Heartworm in Dogs

Heartworm Prevention in Dogs

By Hannah Plaugher


About Heartworm Disease

Heartworm disease is a preventable but very serious and sometimes fatal illness that primarily affects dogs, cats, and ferrets. Not only does heartworm disease affect our pets, but also wild canids such as foxes, wolves, and coyotes; wild felids such as tigers and lions; as well as opossums and raccoons.

What are signs of Heartworm Disease?

Dogs with heartworm disease may present with a mild persistent cough, reluctance to exercise, exercise intolerance (or becoming tired after mild activity), decrease in appetite, and weight loss. As heartworm disease progresses to a more advanced state, animals will develop heart failure, may begin to retain fluid in their abdomen, and can even develop heart murmurs. As more and more adult worms congregate in the vessels, the dog may develop a sudden blockage of blood flow within the heart. This leads to very sudden and serious cardiovascular collapse. Veterinarians refer to this process as “caval syndrome,” which is considered a medical emergency. These animals are in severe distress and will present with labored breathing, pale gums, and dark red-colored urine. In order to survive, these dogs must undergo immediate surgery to remove enough worms for the animal to be able to pump blood through its heart. This is a very complex, serious surgery and the prognosis for “caval syndrome” is often grave.

How to do pets get Heartworm Disease?

Heartworms are transmitted from animal to animal solely by mosquitos. When a mosquito bites a heartworm infected animal, immature heartworms (called microfilariae) develop into infective larva within the body of the mosquito. These infective larvae are then able to be transmitted to another animal from the mosquito once reaching this stage of life. When a mosquito bites an animal, a heartworm larvae travels into the bite wound from the mouth of the mosquito. From here, the larvae migrate further into the body of the animal, reach a blood vessel, and enter circulation. Once entering the circulation, the heartworm larvae then continue to mature and slowly make their way to the blood vessels in the lungs. After reaching the blood vessels in the lungs, the immature heartworms continue to develop into adults, with female worms reaching as much as 14 inches in length. This process takes approximately 6 months. Once these worms are inside the pulmonary vessels, they begin to cause vascular damage, reducing the heart’s ability to pump effectively, ultimately resulting in severe pulmonary and cardiovascular disease.

Are heartworms in my neighborhood?

Although heartworm disease has historically been thought of as a “southern disease,” data shows that dogs have tested heartworm positive in all 50 states in the U.S. With that being said, where there are mosquitos, there is the possibility for heartworms. Environments that often support mosquitos include wooded areas, areas with standing water (including artificial containers such as flower pots, car tires, and gutters), and warm temperatures. Heartworm disease is an emerging disease that is becoming a regular finding in areas that the disease used to be considered less common. Taking that data into consideration, the Heartworm Society recommends that all dogs and cats receive an annual heartworm test as well as heartworm prevention 12 months out of the year.

Why and when should my pet be tested?

Since heartworm disease is a very serious, progressive disease, the earlier it is detected, the sooner your pet can be treated. The sooner your pet is treated the better their overall health will be.

After starting a puppy on heartworm prevention, it is recommended that a veterinarian heartworm test your puppy at 6 months of age. Testing prior to 6 months of age is not recommended because the heartworm larvae may still be present, but not mature enough to cause a positive test. These will eventually develop into adult heartworms. You should then continue heartworm prevention if your dog tests negative at this time and then test again in another 6 months to be sure to catch any heartworms that may have now matured to a recognizable stage of life. If you continue on the same heartworm prevention, it is highly recommended that your pet gets tested annually to ensure your prevention is working effectively. Although heartworm preventatives are highly effective, they are not always 100% effective, often due to issues in administration and owner compliance. If you decide to change heartworm prevention products, you should test prior to changing products as well as 6 months later to ensure that your pet did not get infection during the transition period or prior to. It may seem like testing is performed frequently, but it is all in the best interest to ensure that your pet does not become infected with this life-threatening disease.

What are my prevention options?

Heartworm prevention comes in a variety of forms, including oral, topical, and injectable. Most products last 30 days, but if you’re one that often forgets when your pet is due, there are some options that can last as long as 6 months. Giving heartworm prevention to your pet on time is extremely important in preventing disease. There are also often extra benefits to certain products, such as containing flea prevention and also helping control intestinal parasites.

The three different routes of administration of heartworm prevention include:

  • Oral – often manufactured into a flavored tablet or chewable.
  • Topical – prevention is in a liquid suspension that is applied directly to the skin on the back between the shoulder blades.
  • Injectable – prevention is in a form that is injected with a needle and syringe into the subcutaneous layer of the dog’s skin. This product is approved for longer protection and has a longer dosing interval.

