Heartworm Disease

What causes heartworm disease?

Heartworm disease (dirofilariasis) is a serious and potentially fatal disease in dogs.  It is caused by a worm called Dirofilaria immitis.

How do dogs get infected with heartworms?

The disease is not spread directly from dog to dog.  An intermediate host, the mosquito, is required for transmission.  Spread of the disease therefore coincides with the mosquito season, which in Orange County is year-round.

As many as 30 species of mosquitoes can transmit heartworms.  The female mosquito bites the infected dog and ingests the microfilariae (immature worm) during a blood meal.  The microfilariae develop further for 10 to 30 days in the mosquito and then enter the mouth parts of the mosquito.  The mosquito then bites the dog where the hair coat is thinnest, thereby transmitting the heartworm.  (Note- Having long hair does not prevent a dog from getting heartworms.)

When fully developed, the infective larvae enter the bloodstream and move to the heart and adjacent vessels, where they grow to maturity in two to three months and start reproducing, thereby completing the full life cycle.  One dog may develop as many as 300 worms.

Where are heartworms found?

Canine heartworm disease occurs all over the world.  In the United States, it was once limited to the south and southeast regions.  However, the disease is spreading and is now found in most regions of the United States and Canada, particularly where mosquitoes are prevalent.   

What do heartworms do to the dog?

Adult Worms: Adult worms cause disease by clogging the heart and major blood vessels leading from 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 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, chronic 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.

Severely infected dogs may die suddenly during exercise or excitement.

Microfilariae (young worms):  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.

How is heartworm infection diagnosed?

In most cases, diagnosis of heartworm disease can be made by a blood test that can be run in the veterinary hospital or by a veterinary laboratory.  Further diagnostic procedures are essential, in advanced cases particularly, to determine if the dog can tolerate heartworm treatment.  Depending on the case, we will recommend some or all of the following procedures before treatment is started.

Radiographs (X-rays):  A radiograph of a dog with heartworms will usually show heart enlargement and swelling of the large artery leading to the lungs from the heart.  These signs are considered presumptive evidence of heartworm disease.  Radiographs may also reveal the condition of the heart, lungs, and vessels.  This information allows us to predict an increased possibility of complications related to treatment.

Echocardiography (Sonogram):  An echocardiogram allows us to see into the heart chambers and even visualize the heartworms themselves.  Although somewhat expensive, this procedure can diagnose heartworms when other tests fail. 

How is heartworm treated? 

Treatment for heartworm disease is complicated, risky for the pet, and very expensive for the client.  Therefore, prevention is the key.

How can you prevent heartworm disease? 

The best way to prevent heartworms in dogs is by administering Trifexis, a chewable tablet that must be given monthly.  A heartworm test is recommended once a year.

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