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
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.