Heartworm disease is defined as the presence of heartworms (Dirofilaria immitis) in the pulmonary artery and/or right heart with the presence of clinical signs. These parasites are transmitted from the bite of an infected mosquito and their lifecycle completes within in the canine and feline heart. The consequences of these parasites are inflammation of the pulmonary vasculature and lung parenchyma. Pulmonary arteries become dilated and tortuous and this scenario can lead to decreased oxygenation of the blood within the lungs. Pulmonary hypertension results from the aforementioned changes as the right heart has to generate higher pressures to pump blood through the altered pulmonary vasculature. As a result, the right heart eccentrically hypertrophies in response to an acquired pressure overload. Eventually due to the combined effects of right heart dilation and pulmonary hypertension, right heart failure can occur.
Other complications seen with heartworm disease include chronic immune stimulation which can result in immune-complex glomerulonephritis of the kidneys, vasculitis and polyarthropathies. In addition, the pulmonary parenchyma can undergo detrimental changes as well. Allergic pneumonitis is a pulmonary parenchymal disease that can occur in dogs infected with heartworms. Rarely, a form of non-infectious pneumonia (pulmonary eosinophilic granulomatosis) can occur from excessive infiltration of inflammatory cells into the lung in response to the parasite. Thromboembolic events (most commonly pulmonary thromboembolisms) can also result. Cardiac arrhythmias can also occur as a result of myocardial changes with dilation.
Heartworm disease has four distinct clinical stages and treatment is initiated specifically for the stage diagnosed
Treatment for heartworm disease depends upon the classification aforementioned. In general, all animals that are heartworm positive should be started on an ivermectin-based once monthly oral preventative to help kill of the immature larvae and microfilaria) to halt the lifecycle and prevent a reservoir for disease. An ivermectin-based product should be chosen rather than milbemycin based products in this situation in order to kill the microfilaria slowly rather than all at once which could lead to an anaphylactic shock state.
For class 1 heartworm positive patients, it is recommended to start an ivermectin-based preventative for at least 2-3 months before melarsomine dihydrochloride (Immiticide®) administration. Additionally, doxycyline, an antibiotic is administered for one month after diagnosis for the treatment of Wolbachia, a parasite that lives within the heartworms. The melarsomine is administered as a deep intramuscular injection into the muscles of the lower back. One injection should be administered to kill the most sensitive adult worms and the patient should be strictly cage rested for one month to try to prevent thromboembolic events from the dying worms. The patient should then return in one month after the first injection of melarsomine for two more injections 24 hours apart. Again the patient must be strictly cage-rested for one month. The patient should then have an antigen heartworm test performed six months after treatment and then on an annual basis.
For Class 2 and 3 heartworm positive dogs, the same protocol should be followed as for Class 1 patients with some important alteration in treatment. The oral ivermectin-based preventative should be given for 2-3 months prior to adulticide (melarsomine) treatment. In addition, an antibiotic called Doxycycline and/or steroids may also be prescribed during this time frame. These patients may also have evidence of right-sided heart failure and may require medical treatment such as diuretics, angiotensin inhibitors and pulmonary arterial vasodilators. After 2-3 months of treating the microfilaria and immature larvae, adulticide treatment will be initiated as for Class 1 with heartworm testing performed 6 months after the final treatment with melarsomine.
Class 4 heartworm patients with Caval Syndrome require immediate attention and hospitalization. These animals require heartworm embolectomy, which is surgical removal of the adult heartworms from the right heart. This procedure is performed through intravenous access through the jugular vein and special instruments are placed through this vein into the right heart to remove the adult worms. These patients are unstable and are not the best candidates for anesthesia; however, the worms need to be removed. These patients require intense monitoring and support in an ICU setting post embolectomy and may require blood transfusions, oxygen therapy and medications for right-sided heart failure. If this patient recovers, the same treatment for a Class 3 heartworm positive patient will be necessary.
Cats with heartworm disease should never be administered adulticide treatment as this can cause a severe life-threatening systemic shock response. Cats' clinical signs are controlled by anti-inflammatory doses of steroids and once monthly ivermectin-based preventatives to stop the heartworm life cycle. Adult heartworms are not killed and are left to die on their own. Cats require both antibody and antigen heartworm testing because they often have low burden infections with mostly single sex (female) worms. The antibody test can show exposure and the antigen test can confirm an active infection.
Dogs should be tested every 12 months for heartworm disease, as this is a preventable disease. Once a dog is infected, the recheck examinations will be based upon which class your pet falls into.
ECG recordings from a dog with heartworms can vary based on how severe the disease process is. The ECG can be normal or show signs of right ventricular enlargement such as deep S waves. P waves may also be tall, suggestive of right atrial enlargement.