What Is It?

Heartworm disease is a very common problem in many parts of the world, including Minnesota. It is caused by a parasitic worm called Dirofilaria immitis. In the adult form, this worm resides in the pulmonary vessels and occasionally in the right side of the heart, thus the name ‘heartworm’. Transmission of this disease is via mosquitoes. A female mosquito feeds on an infected dog, cat, or ferret and acquires the first-stage larvae (microfilariae). Larvae develop in the mosquito as long as the average daily temperature is more than 64°F (generally May through Oct in Minnesota). As the mosquito feeds again, third-stage larvae enter into the new host and begin their migration through the subcutaneous tissue to the vascular system. During this period, two more molts occur. Young adults (fifth-stage larvae) reach the pulmonary arteries by approximately 5-6 months post-infection. At this time, worms may begin producing microfilariae. Not all dogs and cats with heartworm disease will produce microfilariae. Unisex infections or infections with immature or sterile heartworms will not be capable of producing microfilariae. Occasionally, the body’s immune response will be able to destroy the microfilariae, leaving just the adult worms. Infections which do not produce microfilariae are termed occult infections and tend to be more clinically severe.

The clinical signs associated with heartworm infection reflect the number of worms present, duration of infection, and host-parasite interactions. Until approximately 50 worms are present, nearly all the worms reside in the pulmonary arteries. As the numbers approach 75, worms may be visualized in the right atrium and ventricle. The vena caval syndrome (discussed below) can occur with a worm burden of greater than 100. Many dogs are asymptomatic at the time of diagnosis. Symptomatic dogs and cats generally have respiratory signs that may include exercise intolerance, coughing, difficulty breathing, weight loss, and episodes of collapse. Some cats will vomit. As the number of worms increase, signs of right-sided congestive heart failure may develop. Severely infected individuals have a large number of worms mechanically occluding the right side of the heart and associated vessels. This is a serious condition called the vena caval syndrome. Rare individuals will develop serious pulmonary sequelae including an allergic pneumonitis and pulmonary eosinophilic granulomatosis. These diseases carry a guarded prognosis. In addition, clinical signs may reflect dysfunction in other organ systems secondary to immune-complex deposition or aberrant migration of worms.


The diagnosis of heartworm disease begins with a complete medical history and a thorough physical examination. Screening for heartworm disease is usually done in the spring in Minnesota, though now that most patients are on year round preventative, testing may be performed at any time. Infections acquired in October or November of the preceding year will not be detectable until April or May of the following spring. Puppies less than 6 months of age need not be tested until the following spring. The American Heartworm Association currently recommends that adult heartworm antigen tests be used as the primary method of screening dogs. These tests detect antigen produced from adult female worms. Immature infections or infections consisting of all male worms will not be detected. Microfilarial detection tests are indicated for heartworm antigen-positive dogs to search for immature larvae. In cats, a combination panel including both a heartworm antigen and a heartworm antibody test is generally performed.

Dogs and cats testing positive for heartworm disease should have a minimum pretreatment database performed in order to classify the severity of the disease and establish a prognosis. Radiographs of the chest are probably the most important diagnostic tool for determining the severity of disease. In addition, a complete blood count (CBC), a serum chemistry profile, and a urinalysis are important to identify any concurrent disease. In some cases, electrocardiography (EKG) and/or echocardiography (ultrasound) may be warranted. Based on results of these tests, patients are classified into one of four classes:

Class 1: Asymptomatic or mild clinical signs

Class 2: Moderate clinical and radiographic abnormalities

Class 3: Severe clinical and radiographic abnormalities including right-sided congestive heart failure

Class 4: Vena caval syndrome


Treatment is aimed at killing both the adult heartworms and the microfilariae while having the least amount of drug toxicity and the fewest side effects possible. The approved available adulticide for dogs is melarsomine dihydrochloride (Immiticide). This is an organic arsenical compound that is effective against both immature and mature heartworms. For most patients, 3 injections will be administered. Immiticide is injected intramuscularly in the lumbar muscles of the back. Initially, a single injection is given, then two follow-up injections are given at 24 hour intervals 1 month after the first. Appropriate supportive therapy which may include oxygen, corticosteroids, fluids, diuretics, and anticoagulants are given as needed for patients in this category. Patients in Class 4 require surgical removal of worms prior to adulticide therapy.

Strict patient confinement is essential for 4-6 weeks post-treatment to decrease the risk of complications. Thromboembolic complications are the most common serious side effect to treatment. This means that dead or dying worms can cause thrombosis (‘clotting’) and pulmonary artery obstruction. This is most likely to occur 7-17 days after treatment. Clinical signs include depression, fever, increased heart and respiratory rates, cough, and occasionally right-sided heart failure, collapse, or death. Side effects associated with the Immiticide itself include behavioral changes, lethargy, respiratory abnormalities, vomiting, anorexia, and injection site reactions. Please alert your veterinarian immediately if you are detecting any of these clinical signs.

Microfilariae are eliminated by administering Heartgard Plus monthly. Six months after adulticide treatment, an antigen test is performed. A strongly positive test indicates that adult heartworms remain and retreatment is necessary. This is rare with Immiticide treatment. A weakly positive test may be the result of a few persisting live worms. Wait an additional month and then retest. Immiticide tends to be highly efficacious and this is not usually an issue.

In cats, no approved therapy is available to treat the adult worms. Immiticide can NOT be given to cats. Supportive care is provided to manage secondary complications while waiting for the adult worms to die naturally. This disease can be fatal in both cats and dogs, but the risks are greater in cats due to the lack of available therapy. All cats should be placed on year round Revolution to prevent further infections.


Heartworm disease is an easy disease to prevent. In our clinic, we recommend Heartgard Plus or Tri-Heart Plus for dogs and Revolution for cats once monthly. The dose of ivermectin in Heartgard is safe even for ivermectin-sensitive breeds such as Collies. Your dog should be tested for heartworm disease annually even if he or she is on year round preventative. Cats may be tested, but this is not routinely performed.

Ask us if you have any questions about heartworm disease in your dog or cat!

Content prepared by St. Francis Animal Hospital, 1227 Larpenteur Ave. West, Roseville MN. 55113