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Treating Heartworm Disease in Dogs

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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.

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

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