Heartworm Disease in Dogs & Cats

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 major vessels of the lungs and occasionally in the right side of the heart, thus the name ‘heartworm’. Heartworm disease is transmitted via mosquitoes.

Life Cycle

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. As the mosquito feeds again on another animal, third-stage larvae enter into the new host and begin their migration through the subcutaneous tissue to the bloodstream. During this period, the immature heartworms molt two more times. Young adults (fifth-stage larvae) reach the pulmonary arteries by approximately 5-6 months post-infection. At this time, the adult worms may begin producing microfilariae. A mosquito then feeds on the infected host and the life cycle starts all over again. 


The clinical signs associated with heartworm infection reflect the number of worms present, duration of infection, and host-parasite interactions. Many dogs are asymptomatic at the time of diagnosis. Symptomatic dogs and cats generally have respiratory symptoms 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. 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. 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 several tests performed in order to classify the severity of the disease and establish a prognosis. Radiographs of the chest are 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 first medication used is an antibiotic called doxycycline, which can help to weaken the heartworms and make them more susceptible to treatment. Next, an adulticide medication is administered to kill the adult heartworms. The only approved adulticide for dogs is arsenic-based compound called melarsomine dihydrochloride (Immiticide or Diroban). For most patients, 3 injections will be administered. Melarsomine is injected into 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. Symptoms include depression, fever, increased heart and respiratory rates, cough, and occasionally right-sided heart failure, collapse, or death. Side effects associated with the melarsomine itself include behavioral changes, lethargy, respiratory abnormalities, vomiting, anorexia, and injection site pain or reactions. Please alert your veterinarian immediately if you are detecting any of these issues.

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 melarsomine 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. Melarsomine 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 Plus to prevent further infections.


Heartworm disease is an easy disease to prevent. In our clinic, we recommend Heartgard Plus for dogs and Revolution Plus for cats once monthly. The dose of ivermectin in Heartgard Plus 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.

If you’d like to read more about heartworm disease, visit the American Heartworm Society.

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