Heartworm 101

What Is Heartworm Disease?

Heartworm disease is a serious and potentially fatal disease in pets. It is caused by worms (heartworms) that live in the heart, lungs and associated blood vessels of affected pets, causing severe lung disease, heart failure and damage to other organs in the body. Adult male heartworms can be 6 inches long while adult females can be 12 inches long. Heartworm disease affects dogs, cats and ferrets, but heartworms also live in other mammal species, including wolves, coyotes and foxes. Because wild species such as foxes and coyotes live in proximity to many urban areas, they are considered important carriers of the disease.


The dog is a natural host for heartworms, which means that heartworms that live inside the dog mature into adults, mate and produce offspring. A dog not on heartworm prevention can be infected repeatedly. If untreated, their numbers can increase, and dogs have been known to harbor several hundred worms in their bodies. The average number per infection is thought to be 15. Heartworm disease causes lasting damage to the heart, lungs and arteries, and can affect the dog’s health and quality of life long after the parasites are gone. For this reason, prevention is by far the best option, and treatment—when needed—should be administered as early in the course of the disease as possible.

How is heartworm disease transmitted from one animal to another?

The mosquito plays an essential role in the heartworm life cycle. Adult female heartworms living in an infected dog, fox, coyote, or wolf produce microscopic baby worms called microfilaria that circulate in the bloodstream. Each female heartworm can produce 2000 microfilaria at a time. These microfilaria can live up to 2 years in the bloodstream. When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which develop and mature into “infective stage” larvae over a period of 10 to 14 days. Then, when the infected mosquito bites another dog, cat, or susceptible wild animal, the infective larvae are deposited onto the surface of the animal’s skin and enter the new host through the mosquito’s bite wound. Once inside a new host, it takes approximately 6 months for the larvae to mature into adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats. Because of the longevity of these worms, each mosquito season can lead to an increasing number of worms in an infected pet.

What are the signs of heartworm disease in dogs?

In the early stages of the disease, many dogs show few symptoms or no symptoms at all. The longer the infection persists, the more likely symptoms will develop. Active dogs, dogs heavily infected with heartworms, or those with other health problems often show pronounced clinical signs.

Signs of heartworm disease may include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, and weight loss. As heartworm disease progresses, pets may develop heart failure and the appearance of a swollen belly due to excess fluid in the abdomen. Dogs with large numbers of heartworms can develop a sudden blockage of blood flow within the heart leading to a life-threatening form of cardiovascular collapse. This is called caval syndrome, and is marked by a sudden onset of labored breathing, pale gums, and dark bloody or coffee-colored urine. Without prompt surgical removal of the heartworm blockage, few dogs survive.

What do I need to know about heartworm testing?

Heartworm disease is a serious, progressive disease. The earlier it is detected, the better the chances the pet will recover. There are few, if any, early signs of the disease when a dog is infected with heartworms, so detecting their presence with a heartworm test administered by a veterinarian is important. The test requires just a small blood sample from your pet, and it works by detecting the presence of heartworm proteins. Some veterinarians process heartworm tests right in their hospitals while others send the samples to a diagnostic laboratory. In either case, results are obtained quickly. If your pet tests positive, further tests such as a blood filter test for microfilaria and a chest radiograph may be ordered.

When should my dog be tested?

Following are guidelines on testing and timing:

  • Puppies under 7 months of age can be started on heartworm prevention without a heartworm test (it takes at least 6 months for a dog to test positive after it has been infected), but should be tested 6 months after your initial visit, tested again 6 months later and yearly after that to ensure they are heartworm-free.
  • Adult dogs over 7 months of age and previously not on a preventive need to be tested prior to starting heartworm prevention.  They, too, need to be tested 6 months and 12 months later and annually after that.
  • If there has been a lapse in prevention (one or more late or missed doses), dogs should be tested immediately, then tested again 6 months later and annually after that.

Annual testing is recommended, even when dogs are on heartworm prevention year-round, to ensure that the prevention program is working. Heartworm medications are highly effective, but dogs can still become infected. If you miss just one dose of a monthly medication—or give it late—it can leave your dog unprotected. Even if you give the medication as recommended, your dog may spit out or vomit a heartworm pill—or rub off a topical medication. Heartworm preventives are highly effective, but not 100 percent effective. If you don’t get your dog tested, you won’t know your dog needs treatment.

What happens if my dog tests positive for heartworms?

No one wants to hear that their dog has heartworms, but the good news is that most infected dogs can be successfully treated. The goal is to first stabilize your dog if he is showing signs of the disease, then kill all adult and immature worms while keeping the side effects of treatment to a minimum.

There are four classes, or stages, of heartworm disease.  The higher the class, the worse the disease and the more obvious the symptoms.

  • Class 1:  No symptoms or mild symptoms such as an occasional cough.
  • Class 2:  Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity.
  • Class 3:  General loss of body condition, a persistent cough, and tiredness after mild activity.  Trouble breathing and signs of heart failure are common. For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest x-rays.
  • Class 4:  Also called caval syndrome.  There is such a heavy worm burden that blood flowing back to the heart is physically blocked by a large mass of worms.  Caval syndrome is life-threatening and quick surgical removal of the heartworms is the only treatment option.  The surgery is risky, and even with surgery, most dogs with caval syndrome die.

Not all dogs with heartworm disease develop caval syndrome.  However, if left untreated, heartworm disease will progress and damage the dog’s heart, lungs, liver, and kidneys, eventually causing death.


There are two methods to treat Class 1-3 heartworm infections. Each method has its advantages.

  • Soft Kill treatment is less expensive but takes longer. The treatment is a combination of a heartworm preventative and Doxycycline (antibiotic). The heartworm preventative kills only the adolescent (L1, L3-L4) larvae. The Doxycycline is believed to weaken the adults (L5) shortening their lifespan and eventually sterilizes the females. This method can take up to 18 months.
  • Fast Kill* treatment is more expensive but kills quickly. This is a 2 part treatment that is done 7-8 weeks apart. Part I,  2 injections of melarsomine to kill the adults are administered 30 days apart. Part II, Ivermectin is administered orally to eliminate the adolescents. Antibiotics and steroids are given during treatment to lessen the effects such as thromboembolism and inflammatory responses caused by the dead worms. This method takes 60 days in most cases. *Our Vets current protocol.

Fast kill is the American Veterinary Medical Association and American Heartworm Society recommended treatment. Soft kill may be recommended in rare cases where the overall health of the animal is so poor they are not deemed a good candidate for fast kill. In all cases, please follow your vets recommendation.

Class 4 infections are life-threatening and surgical removal of the heartworms is the only option.


Preventatives kill the heartworm larvae before they have a chance to grow and mature into adult heartworms. Some contain additional ingredients that will control other parasites, such as roundworms, hookworms, fleas and ticks. All heartworm preventatives are macrocyclic lactones most commonly; Ivermectin, Milbemycin, Moxidectin and Selamectin. A preventative should be given to your pets year round and consistently on a schedule such as the 1st day of each month. Most preventatives are in effect for a full month but some products can protect your pet up to 6 months. Remember that mosquitoes can get indoors, so even though your pet may not go outside, he is still susceptible.