Heartworm in Dogs

Heartworm disease is an increasingly common disease in dogs. It is most common in the southeastern United States but has been spreading further north, particularly due to the relocation of shelter animals from the south. (1) The disease is spread by mosquitoes and can also affect cats, ferrets, and sometimes even humans. 

Heartworm disease in dogs can be fatal. However, your veterinarian can prescribe a monthly parasite preventative, such as Simparica Trio chewable tablets or Revolution topical treatment, to provide year-round protection from the dangers of heartworms.

What is Heartworm in Dogs?

Heartworm disease is a serious illness in dogs caused by Dirofilaria immitis, the heartworm.  The adult worms are up to a foot long and live in the heart, lung, and pulmonary blood vessels. If left untreated, the number of worms increases and dogs have been known to harbor several hundred heartworms in their bodies at one time. The worms can cause heart failure, lung disease, and a serious, often fatal condition called caval syndrome. Worms can also reside in the large vein that carries deoxygenated blood to the heart (the caudal vena cava), which can cause liver and kidney failure. (2, 3) 

How Do Dogs Get Heartworm?

Heartworm disease is transmitted by mosquitoes. When a dog is infected with heartworms, the adult worms reproduce and create offspring called microfilariae. Microfilariae reside in the blood of most, but not all, infected dogs. When a mosquito bites an infected dog, the mosquito ingests the microfilariae. While inside the mosquito, the microfilariae develop into infective larvae.  These infective larvae are then deposited into another dog (the host) when the mosquito bites again.

Once inside the dog, the larvae migrate through the tissues and mature into sexually immature adults. As early as 70 days after infection, the immature worms migrate to the heart and lungs.  The worms then fully mature and mate within the pulmonary vessels, primarily the pulmonary artery. Microfilariae appear in the blood as early as six months post-infection. Adult worms can live in the dog for approximately five to seven years. (2)

Heartworm disease is not contagious from dog to dog—it must be transmitted by a mosquito. In rare cases, humans can get heartworms from being bitten by an infected mosquito, but the infective larvae typically die before maturing into adult worms.

Dog Heartworm Symptoms to Know

Many dogs with heartworm disease have no symptoms. The presence of symptoms depends on several factors, including the worm burden, the size of the patient, the duration of infection, the degree of the inflammatory response to the heartworms, and the patient’s activity level. The longer the infection persists, the more likely it is that the dog will develop symptoms.  

Symptoms of heartworm disease in dogs include:

• Cough
• Weight loss
• Fatigue
• Decreased appetite
• Reluctance to exercise

As the infection progresses and worm burden increases, the infected dog may develop congestive heart failure. Signs of congestive heart failure in dogs include:

• Weakness
• Abdominal distension
• Increased respiratory effort
• Cough
• Lethargy
• Exercise intolerance
• Reluctance to lie down
• Loss of appetite
• Weight loss

Dogs with high numbers of worms can develop a severe, life-threatening condition called caval syndrome. This condition occurs when worms disrupt blood flow through the heart. Caval syndrome can also sometimes occur even in dogs with low worm burdens. Symptoms of caval syndrome include:

• Labored breathing
• Pale gums
• Bloody or brown-colored urine
• Shock
• Death

Stages of Heartworm Disease in Dogs

Heartworm disease in dogs is classified according to its severity. There are four classes of heartworm disease in dogs (4):

Class 1: Dogs with no or very mild signs of disease, such as a mild cough. These dogs have no changes on radiographs (X-rays) or on physical examination. These cases are usually identified on routine screening tests.

Class 2: Dogs with moderate heartworm disease but no signs of heart failure. Dogs in class 2 may have coughing, weight loss, loss of appetite, and fatigue. Changes may be present on radiographs (X-rays) and blood work.

Class 3: Dogs with severe heartworm disease and heart failure. Symptoms include weight loss, persistent cough, abnormal respiratory rate and effort, abnormal heart and lung sounds, visible jugular pulse, signs of heart failure, fluid in the abdomen, and fainting. Severe changes are noted on radiographs and blood work. Dogs in Class 3 have a guarded prognosis.

Class 4: Dogs with caval syndrome. Heartworms are present in large numbers in the right atrium of the heart and in the vena cava, the large vein that carries deoxygenated blood to the heart. Many dogs in Class 4 are also suffering from liver failure. Dogs in Class 4 have a grave prognosis.

Heartworm disease is a progressive condition and damage to the heart and lungs worsens the longer the infection is present. Complications during treatment are also more likely to occur with more severe infections. It is essential to diagnose and treat the infection early, when symptoms are minimal, to reduce the risk of complications and minimize damage to the organs. 

How to Diagnose Heartworm in Dogs

Here is what to expect when it comes to diagnosing heartworm in dogs:

History and Physical Examination. Your veterinarian will ask you about your dog’s history, including any travel history, to ascertain whether your dog could have been exposed to heartworm disease. Your veterinarian will also perform a full head-to-tail physical examination on your dog, which will include listening to your dog’s heart and lungs.

