Keeping your dog healthy includes regular preventive health care.
Timely administration of heartworm preventive medication is one of the preventive measures that all dog owners should be practicing.
What are heartworms?
Heartworms are mosquito-borne parasites that live in the heart and blood vessels of an infected dog. These worms cause damage, often significant, to the cardiovascular structures and can lead to serious illness in the infected dog. In severe cases, heartworms can be fatal. Their scientific name is Dirofilaria immitus.
What are the symptoms of heartworm disease?
Heartworms can damage both the pulmonary arteries and the heart. Symptoms seen may include:
- exercise intolerance
- hemorrhage from the nose
- right-sided heart failure
- cardiac arrhythmia, which may lead to sudden death in some cases
Caval syndrome (CS)
A particularly devastating complication of heartworm disease is caval syndrome.
Caval syndrome refers to an infestation by a vast number of adult worms (100 or more). The heart cannot contain all of them, and the worms overflow into the vena cava. Vena cava is one of the large vessels that leave the right side of the heart. The worms obstruct blood flow.
CS is a life-threatening condition which can lead to
- destruction of red blood cells
Death may occur within one to two days without veterinary intervention.
The survival of the dog requires surgical removal of the worms from the heart and blood vessels. The physical removal of the worms helps restore blood flow to give the dog a chance of survival. Without surgery, the chance of survival is small.
How can heartworms be prevented?
Numerous medications are available that prevent heartworm infections from occurring in a susceptible dog. Most of these medications are either tablets or topical medications that must be applied monthly. Though every medication can have unwanted side effects, these drugs are generally safe and effective if used correctly.
Alternatively, there is an injection that provides protection for 6 months.
Why prevent heartworms rather than treat the infection if it occurs?
- Heartworms can cause significant damage to the circulatory system before they are detected. The damage is likely to be irreversible.
- Treating an existing infection carries the risk of complications, even though modern treatments are safer than those used in the past.
- Strict cage rest is required for long periods of time during the treatment—sometimes as long as 6-8 weeks or longer.
- Heartworm treatment is not guaranteed to kill all of the infecting heartworms.
Isn’t the administration of a monthly preventive effective in killing the worms in an infected dog?
Monthly preventive medications only address the immature form of the heartworm. They do not kill adult worms directly. With time, the adult worms will age and die of natural causes. Therefore, monthly heartworm preventive medications can eventually be effective in eliminating heartworm infections. However, elimination may take up to 2 years to occur. Meanwhile, the worms remain in the heart and blood vessels causing additional damage and disease.
In some instances, such as when a dog is debilitated or suffering from concurrent disease, he may not be considered a good candidate for heartworm treatment with a medication that kills adult worms. In these cases, treatment with a monthly preventive may be the optimal form of treatment. However, in most cases where the dog is otherwise healthy and deemed a fit candidate for treatment, the drug of choice is Immiticide® (melarsomine dihydrochloride).
Immiticide® is the only drug available that will kill adult heartworms. The major risk with Immiticide® injection is the embolization of dying worms to the lungs. That can cause serious side effects for the dog being treated. Strict cage rest is necessary to minimize these complications. Before treatment with Immiticide®, a monthly preventive medication is usually administered to remove the immature heartworms before addressing the adults.
Heartworm in Dogs