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Treating Heartworm Disease in Dogs

When and how to treat depends on the number of heartworms, their location, any medical complications (such as congestive heart failure or liver or kidney disease), the age and condition of the dog, and the presence of circulating microfilariae. After a thorough medical examination, your veterinarian will discuss these options and recommend a treatment program based on the findings.

For dogs with uncomplicated heartworm disease, the objectives are to eliminate all adult worms, kill the microfilariae (if present), and initiate preventive measures. At the same time, it is important to avoid complications associated with drug toxicity and the passage of dead worms into the lungs. Some veterinarians may choose to reduce the microfilaria numbers first, then go after the adult worms.

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If you and your veterinarian decide to eliminate the adult worms first, the first step in dealing with uncomplicated heartworm infection is to administer an agent that will poison the worms. Two drugs that are FDA-approved and commonly used are thiacetarsamide (Caparsolate) and melarsamine (Immiticide). Both contain arsenic. Caparsolate is given intravenously twice a day for two days. Significant toxic reactions can occur, and include loss of appetite, vomiting, diarrhea, jaundice, kidney failure, and death. Caparsolate does not always kill all the worms. Immature worms, especially females, are relatively resistant. Treatment with Caparsolate must be followed by treatment for microfilaria. The drug is not safe to use on high-risk dogs with congestive heart failure, liver failure, or kidney impairment.

Immiticide eliminates more than 90 percent of worms, making it more effective than Caparsolate. It has a higher margin of safety and can be given to dogs at high risk. Immiticide is given by intramuscular injection once a day for two days. If the dog is severely debilitated by heartworms, the drug can be given in divided doses 30 days apart.

Approximately 10 percent of dogs are poor candidates for immediate drug treatment because of severe pulmonary artery infestation and congestive heart failure. These dogs will benefit from complete rest and confinement for a minimum of two to three weeks before and after drug therapy. Aspirin, a mild anticoagulant, is given to help prevent respiratory failure due to worm thromboembolism.

Elderly dogs with heartworms are at high risk of death from therapy to kill the adult worms. Some old dogs may be better off without treatment. An acceptable alternative is to restrict exercise and administer a low dose of aspirin daily to prevent further damage to the lungs. Give the dog a monthly heartworm preventive to prevent new worms from being acquired.

Surgical removal of worms is reserved for critically ill dogs with vena cava syndromewho are not candidates for drug therapy because of the risk of liver failure or thromboembolism. To remove the worms this way, an incision is made over the jugular vein in the neck. The vein is opened and a long grasping instrument is passed down through the superior vena cava into the right atrium and the inferior vena cava. The worms are grasped one by one and removed. The procedure requires X-ray equipment and special skills. Residual worms are eliminated with drug therapy after the dog improves.

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

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