Heartworm Antigen Test. A heartworm antigen test is often performed as a routine screening test and may also be performed if your veterinarian suspects that your dog may have heartworm disease. This test detects a protein secreted by mature female heartworms as early as five to seven months post-infection. The American Heartworm Society recommends that this test be performed annually on all dogs over 7 months of age to screen for heartworm disease (5).

Microfilaria Test. In this test, a drop of fresh blood is examined under a microscope to detect microfilariae. Microfilariae can be detected in the blood as early as six months post-infection. The American Heartworm Society recommends that this test be performed annually on all dogs over 7 months of age to screen for heartworm disease (5).

Radiographs (X-rays). Your veterinarian may recommend taking radiographs of your dog’s heart and lungs to look for changes that can be caused by heartworms, such as enlargement of the right side of the heart, dilation of the pulmonary arteries, and enlargement of the caudal vena cava. Even if your dog does not have symptoms of heartworm disease, X-rays should be evaluated prior to starting treatment to assess the extent of heartworm disease.

Blood Work. Your veterinarian may recommend a complete blood count and biochemistry panel to assess your dog for changes such as anemia, elevated liver values, and azotemia, which can occur secondary to heartworm infection.  

Echocardiography. Your veterinarian may recommend an echo (ultrasound) of the heart to evaluate the extent of heartworm disease and confirm a positive antigen test result. Dogs with mild heartworm disease may have normal results on echocardiography, while those with more significant disease may have increasingly severe changes. With heavy worm burdens, worms may be visualized in the heart and pulmonary arteries on echocardiography. 

Heartworm Treatment for Dogs

Heartworm disease is a serious illness and must be detected early and treated promptly to prevent damage to the heart, lungs, and other organs. Heartworm disease is progressive and delays in treatment will result in increasing worm burdens and worsening of damage to the heart and lungs.  

As soon as your dog is diagnosed with heartworms and throughout treatment, you will need to restrict your dog’s activity and keep your dog very quiet. This is necessary because physical exertion increases the damage done to the heart and lungs by heartworms (6). According to the American Heartworm Society, there is a distinct correlation between the severity of infection and the activity level of the dog (5). Activity restriction is therefore essential to minimize complications associated with heartworm infection.

There are two methods for treating heartworm disease: adulticide therapy (see “Melarsomine” in the Heartworm Medicine section) and the “slow kill” method. The slow-kill method is not recommended, as it often takes more than two years to kill the adult heartworms, during which time the damage to the heart and lungs of the dog is progressing. The slow-kill method also may not kill all of the adult worms and there is concern that this method may create resistant subpopulations of heartworms. Due to the high risk of complications with the slow-kill method, the American Heartworm Society does not recommend it (5).

If your dog has significant symptoms of heartworm disease, your dog may need to be stabilized prior to starting heartworm disease treatment. This may include the use of diuretics (medications that help remove sodium from the body), vasodilators (medications that open blood vessels), positive inotropic agents (medicines that strengthen the force of the heart’s contractions), glucocorticoids (steroid hormones used to treat inflammation), and fluid therapy.  

The goal of heartworm treatment is to eliminate all stages of heartworms in the dog and improve the dog’s symptoms. Following treatment, your dog will need to be tested again with an antigen test and a microfilaria test to confirm that the heartworms are no longer present. If your dog tests positive again, a second round of treatment may be necessary.  

Heartworm Medicine for Dogs

The American Heartworm Society recommends a heartworm treatment protocol using the following medications (5):

Doxycycline. Dirofilaria immitis harbors a bacteria called Wolbachia in its body. This bacteria has a symbiotic relationship with the parasite. While its exact function is unknown, we do know that the parasite needs this bacteria to survive. Treating your dog with the antibiotic Doxycycline kills the Wolbachia in the heartworms and this weakens the heartworms, making them easier to kill. Doxycycline is administered to your dog on Days 1-28 of treatment.

Macrocyclic Lactones. A macrocyclic lactone parasiticide is administered to eliminate susceptible larvae and prevent new infections. Macrocyclic lactones commonly used in veterinary medicine include avermectins and milbemycins. A macrocyclic lactone is administered on Day 1 and then every 30 days thereafter. 

Antihistamines. Antihistamines such as diphenhydramine (Benadryl) may be administered at the same time as macrocyclic lactones in dogs with high microfilarial counts. Because macrocyclic lactones rapidly reduce the numbers of microfilaria, there is a risk of a reaction.  Pre-treatment with antihistamines and steroids such as prednisone reduces the risk of reaction. 