When trying to decide which product works best for you, it is important to think about the route of administration and the dosing frequency. If your dog has food hypersensitivity and is allergic to certain protein sources, you may want to look further into the ingredients within an oral chewable preventative to make sure it does not interfere with your dog’s current dietary restrictions. In these cases, a topical or injectable product may better suit your pet. If you have a pet that is mainly outdoors and swims daily, applying and letting a topical heartworm prevention completely dry before the dog gets wet may be extremely difficult, therefore, oral or injectable products may be more ideal.

Common Veterinarian recommended canine heartworm prevention products

Name of Product Minimum Age Dose Frequency Active Ingredient

Also Prevents

Heartgard® Plus for Dogs 6 weeks Every 30 days, oral Ivermectin Roundworms and hookworms
Sentinel® for Dogs 4 weeks, 2 lbs Every 30 days, oral Ivermectin Hookworms
Trifexis 8 weeks, 5 lbs Every 30 days, oral Mibemycin oxime and spinosad Fleas, roundworms, hookworms, and whipworms
Revolution® for Dogs 6 weeks Every 30 days,  topical Selamectin Fleas, flea eggs, ear mites, sarcoptes mange, and American Dog Ticks
Advantage Multi™ for Dogs 7 weeks, 3 lbs Every 30 days, topical Moxidectin Fleas, roundworms, hookworms, and whipworms
ProHeart®6 6 months 6 months, injectable Moxidectin Roundworms, hookworms, and whipworms

Oh no! My dog tested positive! Now what?

If your dog tests positive, the test needs to be confirmed with a second test of a different type and brand. If heartworms are confirmed, you will need to begin to restrict your dog’s exercise considerably. Dogs should only go outdoors to relieve themselves and stay on a leash at all times. Indoors, your dog should stay in a small enclosed room or, ideally, crated. If your dog is showing severe signs or has a secondary illness, those must be addressed and your dog should be stabilized prior to treatment. Again, some dogs may show no signs of illness but exercise restriction should still be taken seriously. The next step is to begin heartworm treatment.

Heartworm treatment is most effective in mildly infected dogs. Although dogs with severe heartworm infections can be treated, the complications are typically much greater. The most common complication associated with heartworm treatment is pulmonary thromboembolism, or “throwing a clot/worm.” This is exhibited by lethargy, weakness, fever, coughing, right-sided heart failure, coughing up blood, or sudden death. Pulmonary thromboemboli are exacerbated by exercise, therefore, it is extremely important to keep dogs as calm and quiet as possible.

The American Heartworm Association Protocol for Heartworm Disease is considered the “gold standard” for heartworm treatment in the United States. Dogs should start (or continue) heartworm prevention throughout the process of treatment. After starting heartworm prevention, dogs should be started on 30 days of the antibiotic, doxycycline or minocycline. One month after finishing antibiotic treatment, dogs will receive an injection of melarsomine dihydrochloride. In 30 days, dogs will receive another melarsomine dihydrochloride followed by one more injection in 24 hours. This is crucial time for activity restriction, as the drug being injected is now “killing” the heartworms. Your dog is to then to be tested for microfilariae presence 30 days later. If your pet is positive, treat with a microfilaricide and retest in 4 weeks. At this point, you can slowly begin to lift exercise restriction. Perform an antigen test 6 months after the last melarsamine dihydrochloride injection and screen for microfilariae.


Heartworm disease is a serious, emerging parasitic disease in the canine that is easily preventable. Although heartworm disease may seem like a complicated process, choosing the prevention that best fits your lifestyle and pet is crucial. If you feel overwhelmed with all of your different options, it is important to communicate with your veterinarian about the questions you may have. No matter where your location in the United States, keeping your pet on heartworm prevention is fundamental to their health and wellness.


"Heartworm Basics." American Heartworm Society -. American Heartworm Society, n.d. Web. 14 Aug. 2016.Resources:

  1. "Heartworm - Canine Heartworm." Companion Animal Parasite Council. CAPC, July 2015. Web. 01 Aug. 2016.
  2. "Heartworm Disease." Heartworm Disease. American Veterinary Medical Association (AVMA), n.d. Web. 01 Aug. 2016.
  3. "Heartworm Preventive Comparison Chart." Dog Heartworm Preventive Products Comparison Chart. Petco Wellness, LLC, n.d. Web. 02 Aug. 2016.