Heartworms - Laura Anna See, DVM

What are heartworms?

Heartworms are parasitic worms (nematodes) that live in the heart and arteries of the lungs of dogs and other mammals, including cats.

While adult worms live in the heart and arteries, baby worms (microfilaria) are found in the blood of infected animals. When a mosquito bites a heartworm-infected animal, the mosquito eats the microfilaria along with the blood of the animal. In the mosquito, the microfilaria mature over 10 to 14 days to become larvae that are infective and dangerous to other animals. When this infected mosquito bites an unprotected dog (or cat), the pet can become infected with the larvae from the mosquito.

In the dog, the larvae will mature and become adult worms that live in the heart and lung arteries. It takes approximately 6 months for the larvae to become adult worms in the heart (and consequently 6 months for the dog to test positive on a heartworm test even though he was actually infected when the mosquito bit him). Cats can also become infected with heartworms, although it occurs more frequently in dogs, so we recommend that all dogs and all outside cats be on monthly heartworm prevention year-round .

What happens when an animal is infected with heartworms?

For both dogs and cats, the clinical signs of heartworm disease may not be seen during early infection. Over time, the worms in the heart and lung arteries cause much inflammation and damage and lead to heart failure. Clinical signs of heart failure include a mild, persistent cough, reluctance to move or exercise, fatigue after only moderate exercise, reduced appetite, and weight loss. In cats clinical signs can be very subtle and often infected cats die suddenly without any apparent signs of sickness. Heartworm disease is a serious and fatal condition if left untreated.

How do we prevent heartworm infection?

Fortunately, prevention of heartworm disease is easy and only requires a once-a-month medication, either in the form of a tablet or chew (Heartgard/Interceptor/Sentinel) or in the form of a liquid that is put directly on the skin (Advantage Multi). Some preventatives also treat fleas in addition to heartworms.

We recommend having all dogs and all outside cats on monthly heartworm prevention year-round as Texas is one of the states with the highest occurrence of heartworm disease. As long as the pet receives heartworm prevention each month, the risk of disease is very low. However, because heartworm disease is so dangerous if undetected and untreated, we test all dogs for heartworm disease on a regular basis, even if they are receiving heartworm prevention monthly .

When does my pet need to be tested for heartworms?

If a dog is on heartworm prevention every month year-round, we test every 2 years.

Remember that it takes up to 6 months for an infection to test positive on our heartworm test because the test checks for adult worms, not larvae. That means that if your dog missed several doses of prevention in April and May and became infected through a mosquito bite, the test would not show positive until October or November.

If your pet has missed more than one dose of heartworm prevention, we will want to check for heartworm disease 6 months after the date you begin using prevention again on a monthly basis. For example, if it is now July and you missed 3 months of prevention in April, May, and June, we would start heartworm prevention and then test for heartworms in January, 6 months after your dog has been back on heartworm prevention continually.

Usually, if your dog has gone a long time (greater than 6 months) without any heartworm prevention we will test for heartworms before putting your pet on prevention. This is because heartworm positive dogs can have adverse reactions to some heartworm preventatives. We would also check your pet again 6 months later to make sure your pet is clear of infection.

Unfortunately, testing is not very accurate in cats and they can be infected with a very low number of worms that do not always show up on our tests. If you are concerned that your cat may be infected with heartworms we would recommend taking x-rays of the heart to see if any changes have occurred that would be consistent with heart disease from heartworms. Another option is to ultrasound the heart to look for the worms in the large blood vessels.

How do we treat heartworm disease?

Heartworm disease can be treated in dogs with a series of injections that kill the adult worms in the heart. This procedure has risks because the dead worms travel in the bloodstream from the heart to the arteries in the lungs to decompose. This can cause serious respiratory distress if a large number of worms go to the lungs at once. For this reason it is vital that your dog be confined to a small area and not be allowed to run, jump, or play while the treatment is being performed and for 6-8 weeks afterwards. If your dog is exercising at all, it increases the risk of a large number of dead worms being carried by the blood to the lungs. By being quiet and calm, the blood flow is slower and the risk is decreased.

The only method of treating heartworm disease in cats is to manually extract the worms through a very difficult surgery that must be performed at a specialty hospital. The medicine we use in dogs to kill adult worms is very dangerous to cats and likely to cause death.

As you can see it is much easier, safer, and less expensive to prevent heartworm disease than to treat it.

What happens if my dog has heartworm disease but I do not treat?

Over time, the worms damage the heart and this will eventually cause heart failure. If you choose not to treat your heartworm positive dog, then we recommend keeping your dog on prevention monthly year-round to prevent the infection from getting worse. Some monthly preventatives may help with the current heartworm infection over a very long period of time. The lifespan of a heartworm is 5 to 7 years in a dog and even after the worm dies the damage that has been done to the heart is not necessarily reversible, so treating the disease is by far the best option.