MONDAY MUSING: Heartworm Prevention


Just the facts

It’s heartworm season!  At this time of year veterinary clinics in Ontario are busy drawing blood samples for heartworm tests and prescribing parasite prevention.  We downloadalso answer lots of questions about heartworm – here are the most frequently asked questions!

How does a dog get heartworm?

Heartworm is transmitted when a mosquito bites an infected animal and takes a blood meal, taking microfilariae (the heartworm larvae) with it.  The larvae undergo two stages of maturation in the mosquito and can then infect another animal when the mosquito takes another meal.  While dogs are the natural host for heartworm, they can be carried by wild canids like coyotes and foxes, and can also infect cats.

The larvae migrate through the tissues and eventually mature in the pulmonary vessels.  This maturation process takes approximately six months.  Adult male heartworms are 15-18 cm in length, while adult females can reach 20-23 cm.  A heartworm can live up to 7 years in their natural host, the dog, and can live to be 3 years in cats.

What are the symptoms of heartworm disease?

 During the initial stages of infection, heartworm disease does not have any clinical signs!  Over time however, the adult heartworms cause damage to the pulmonary vessels and affects the blood flow through the heart and body.  Symptoms at that time can be variable but often include coughing, exercise intolerance, lack of appetite and subsequent weight loss.  Heart failure can develop leading to fluid accumulation in the chest or abdomen.

 Dogs with a heavy burden of worms can experience a life-threatening condition called “caval syndrome” where the worms cause an obstruction of the blood vessel.  Symptoms of caval sydrome include sudden collapse, pale gums and respiratory distress.  There can also be destruction of the red blood cells in the blood vessels (intravascular hemolysis).  Treatment for caval syndrome primarily involves direct removal of worms from the heart and major blood vessels.  Even with therapy, many dogs do not survive.

 Do we have heartworm in Ontario?

 The prevalence of heartworm has been increasing in recent years.  Because an animal can be infected for some time without clinical signs, undiagnosed carriers of the parasite are a source of microfilariae to be picked up by mosquitoes.

 In a study published in 2010 by Dr. J. Slocombe at the Ontario Veterinary College reported 564 diagnoses of heartworm across the province that year, and 51% of the dogs had no travel history outside of Ontario.  This is a significant increase from the previous survey performed in 2002, where 268 heartworm diagnoses were reported in Ontario.

 Of course, in many cases we do travel with our dogs. Most of the United States is now considered endemic for heartworm. and the American Heartworm Society recommends year round heartworm prevention throughout the US.

 How is heartworm diagnosed?

 The first step in the detection of heartworm is the antigen test.  This is the small blood sample that we draw from hundreds of dogs every year to screen for the disease.  The antigen that we test for is produced by an adult female heartworm.  Because we know a heartworm takes about six months to mature, we begin heartworm testing six months after mosquito season ends.

 A positive heartworm test is usually followed by a screen for circulating microfilariae in the blood.  The presence of microfilariae confirms the diagnosis of heartworm.

 Can heartworm be treated?

 Treatment of heartworm disease can be difficult and expensive.  There is only one medication labelled for treating adult heartworms, melarsomine dihydrochloride (Immiticide).  The drug is given by injection deep into the dog’s muscle – usually two or three injections total are required over two months.  After each injection, absolute exercise restriction (strict cage rest) is necessary to reduce the possibility of thromboembolic disease – significant obstruction of blood vessels by dead or dying heartworms.

 Dogs that have more advanced clinical signs at the time of diagnosis may also require hospitalization prior to starting treatment.

 Prevention is much better than treatment as it is easy, safe and effective.  There are multiple products available for the prevention of heartworm, and can be administered topically or orally depending on the preference of the owner and dog!  Many of these products also offer protection against fleas and other parasites.  Ask your veterinarian to make a recommendation for your specific pet.

I’m using the preventative.  Do I still need to test every year?

 Yes!  There are a few reasons to continue testing annually.  First, preventatives are applied by humans, and we humans are not infallible!  We apply doses late, and we miss doses.  A lapse in treatment can allow for infection.  In addition, although the preventatives we use are extremely effective, nothing is 100%, and earlier we diagnose heartworm, the better.   Undiagnosed heartworm positive dogs will increase the prevalence of heartworm as mosquitoes bite and transmit disease.

I’m a cat person.  Can cats get heartworm?

 They can.  While they are not the natural host, heartworm disease can rarely occur in cats.  Unfortunately there is no safe and effective treatment for heartworm in cats.  Prevention is the best option for our feline friends!

 What’s the bottom line?

While the risk of heartworm infection in Ontario is still small, it is present.  Heartworm prevention is easy, effective and inexpensive compared to treatment.  Prompt identification of heartworm positive animals will help limit the prevalence of heartworm in our region by allowing proper medical management to begin immediately.

 I want to learn more – where can I find more information?

 There is excellent information from these sources:

 American Heartworm Society:

Companion Animal Parasite Council:

Dr. Slocombe’s study:


SOURCE: Dr. Erin Marley of Bellbrae Animal Hospital


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