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Aina Meducci 2012


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Canine Heartworm: Dirofilaria immitis

I suddenly remember Dr Bern's post regarding heartworm on his blog few days ago. However I didn't have much time to read and eventually forgot the post. I haven't post anything regarding parasitology lately so here's the chance.


Canine Heartworm

Dirofilaria immitis in the heart of a dog


Dirofilaria immitis is a filarial parasitic nematode that infects primarily dogs and other canid species by bitting from infected mosquitoes. It usually resides in the pulmonary, femoral, and hepatic arteries, as well as other blood vessels including veins and right atrium of live dogs. Heartworms cause serious vascular damages and can be fatal, especially in working dogs with high level of physical activity. It may also infect human (rare case).

D. immitis is a vector borne disease that is transmitted via the mosquito. Aedes, Anopheles, and Culex spp. are the most prominent mosquitos responsible for transmitting this disease.

Anopheles sp (female)

Life cycle of D.immitis

Dirofilaria immitis has an indirect life cycle. The adult parasite sexually reproduces in its vertebrate host, and the offspring are transferred to the intermediate host, which is usually a mosquito or a flea. The larva develops inside the intermediate host and molts twice.When the intermediate host feeds, the larvae enter the new vertebrate host through the wound. The parasite remains dormant in the vertebrate hosts muscle tissue for 85 to 120 days. After this time period, the parasites enter the host's blood stream, where they are carried to the heart.Completion of the life cycle in the heart requires 7 to 9 months.

Clinical signs

Infection with adult worm:

Adult heartworms cause disease by clogging the heart and major blood vessels leading from the heart. They interfere with the valve action in the heart. By clogging the main blood vessels, the blood supply to other organs of the body is reduced, particularly the lungs, liver and kidneys, leading to malfunction of these organs.

Most dogs infected with heartworms do not show any signs of disease for as long as two years. Unfortunately, by the time clinical signs are seen, the disease is well advanced. The signs of heartworm disease depend on the number of adult worms present, the location of the worms, the length of time the worms have been present, and the degree of damage to the heart, lungs, liver, and kidneys from the adult worms and the microfilariae.

The most obvious signs are a soft, dry cough, shortness of breath, weakness, nervousness, listlessness, and loss of stamina. All of these signs are most noticeable following exercise, when some dogs may even faint.Listening to the chest with a stethoscope will often reveal abnormal lung and heart sounds.

In advanced cases, congestive heart failure may be apparent and the abdomen and legs will swell from fluid accumulation (ascites). There may also be evidence of weight loss, poor condition, and anemia.Severely infected dogs may die suddenly during exercise or excitement.

Infection with microfilariae (young worm):

Microfilariae circulate throughout the body but remain primarily in the small blood vessels. Because they are as wide as the small vessels, they may block blood flow in these vessels. The body cells being supplied by these vessels are deprived of the nutrients and oxygen normally supplied by the blood.

The lungs and liver are primarily affected.Destruction of lung tissue leads to coughing. Cirrhosis of the liver causes jaundice, anemia, and general weakness because this organ is essential in maintaining a healthy animal. The kidneys may also be affected and allow poisons to accumulate in the body.


Death of the host may occur during cardiac hypertrophy due to microfilarial thrombi, or may not occur until severe dilatation and cardiac failure are prominent.Expected necropsy and histopathology related changes may include the following:

1)Cardiac hypertrophy due to obstruction of the right ventricle and pulmonary arteries with adult heartworms,
2)Chronic multifocal granulomatous pneumonitis caused by adult worm and microfilarial thrombi,
3)Bronchiectasis with mucous hyperplasia of the bronchial epithelium,
4)Centrolobular hepatopathy

Globose or rounded heart from dilation of the right atrium in a dog with advanced heartworm disease

Pulmonary hemorrhage (left, arrows) and roughened, fibrotic liver from a dog with advanced heartworm disease.


  • Serological test for antigens to adult worms
  • Blood test for microfilariae
  • Blood count
  • Radiographs (x-ray)
  • ECG- detection for abnormal heart sound


The first adulticide (drug to kill the adult heartworms) for dogs that was developed was thiacetarsamide sodium (Caparsolate), which contained arsenic. It was given in the vein through a catheter. If any drug got outside of the vein, severe tissue damage was possible. Some animals became quite ill from this drug, and therapy sometimes had to be stopped.

There are two options for treatment of heartworm disease. The first is via a series of deep intramuscular injections of melarsomine dihydrochloride into the epaxial lumbar muscles. This currently is the only adulticide agent available. A gradual two stage elimination of the heartworms are recomended. One injection is given initially. In four weeks, two more injections are given within 24 hours of each other. This protocol will kill the adult worms slowly, decreasing the chance of post-adulticide thromboembolic complications. Treatment of the microfilariae should begin immediately after the dog is diagnosed with the disease. Administering the monthly prophylactic dose of Ivermectin will eliminate microfilariae within a couple of weeks.

The second option for treatment of adult heartworms is continuous prophylactic doses of Ivermectin. This process can take up to 2 years for complete elimination of adult worms. It is important to keep in mind that with this method, the adult worms (and any microfilariae) are killed slowly and lung pathology continues.

Despite the method chosen to treat the adult heartworms, it is very important to limit the dog’s activity for 4-6 weeks. Exercise restriction decreases the chance of pulmonary thromboembolic complications as potions of dead heartworms are carried from the heart and pulmonary arteries to the lungs. Six months after treatment is complete, a heartworm antigen test and a microfilariae test should be used to confirm efficacy of the treatment.


Heartworm disease can be easily prevented by periodic medication. All dogs in endemic areas should be on macrolide prophylaxis throughout the year and especially during the region’s vector season when mosquitos are prevalent and active. Puppies should begin monthly treatments for heartworm prophylaxis before 8 weeks of age. Heartworm antigen tests also should be evaluated annually.

Sources: Vet.Clinical pathology clerkship program.Heartworm;peteducation.com

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