Melarsomine. Melarsomine is the only drug approved by the FDA for the treatment of adult heartworms. It is administered by intramuscular injection at Days 61, 90, and 91. According to the American Heartworm Society, this three injection protocol kills about 98 percent of adult heartworms. Following the first melarsomine injection, it is essential to further reduce the dog’s activity level to reduce the risk of cardiopulmonary complications due to the dying worms.  Exercise restriction must continue for 6-8 weeks following the last melarsomine injection.

Prednisone. Prednisone is a steroid used in heartworm treatment for its anti-inflammatory properties. Pulmonary thromboembolism—a clot that gets stuck in an artery in the lung, blocking blood flow to part of the lung—is an inevitable consequence of heartworm disease, particularly after treatment with melarsomine. Prednisone reduces the symptoms of pulmonary thromboembolism by controlling inflammation. Prednisone may also be used as pre-treatment prior to the use of macrocyclic lactones in dogs with high microfilarial counts to reduce the risk of a reaction.

Heartworm Surgery for Dogs

Caval syndrome is an emergency condition in which heartworms suddenly obstruct blood flow through the tricuspid valve of the heart. This requires surgery to remove the worms and restore proper blood flow through the heart. If the worms are not removed promptly—usually within two days of the onset of symptoms—the dog will die. Symptoms of caval syndrome in dogs include a sudden onset of severe lethargy, abnormal breathing, pale gums, weakness, and bloody or brown-colored urine. If your dog experiences these symptoms, see your veterinarian immediately.  

Even with surgery, the prognosis for caval syndrome is guarded. For some dogs, surgery is successful at removing the mass of worms blocking the heart and the dog becomes clinically normal in about 24 hours. However, in some cases, surgery is unsuccessful and not enough worms are able to be removed to relieve the obstruction. In other cases, damage to organs is irreversible and the patient does not recover even though surgery was successful at relieving the obstruction. The pet owner should be aware that the prognosis following surgery is variable and the recovery period is a critical time for their dog.

While surgery can remove the mass of worms that is blocking blood flow through the heart, it cannot remove worms from the pulmonary arteries. Your dog will need to recover from surgery for several weeks and will then need to undergo heartworm treatment with adulticide therapy (melarsomine) to kill the remaining worms.  

Cost of Treating Heartworms in Dogs

The American Heartworm Society estimates that the average cost for treatment of heartworm disease for a 40-pound dog is $1,200-$1,800 (7). Compare that to heartworm prevention, which costs on average $70-$200 for a year’s supply, and you can see that it is much cheaper (and safer!) to prevent heartworm disease than it is to treat it.

This cost estimate does not include the cost of surgery for caval syndrome, which is an emergency procedure and costs $4,000-$6,000 in addition to the costs to treat heartworm disease.

Heartworm Prevention for Dogs

Fortunately, heartworm disease can be easily prevented with medication prescribed by your veterinarian. Heartworm preventative medications come in a variety of forms, from injectable medication your vet administers every 6-12 months to topical treatments like Revolution that you apply once a month at home. Then there are combination products, like Simparica Trio, a convenient monthly chewable that prevents heartworm disease and protects against other parasites, including fleas and ticks.

Note that consistency is key when it comes to administering monthly heartworm preventatives at home to ensure your dog is protected year-round. Also, most heartworm preventatives require a negative heartworm test before starting, and yearly thereafter.

Be sure to consult with your veterinarian to determine the best heartworm preventative for your pet’s needs, your budget, and your personal preferences, too.

Related Conditions 

• Congestive Heart Failure
• Caval Syndrome

References

1. Self, S.W., Pulaski, C.N., McMahan, C.S. et al. Regional and local temporal trends in the prevalence of canine heartworm infection in the contiguous United States: 2012–2018. Parasites Vectors 12, 380 (2019) doi:10.1186/s13071-019-3633-2
2. Heartworm. Companion Animal Parasite Council. Retrieved from https://capcvet.org/guidelines/heartworm/
3. Heartworm in Dogs. American Heartworm Society. Retrieved from https://www.heartwormsociety.org/heartworms-in-dogs
4. Keep the Worms Out of Your Pet’s Heart! The Facts About Heartworm Disease. U.S. Food and Drug Administration.  Retrieved from https://www.fda.gov/animal-veterinary/animal-health-literacy/keep-worms-out-your-pets-heart-facts-about-heartworm-disease
5. American Heartworm Society. (2018). Current Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs [PDF file]. Retrieved from https://www.heartwormsociety.org/images/pdf/2018-AHS-Canine-Guidelines.pdf
6. Heartworm Positive Dogs. American Heartworm Society. Retrieved from https://www.heartwormsociety.org/heartworm-positive-dogs
7. American Heartworm Society. Weigh the Costs: Heartworm Treatment vs. Heartworm Prevention [PDF file]. Retrieved from https://d3ft8sckhnqim2.cloudfront.net/images/infographics/0010-weigh-the-costs.jpg