Don't Let Heartworm Become A Heartbreak!

Guest post by Lorie Huston, DVM

Keeping your pet healthy requires 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 damage, 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:

• coughing
• exercise intolerance
• hemorrhage from the nose
• pneumonia
• right-sided heart failure
• cardiac arrhythmia, which may lead to sudden death in some cases

A particularly devastating form of heartworm disease is known as caval syndrome. In caval syndrome, there are extremely large numbers of adult worms (approximately 100 or more) and the right heart chambers are too small to contain all of them, allowing the worms to overflow into the vena cava, one of the large vessels that leaves the right side of the heart. Collapse, shock and the destruction of red blood cells are the symptoms associated with caval syndrome and death may occur within one to two days without veterinary intervention. Survival of the dog requires surgical removal of the worms from the heart and blood vessels. If enough worms are removed, blood flow is resolved and the dog has 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 the risk of side effects can occur with any medication, these drugs are generally safe and are very effective in preventing heartworm infections if used correctly.

Alternatively, there is an injection that can be administered every 6 months which is also capable of preventing heartworm.

Why prevent heartworms rather than treat the infection if it occurs?

• Heartworms can cause significant damage to the circulatory system before they are detected and this damage is likely to be irreversible.
• Heartworm treatment (treating an existing infection) carries the risk of complications, even though the medications used today are safer than those used in the past.
• Strict cage rest is required for long periods of time while the dog undergoes treatment, sometimes as long as 6-8 weeks or longer.
• Heartworm treatment is not guaranteed to kill all of the infecting heartworms.

Isn’t 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 the 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 embolization of dying worms to the lungs, which 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.