Dog Owner's Home Veterinary Handbook.pdf - Mr. Walnuts

346 •DOG OWNER’S HOME VETERINARY HANDBOOK
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 (see page 349).
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 (see
Ivermectin Sensitivity, page 349). Ivermectin should not be used in these dogs.