Heartworm in Dogs

Heartworm disease is an increasingly prevalent condition in dogs throughout North America. Heartworm disease also affects cats, ferrets, and even humans. Transmitted by mosquitoes, heartworm disease is a serious condition that can be fatal if left untreated. This disease is costly to treat, but relatively inexpensive and easy to prevent.

What is Heartworm in Dogs?

Heartworm disease is a serious, potentially fatal illness caused by the parasite Dirofilaria immitis. Adult heartworms live in the vessels that supply blood to the lungs and can be up to a foot in length. An infected dog can harbor hundreds of heartworms at once and the worms can live approximately 5-7 years in the dog if left untreated. The worms cause damage to the heart and lungs and can also impact the liver and kidneys.

How Do Dogs Get Heartworm?

Heartworm disease is transmitted by mosquitoes. In a dog infected with heartworms, male and female heartworms mate, producing offspring called microfilariae. Microfilariae reside in the blood of most, but not all infected dogs. When a mosquito bites an infected dog, it ingests some microfilariae. While inside the mosquito, the microfilariae then develop into infective larvae. These infective larvae are then deposited into the skin of a new host when the mosquito bites again.

Once inside the new host, the larvae migrate through the tissues to the heart and pulmonary arteries, where they finish maturing into adults. The adults mate and produce microfilariae, which appear in the blood as early as 6 months post-infection.

Dogs and some wild canids are the natural hosts for heartworms, but other animals such as cats, ferrets, and even humans can become infected. Heartworm disease is not contagious from dog to dog; it must be transmitted by a mosquito.

Symptoms of Heartworm in Dogs

Many dogs with heartworm disease have no symptoms, particularly early in the course of infection. The presence of symptoms will depend on several factors, including the number of worms present, the size of the patient, the degree of inflammatory response the body mounts to the worms, the duration of the infection, and the dog’s activity level. The longer the infection persists, the more likely it is that damage will be done to the heart and lungs and symptoms will develop.

Symptoms of heartworm disease in dogs include:

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

As the infection progresses and the number of worms in the heart and pulmonary arteries increases, the dog may develop right-sided congestive heart failure. Symptoms of congestive heart failure in dogs include:

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

Dogs with high numbers of worms can develop a severe life-threatening condition called caval syndrome. This condition occurs when worms block blood flow through the heart and interfere with the function of the tricuspid valve of the heart. Caval syndrome requires emergency surgery to remove the heartworms and restore proper blood flow through the heart. Symptoms of caval syndrome in dogs include:

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

If you suspect your dog has symptoms of heartworm disease, see your veterinarian right away. Because these symptoms can be present with several types of cardiac disease, your veterinarian may recommend several diagnostic tests to evaluate your dog’s heart and lungs.

Diagnosing Heartworm Disease in Dogs

Heartworm disease may be diagnosed in your dog using some or all of the following diagnostic tests:

• Physical examination. Your veterinarian will perform a full head-to-tail physical examination on your dog, including listening to your dog’s heart and lungs. Your veterinarian will also ask you questions about your dog’s history, including your dog’s symptoms, medication history, and travel history, to determine your dog’s risk for exposure to heartworm disease.
• Heartworm Antigen Test. A heartworm antigen test detects a protein secreted by female heartworms as early as 5-7 months post-infection. A sample of your dog’s blood will be needed to run this test. The American Heartworm Society recommends that this test be performed annually on all dogs over 7 months of age to screen for heartworm disease. This test may also be performed if your veterinarian suspects that your dog may have heartworm disease.
• Microfilaria Test. In this test, a drop of your dog’s blood is examined under a microscope to detect microfilariae, the heartworm offspring. Microfilariae can be detected in the blood as early as 6 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. This test may also be performed if your veterinarian suspects that your dog may have heartworm disease.
• Radiographs. Your veterinarian may recommend radiographs (x-rays) 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. Radiographs are also recommended prior to starting heartworm disease 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 that can occur secondary to heartworm infection, such as anemia, elevated liver values, and azotemia. Baseline blood work is also recommended prior to starting treatment for heartworm disease.
• Echocardiography. An ultrasound of the heart may be recommended to evaluate the extent of heartworm disease and to confirm a positive antigen test result. Dogs with mild heartworm disease may have normal results on echocardiography, while those with more severe disease may have more severe changes. In some cases, worms may be visualized in the heart and pulmonary arteries on echocardiography.

Treatment for Heartworm in Dogs

Even though some dogs have no symptoms of heartworm disease, treatment is still imperative. Heartworms do extensive damage to the heart, lungs, liver, and kidneys, and delays in treatment will result in increasing worm burdens and worsening of damage to these organs. Treatment for heartworm disease involves strict exercise restriction, medication, and in severe cases, surgery to address correct caval syndrome.

