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Heartworms: Life Cycle & Prevention – Dr. Ingrid Judge

Heartworms are known to occur throughout the United States and though they have been 100 percent preventable for decades they are still a common problem in dogs and cats. Studies of owner compliance indicate that only 55% of dog owners that use veterinary care purchase heartworm preventatives. Moreover, in the gulf coast states following hurricane rescues, dogs sampled in rescue shelters had infection rates greater than 75%. Heartworm disease is an underestimated problem primarily because clinical symptoms are not readily apparent until the disease has advanced.

Heartworm disease is the result of prolonged infection with the parasite Dirofilaria immitus. Dirofilaria immitus affects members of the family Canidae including the domestic dog, wolves, foxes, coyotes, sea lions, ferrets, muskrats, non-domestic cats and domestic cats. Transmission of heartworm infection occurs by over 60 species of mosquitos. Adult heartworms reside in the pulmonary arteries and in heavy infections in the right ventricle of the heart. After mating, the microfilariae (L1) are produced by the mature adult female heartworm and released into the circulation. L1 are ingested by feeding female mosquitos and undergo two molts (L1 to L2 to L3) over an 8-17 day period. This process is temperature dependent and in times of the year when insufficient numbers of days occur in which the ambient temperature is adequate, molting does not occur during the lifetime of the mosquito and transmission does not occur. Larval molts and maturation are also dependent on a symbiotic bacterium Wolbachia pipientis. The successfully molted infective L3 is then transmitted by the feeding mosquito to the original or another host. Another molt, L3 to L4, occurs in the subcutaneous, adipose and skeletal muscular tissues shortly after infection (1-12 days) with a final molt, L4 to L5 (immature adult) occurring 2-3 months after infection. The transition from heartworm infection to heartworm disease occurs when the immature adult L5 heartworm (1-2cm length) enters the vascular system and migrates to the heart and lungs where the final maturation and mating process occurs. Mature adult heartworms are 15-18cm (male) and 25-30cm (female). With optimum conditions this life cycle is completed in 184-210 days. Dogs are generally microfilaremic as early as 6 months (Avg. 7-9 months) post-infection. Adult heartworms can live 5-7 years while the microfilariae can live up to 30 months. The severity of the clinical symptoms associated with heartworm disease varies with the relative number of worms, duration of infection and the host parasite interaction. Click HERE for a great video on the heartworm life cycle.

The most important clinical effect of heartworm disease is not the obstruction of the pulmonary arteries by the living worms but is the damage to the pulmonary arteries produced by the worm induced toxic substances, immunologic responses and physical trauma. Pulmonary vascular lesions begin to develop within days of worm arrival, as early as 3 months post-infection. The pulmonary arteries become enlarged, thick-walled and tortuous with roughened endothelial “lining” surfaces. This may be complicated by arterial obstruction and constriction caused by dead worm thromboemboli. This in turn results in pulmonary hypertension and vascular leakage. The pulmonary arteries are functionally incompetent and hence exercise capacity is diminished. Clinically affected dogs will show weight loss, diminished exercise tolerance, lethargy, poor condition, coughing, respiratory distress and abdominal distension associated with right heart failure. The clinical signs noted will depend on the severity and duration of the heartworm infection with the vast majority of dogs being asymptomatic until advanced stages of the disease.

The diagnosis of heartworm disease relies on routine evaluation prior to the onset of symptoms and in endemic areas should be done yearly. Microfilariae detection is not routinely used to identify infection as the potential for amicrofilaremic infections is 5-67% and false negative results may occur. Immunodiagnostic antigen testing has been the most reliable form of testing used for screening evaluations because of the high sensitivity and specificity of these tests and the ease of performing such tests in the clinical setting. Microfilariae evaluation is usually performed in antigen-positive dogs to determine microfilarial status.

The staging of the severity of heartworm disease relies on radiographic evaluation, laboratory evaluation (CBC, Chemistry Panel), and echocardiography (cardiac ultrasound). Radiographic evaluation assesses the severity of pulmonary tissue changes. Radiographic changes develop relatively early in the course of disease and are present in 85% of cases. The most common radiographic changes include right ventricular enlargement (60%), increased prominence of the main pulmonary artery (70%), increased size and density of the pulmonary arteries (50%) and pulmonary artery tortuosity and blunting (50%). Common laboratory abnormalities include a low grade non-regenerative anemia (10-60% of affected dogs), neutrophilia (20-80% of affected dogs), eosinophilia (85% of affected dogs), basophilia (60% of affected dogs), and thrombocytopenia. With severe heartworm disease especially if heart failure is present, liver enzyme elevations may be noted and occasionally hyperbilirubinemia. Kidney enzyme elevations may be noted if dehydration or heart failure is present but may also be secondary to glomerulonephritis, a severe immune-mediated inflammatory condition of the kidney associated with prolonged and severe heartworm disease. If glomerulonephritis is present, severe hypoalbuminemia may result and complicate the clinical picture. Cardiac ultrasound is sensitive in the detection of right heart enlargement and can be used to demonstrate worms in the pulmonary arteries. See a real picture of a heart with heartworms by clicking to our Pinterest page HERE.

Medical management of heartworm disease relies on the use of macrocyclic lactone (macrolide) antibiotic preventative therapy. There are several macrocyclic lactone antibiotic preventatives in use today including Ivermectin (Heartgard), Milbemycin Oxime (Trifexis, Interceptor), Selamectin (Revolution) and Moxidectin + Imidacloprid (Advantage Multi). These preventatives interrupt larval development (L3 and L4) during the first two months of infection, have a large temporal window of efficacy and are administered monthly. These medications cause few adverse reactions even when given to microfilaremic dogs and allow a grace period, “reach back” or “safety net”, for inadvertent lapses in administration because of the sterilization effect on the adult worms. Recently, studies have also shown adulticidal activity if these products are used continuously for prolonged periods. Most of these products can and should be administered as early as 6-8 weeks of age.

If you have any questions or concerns regarding heartworm disease and prevention please consult with one of the PMC doctors or technical staff members for assistance. Heartworm testing and preventative programs can be initiated in a single visit requiring only a small blood sample and physical examination.