slowly. A month later, two more injections are given within 24 hours of each other. These injections kill the "stragglers." The goal is to kill the heartworms slowly but surely, because an immediate kill of a large number of worms can cause a fatal blood clot or heart attack if the dead worm fragments clog blood vessels.
The other method is not FDA-approved, but is often used because it is less expensive and may be easier on the dog. The preventative immiticide (the active ingredient in the commonly prescribed HeartGard) is given monthly, often at a higher than usual dose. This method can take up to two years, and the University of Georgia College of Veterinary Medicine cautions that the dog should be considered ill, with continuing lung pathology, the entire time.

Considerations

Treating canine heartworms can be expensive, although cost may vary considerably. After treatment, the dog must be rechecked and monitored, and during treatment it should be kept quiet for four to six weeks.
While several alternative and herbal treatments are sold for canine heartworms, the American Heartworm Society states that there are "no 'natural' or herbal therapies that have been shown to be safe and effective treatment for heartworm disease."

References

• American Heartworm Society: Canine Heartworm Disease
• University of Georgia College of Veterinary Medicine: Dirofilariasis in the Dog: An Overview
• American Heartworm Society: Canine Guidelines

Resources

• American Heartworm Society: Frequently Asked Questions