Exercise Restriction for Heartworm Disease

As soon as your dog is diagnosed with heartworm disease and throughout treatment, it is essential that you decrease your dog’s activity level. Strict exercise restriction is necessary because physical exertion increases the damage done to the heart and lungs by heartworms. According to the American Heartworm Society, there is a distinct correlation between the severity of the infection and the activity level of the dog. Activity restriction is therefore critical to prevent complications associated with heartworm infection. Infected dogs should be taken on controlled leash walks to relieve themselves only and should not be allowed to run freely. Cage rest may be necessary for energetic dogs or those that are severely infected.

Heartworm Medicine for Dogs

The goal of heartworm treatment is to eliminate all stages of heartworms in the dog and improve the dog’s symptoms. The American Heartworm Society recommends a heartworm treatment protocol using the following medications:

• Doxycycline. Heartworms contain bacteria called Wolbachia. These bacteria help the heartworm mature, reproduce and survive. By killing these bacteria, heartworms are weakened and gradually die. Doxycycline is an antibiotic administered to dogs with heartworm disease to kill these Wolbachia bacteria and weaken heartworms, making them more susceptible to adulticide treatments. According to the American Heartworm Society protocol, doxycycline is administered on Days 1-28 of treatment.
• Macrocyclic Lactones. Macrocyclic lactones are a type of anti-parasitic drug. They are used in the treatment of heartworm disease to kill heartworm microfilaria and developing heartworm larvae. Marcocylic lactones are also administered to prevent new heartworm infections, which can occur at any time. The American Heartworm Society recommends administering a macrocyclic lactone on Day 1 of treatment and every 30 days thereafter.
• Glucocorticoids. Steroids such as prednisone may be administered to a heartworm disease patient to reduce the symptoms associated with pulmonary thromboembolism (PTE), which is an inevitable consequence of adulticide therapy. If the patient has microfilariae present in the blood stream, glucocorticoids are also recommended by the American Heartworm Society prior to the first dose of a macrocyclic lactone to prevent an anaphylactic reaction.
• Antihistamines. Antihistamines such as diphenhydramine may be administered prior to the first dose of a macrocyclic lactone in a patient with microfilariae present in the blood. This reduces the risk of an anaphylactic reaction.
• Melarsomine. Melarsomine is the adulticidal drug used to kill adult heartworms. It is the only adulticidal drug approved by the FDA. It is administered by intramuscular injection on Days 61, 90, and 91 of treatment. Exercise restriction following adulticidal treatment is critical to reduce the risk of complications.

Heartworm Surgery for Dogs

Caval syndrome is a severe and life-threatening condition in which adult heartworms block blood flow through the heart at the level of the tricuspid valve. This is an emergency condition and requires surgery to remove the mass of worms and restore blood flow through the heart. If the worms are not removed promptly – usually within 2 days of the onset of symptoms – the dog will die.

Even with surgery, the prognosis for dogs with caval syndrome is guarded. For some dogs, surgery to remove the worms is successful, proper blood flow is restored, and the dog improves within 24 hours following the procedure. However, in some cases, surgery is unsuccessful and not enough worms can be removed to relieve the obstruction. In other cases, damage to the organs due to lack of adequate blood flow is irreversible and the dog does not recover despite a successful surgery. The pet owner must be aware that the prognosis for caval syndrome is variable and the recovery period is a critical time for their dog.

Surgery also cannot remove the worms that live in the dog’s pulmonary arteries. Therefore, the dog must still undergo adulticide therapy to kill the remaining worms. This is typically done several weeks after the surgical procedure, once the dog has had time to fully recover from the procedure.

Follow-up Testing for Heartworm in Dogs

After your dog has completed treatment according to the American Heartworm Society protocol, it is important that you continue strict exercise restriction for 6-8 weeks. Do not allow your dog to run, jump, or get excited during this time. While this may be difficult for both you and your dog, exercise restriction is critical to prevent complications and ensure your dog’s complete recovery.

Following treatment, your dog will receive a microfilaria test on Day 120. If positive, your dog will be treated with a microfilaricide and retested in 4 weeks.

9 months after the last melarsomine injection, your dog will receive an antigen test and a microfilaria screening. If negative, it’s time to celebrate! Your dog is officially cleared of heartworm disease!

Heartworm Prevention for Dogs

According to the Companion Animal Parasite Council, heartworm disease infections continue to increase in number and geographic distribution. Fortunately, heartworm disease can be easily prevented with medication available from your veterinarian. Heartworm prevention medications come in the form of oral tablets, topical products, or injections administered by your veterinarian. Your veterinarian can help you determine which product is best for your pet’s needs.

Even if your pet is maintained on preventive medications year-round, annual antigen and microfilaria testing is still recommended by the American Heartworm Society. This is necessary to identify and address heartworm disease as quickly as possible. For dogs living in areas where heartworm disease is particularly prevalent, testing twice annually may be recommended.

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