Mayfair Animal Hospital      

Heartworms: Disease and Prevention

The Heartworm Lifecycle
Heartworms are transmitted from one infected pet to another pet by mosquitoes. If a mosquito that has taken a blood meal from an infected animal bites your pet, then your pet becomes infected. When the mosquito draws blood from an infected pet, the microfilariae (larvae) in the pet's bloodstream are transferred to the mosquito. The microfilariae then develop and mature in their new transportation, the mosquito. As the mosquito then goes from animal to animal, drawing a blood meal, it injects some of the mature microfilariae into those animals. Once the microfilariae are injected into an animal, they circulate in the bloodstream and migrate to the heart. Over the next 6 months, the larvae develop into adults in the heart.

Heartworm Disease: Symptoms and Testing
A dog can have as many as 250 adult heartworms living in its heart at one time, and the worms can live up to 7 years. Cats typically only have 1 to 3 adult worms that can survive 2 to 3 years. As adult worms live in a pet’s heart, they damage the muscle walls and valves of the heart as well as the surrounding pulmonary arteries.

Pets may exhibit outward symptoms such as coughing, lethargy, exercise intolerance, weight loss, and rapid or difficulty breathing. Bloodwork abnormalities indicating kidney or liver disease may be secondary to heartworm disease. However, it is also very likely that your pet may not exhibit any symptoms at all; especially cats and dogs that get little exercise and have few worms.

Mayfair recommends testing dogs yearly for heartworms even if preventatives are used consistently. Our tests detect antigens that are shed by adult female heartworms. Because it takes 6 months for larvae to develop to adulthood, we also recommend testing 6 months after a lapse in preventative, and resuming the use of prevention immediately. Another good reason to test yearly for heartworms is that preventative manufacturers will pay for all heartworm disease treatment if preventatives have been given consistently and testing has been done yearly.

Mayfair does not routinely test for heartworms in cats, but it may be recommended if a cat is exhibiting symptoms. Currently there is no treatment for heartworm disease in cats, only preventatives.

Diagnosis and Treatment (in dogs)
If a heartworm test returns a positive result, Mayfair recommends running further testing such as full bloodwork, urinalysis, chest x-rays, and possibly performing a cardiac ultrasound to determine the severity of the disease. The extended bloodwork reveals if there has been any damage to organs such as the kidneys, the liver, or the heart. Chest x-rays and an ultrasound will show any changes to the heart and lungs due to the worms.

Once the severity of disease is determined, a treatment plan is established. There are 4 classes of disease severity, and treatment is 99% successful for Classes 1, 2, and 3. We typically see Class 1 or Class 2 severity at Mayfair, but Class 3 is also seen occasionally. Class 4 severity is so progressed that it is typically not treated due to the increased risks involved.

Goals for treatment of heartworm disease in dogs are to kill the worms, prevent more worms from developing into adults, and avoid potential complications. At Mayfair Animal Hospitalm we follow the protocol recommended by the American Heartworm Society. Treatment consists of a series of medications to try and decrease the risk of complications with the main treatment, which is a series of three injections of Immiticide. The Immiticide injections consist of three deep intramuscular injections spread out over a one-month period. During the injections and for 6-8 weeks afterward, the dog is to stay strictly confined and as calm as possible. As the worms die, they slowly begin to disintegrate into the bloodstream. A sudden increase in heart rate or activity can cause a large piece to break off and travel down the bloodstream to the lungs, causing pulmonary embolism. This can be potentially fatal, thus one of the potential dangers of treatment and another reason why prevention is best.

Heartworm Preventatives
Preventatives are almost 100% effective and are inexpensive. Treatment of heartworms is very expensive and dangerous to your pet. Therefore it is very beneficial to your pocketbook and your pet’s health for your pet to be on monthly preventatives.

Mayfair recommends ProHeart 6 injectable heartworm preventative for dogs. This is given in our office and lasts for 6 months. The active ingredient in ProHeart 6 is Moxidectin.

If your dog is not a candidate for receiving ProHeart 6, we would be happy to discuss the best options available for your specific situation.

For the convenience of heartworm and flea prevention in one tablet, we recommend Trifexis. It provides protection against heartworms, fleas, hookworms, roundworms and whipworms. The active ingredient in Trifexis is the ultra-safe Spinosad plus Milbemycin Oxime.

For cats, we recommend Heartgard or Revolution. Heartgard is a chewable tablet that also controls hookworms. Revolution is a topical heartworm preventive that also controls ear mites, fleas, roundworms, and hookworms in cats.

Remember, heartworm disease is a killer of pets that can be prevented. For your pet’s health, please use a monthly preventative for your dogs and cats, and have all dogs tested yearly.