• Robert Skidmore, DVM

Heartworm Disease in Dogs

Heartworm disease is caused by infestation with the parasite Dirofilaria immitis. Dogs become infected by mosquitoes. The microfilaria (immature form of the heartworm) from the mosquito develop into adult heartworms and reside in the pulmonary artery, right side of the heart and sometimes the venae cavae. The disease is classified based on clinical signs. In Class 1, there are few or no signs of disease. Class 2 disease is where there are moderate clinical signs. Class 3 is where there are severe clinical signs. Class 4 disease is where there is caval syndrome present. Most dogs are Class 1 at the time of diagnosis through routine screening. Heartworm disease has been diagnosed in all 50 states.

Clinical signs/symptoms:

Early: usually no symptoms

Mild: cough

Moderate: Coughing is the most common sign. There is often weight loss and exercise intolerance also. Other symptoms that may occur are collapse, abdominal distention due to right sided heart failure and pulmonary hypertension (which causes enlarged spleen and liver). Dyspnea (respiratory distress) can develop from pulmonary thromboembolism (PTE). This is where a worm or part of a worm lodges in the lung tissue.

Severe (caval syndrome): This is where there are so many worms in the right side of the heart that they interfere with valve function and migrate into the large vein (caudal vena cavae) bringing blood to the heart. This is the most serious form of the disease and dogs will die quickly. Surgical removal of the worms may increase the chance of survival.


Diagnosis is easily made through an in-clinic ELISA test using a few drops of blood. Blood can also be check for microfiliaria. The American Heartworm Society recommends annual testing. The earliest a positive result can occur post infection is around five months. This is why heartworm testing is not needed in puppies less than 5-6 months of age before putting them on a heartworm preventative. Similarly, if a dog has missed a few months of prevention it should be tested around 7 months post possible exposure. Immediate testing is not needed since enough time has not elapsed for the worm to mature to where it can be detected by testing.


There are two main protocols using the drug Immiticide (melarsomine). The traditional protocol involves 2 Immiticide injections 24 hours apart. Heartworm prevention can then be started. The heartworm prevention will take care of the microfiliaria which Immiticide will not kill. Heartgard will kill the microfilia over a few months. Interceptor kills very quickly. This has the potential to cause an allergic reaction to the dying microfilaria. So the veterinarian may want to hospitalize the patient for a day when the Interceptor is given. A steroid may also be given to guard against an allergic reaction. Rescue groups often use this protocol due to the reduced expense.

The preferred protocol with Immiticide involves three injections and is the treatment of choice of the American Heartworm Society. This is the safer protocol and results in a better kill rate (90% versus 98% with alternate protocol) of heartworms. Once heartworms are diagnosed a heartworm preventative (Heartgard is my preference due to the slower and safer microfilarialcidal activity) for three months. This allows immature forms of the heartworm to mature to the adult state so that the drug can kill it. After three months of heartworm preventative one Immiticide injection is given. Thirty days later, two Immiticide injections are given 24 hours apart as in the first protocol. Blood work is checked before the first injection to ensure liver and kidney function is normal. A steroid such as prednisone may be given after injections to help prevent allergic reactions due to the heartworms dying. It is very important that the pet’s exercise be restricted for at least 8 weeks after the injections. If the pet is too active it becomes more likely for the dying heartworms to embolize the lungs. The antibiotic doxycycline may also be given concurrently due to heartworms often harboring a bacteria called Wolbachia. This bacteria may make heartworm disease worse by secreting inflammatory proteins and/or endotoxins. As an added bonus, doxycycline may have an effect on microfiliaria that causes the adult worm that develops to be infertile.

Treatment for heartworms can be expensive and because of this some people cannot afford treatment. In these cases heartworm preventative can be prescribed and over 18-24 months the heartworms may die and they then test negative. This is not ideal treatment because the longer the heartworms are there the more likely they are to cause permanent damage. If this treatment is chosen doxycycline should also be given every three or four months.

The most common side effect of Immiticide treatment is coughing. This often occurs as a result of the pet being too active. Other side effects include injection site reactions (e.g. pain, swelling), depression/lethargy, decreased appetite, respiratory complications and vomiting. There is also a risk of death due to pulmonary emboli (pieces of the heartworms lodging in the lungs).


Heartworm disease is one of those diseases that are best prevented. Treatment can be risky and expensive. Depending on where in the course of disease it’s treated there may be long term effects. Heartworm preventatives are given monthly either orally or topically. Oral products include ivermectin (e.g. Heartgard®) and milbemycin oxime (e.g. Interceptor®, Sentinel®, Trifexis™). Topical products include selamectin (e.g. Revolution®) and moxidectin (e.g. Advantage Multi®). Most of the preventatives do other things as well such as flea prevention and prevention/treatment of intestinal parasites (roundworms, hookworms, etc.). All of them are effective at preventing heartworm disease. Adult dogs should be tested prior to starting a preventative.

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