Heartworm Disease in Dogs: Symptoms, Diagnoses, and Treatment

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    What is Heartworm Disease in Dogs?

    Photo Credit: Longwood Veterinary Center

    Heartworm disease is a treatable but possibly life-threatening parasite that predominantly affects dogs, cats, and even ferrets. It can also infect wild animals from the dog and cat family, raccoons, opossums, and seals/sea lions among others. While human infections have been documented, it’s not common.

    By blocking the heart and the main blood arteries that flow from it, especially the pulmonary artery, adult heartworms create heartworm disease. They also interfere with how well the heart valves work. Blood flow to multiple organs is decreased when the main blood arteries get clogged, most notably blood flow to the lungs, liver, and kidneys. Due to a reduction in blood flow and oxygen availability, these organs may fail.

    What Causes Heartworm Disease in Dogs?

    Heartworm disease is caused by a blood-borne parasite called Dirofilaria immitis which is transmitted exclusively via mosquito bites.

    Microfilaria, which are tiny about 1/100th of an inch long, are transported from infected to uninfected animals by residue on the mosquito’s mouthpiece. After about two months, the young worms migrate through circulation and settle in the right side of the heart, where they begin to mature.

    The affected animal is the sole host, meaning the worms grow into adults, mate, and reproduce while living within the animal. The worms are known as “heartworms” because the adults dwell in an infected animal’s heart, lungs, and blood vessels.

    Who’s at Risk of Developing Heartworm Disease?

    Heartworm disease is prevalent nearly everywhere and doesn’t discriminate on breed, sex, age, or where the animal lives. It’s equally prevalent in both dogs and cats, indoors and outdoors. The condition is mostly found in dogs aged two to eight years. Because microfilariae take 5 to 7 months to grow into adult heartworms 

    There’s an increased risk in areas with significant concentrations of wild or stray animals or a high mosquito population. Heartworm is also most prevalent along the Mississippi River and its major tributaries, as well as along the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey.

    What are the Symptoms of Heartworm Disease?

    If a dog shows symptoms of heartworm, it often means the disease has progressed. Signs that your dog has contracted heartworms include coughing, being sluggish, having a loss of appetite, tiring quickly with activity, having respiratory problems, or having a swollen stomach. Dogs who are more active will often show more signs of having the disease over not so active dogs due to the change in their activity level and tolerance. 

    How Can Heartworm Disease in Dogs Be Prevented?

    Early detection is the key to preventing the disease from progressing too far to be treated. Annual testing for dogs over 7 months old is also a key to early detection. One of the first ways to prevent heartworms is limiting your dog’s exposure to mosquitos.

    Dogs should also be given heartworm prevention year-round which treats heartworms that have already infected the dog sometime within the last month or so. Heartworm prevention is available in multiple forms and can only be prescribed by a veterinarian. It’s available as an injection every 6 or 12 months, topical medications, or chewable pills.

    Different heartworm preventions can also function in different ways. Some of them will protect against many types of parasites such as fleas and ticks on top of heartworms while others just protect against internal parasites.

    How is Heartworm Disease Diagnosed?

    Heartworm disease can be diagnosed via a blood test as well as an antibody test but further tests may be done to ensure it’s safe for the dog to receive treatment. Antigen testing can detect female adult heartworms.

    Your veterinarian will run blood tests on your dog to check for adult heartworm infections (infections older than 6 months). Heartworm adult females may be found using antigen testing, and heartworm exposure can be determined using antibody tests on your pet. The most often used and very reliable test in dogs is the antigen test. Additional examinations, such as x-rays of the chest, blood work, and echocardiograms (heart ultrasounds), may be required to confirm the diagnosis, gauge the severity of the condition, and choose the most effective course of therapy for your dog.

    What is the Treatment for Heartworm Disease?


    While prevention is always better than needing to treat the disease, if your dog does become infected, there are treatment options available. Keep in mind that there is substantial risk in treating dogs who have heartworm disease but dogs who are in good health have less of a risk.

    Adult heartworms are eradicated from the heart and surrounding regions with the injectable medicine melarsomine (brand name Immiticide®), which is administered in a series. Your veterinarian will determine the precise injection regimen based on the condition of your dog. Most dogs have their initial injection, then 30 days of rest, followed by two further injections spaced 24 hours apart. Many dogs will also receive therapy with an antibiotic in order to avoid infection with the bacteria (Wolbachia) that reside inside the heartworm (doxycycline).

    Prior to receiving treatment for the heartworms, dogs with severe heartworm disease may need to take antibiotics, painkillers, special diets, diuretics to eliminate fluid buildup in the lungs, and/or medicines to enhance heart function. Some dogs could need lifelong heart failure medication even after the heartworms have been eradicated. Diuretics, heart drugs like beta-blockers, ACE-inhibitors, or cardiac glycosides, as well as specialized low-salt diets, are all examples of this.

    What is the Prognosis for Heart Worm Disease?

    The recovery of your dog following heartworm treatment usually astounds dog owners, especially if the dog had been exhibiting clinical heartworm disease signs. Increased energy, better appetites, and weight gain are common in dogs.

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