Heartworm Disease and Prevention

What are heartworms, and how do pets get infected?

Heartworms are parasites that invade a pet’s circulatory system and heart. The adult heartworms produce offspring called microfilariae, which circulate in the blood. Mosquitoes feed on an infected pet, serve as a host while the microfilariae mature, then transfer infective heartworm larvae when they bite healthy pets.

These heartworm larvae migrate through the pet’s tissues and circulatory system and reach the heart in 5-6 months, where they arrive as small worms about two inches in length. Once they are in the heart, the adult worms reproduce and grow. They may reach a length of 12 or more inches and may stay in a pet’s heart for several years. In a severe infestation, a dog’s heart may contain up to 300 worms.

Breed, sex, and age do not affect your pet’s susceptibility to heartworms.  Dogs get heartworms much more commonly than cats, although the incidence of heartworms in cats is higher than once thought. Outdoor pets come into contact with mosquitoes more frequently than indoor pets and may be at increased risk for heartworms.

How to prevent heartworm disease:

Highly effective preventative medications are available for dogs and cats that are not infected and for pets that have been treated successfully for heartworms in the past. Your veterinarian will test your pet for the presence of heartworms before prescribing medication. If your pet becomes infected, these medications will prevent heartworm larvae from developing into adult heartworms. As a safeguard, your vet may recommend annual or biannual screening tests- even for pets that are on heartworm preventative medicine.

Physical signs when pets are infected with heartworms:

There are several signs that might indicate the presence of heartworms in your pet, such as:

• Sudden weight loss
• Developing a rough hair coat
• Difficulty breathing
• Soft, persistent cough
• Tiring easily
• Listlessness or weakness

Many of these symptoms are most noticeable after exercise. Heartworms clog the heart and/or main blood vessels, reducing the blood supply to other organs like the lungs and liver. Major bodily functions, including breathing, are affected.

How heartworm disease is treated:

Once a blood sample has been tested, and the presence of heartworms is detected, further tests may need to be done to determine whether your pet can handle heartworm treatment or not.

Dogs

If heartworm infection is not serious, an injectable drug will be administered to kill the adult heartworms. Complete rest is necessary for a month after treatment because pieces if the dead, decomposing heartworms are circulating through the heart and lungs. The pieces are eventually reabsorbed into the body, but if they are dislodged, your dog could suffer heart or respiratory failure.

After a month, your dog will need to be treated for microfilaria, heartworm offspring. This may need to be repeated if all microfilaria are not killed.

Dogs with severe heartworm disease may not tolerate the heartworm treatment. In this case, it may be safer to treat the resulting organ failure than the heartworms. Dogs in this advanced stage are likely to not survive more than a few months.

Cats

No heartworm treatment medications have been approved for cats because only recently have heartworms been discovered to be prevalent in cats. Once diagnosed, there are two options:

1)      Treat the cat with drugs only approved for dogs. These drugs have side effects, such as respiratory failure and death, in a small percentage of cats.

2)      Treat the symptoms of the disease and hope the cat outlives the heartworms.