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.
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.
Complications are similar to those of Caparsolate, but occur less often. Both drugs can cause a local reaction at the site of injection. Thromboembolism is a complication associated with the death of adult worms, and can occur with either drug.
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 syndrome who 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.
A heartworm antigen test should be performed three to five months after drug therapy. If all worms have been eliminated, the test will be negative. If the test is positive, consider retreatment.
The next step is to kill circulating microfilaria. This step is omitted if parasites are not found on a microfilaria concentration test. Most veterinarians wait four weeks to allow the dog to recover from the effects of killing the adult worms before beginning therapy to kill the microfilaria. Currently there are four drugs used - although none is licensed for this purpose. They are ivermectin, selamectin, moxidectin, and milbemycin. Ivermectin is considered the most effective and has the fewest complications, except in dogs with drug sensitivity.
Currently many veterinarians choose to simply give the monthly preventive drugs to dogs with circulating microfilaria, knowing that the microfilaria will slowly die off over six to nine months. Since the dogs are heartworm carriers during that time, they should be kept indoors during times of high mosquito activity and wear bug repellant when outside.
If the veterinarian decides the microfilaria must be eliminated as quickly as possible, the dog is admitted to the hospital on the morning of treatment. Ivermectin is given orally and the dog is observed for 10 to 12 hours for signs of toxicity, including vomiting, diarrhea, lethargy, weakness, and shock. Most reactions are mild and respond to intravenous fluids and corticosteroids. Shock and death have occurred in Collies, Shetland Sheepdogs, Australian Shepherds, Old English Sheepdogs, and other herding breeds and their crosses with the genetic defect that allows these drugs to pass into the brain. Ivermectin should not be used in these dogs.
Dramatic declines in microfilaria counts occur over the next few days. Ninety percent of dogs are free of all microfilaria at three weeks. At this time the dog should return for a microfilaria concentration test. If positive, the protocol is repeated. If negative, begin heartworm prevention. A positive microfilaria concentration test after two treatments strongly suggests that adult worms are still present in the dog. Confirm this with a heartworm antigen test and treat accordingly.
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