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Signs of Heartworm

How to cHeck for Heartworm

At Gray Street vet clinic we can diagnose heartworm through a simple blood test. X-rays and ultrasounds may also be required.


Heartworm disease is caused by an infection with a parasite called Dirofilaria immitis which is spread by mosquitoes. The adult worms live in the right side of the heart and pulmonary blood vessels. Due to their location, heartworms can cause significant congestive heart disease. The severity of the disease is directly related to the number of worms present, duration of infection and the dog’s immune system response.

Clinical signs:

Initially no signs of heartworm are seen since the disease has a slow insidious onset. Once signs are noticed they are usually clinical signs of heart disease. This is because worms interfere with the normal movement of heart valves causing turbulence. They also cause an inflammatory reaction within the blood vessels themselves. This means the heart has to work harder to pump blood and as a result the heart enlarges, becomes exhausted and less effective at pumping blood to the rest of the body.

Early signs of heartworm disease are lethargy, weakness, loss of stamina, shortness of breath and a dry nagging cough.

As the disease progresses, the dog will lose weight, breathing becomes very difficult, they may develop a potbellied appearance due to fluid accumulation within the abdomen and are likely to stop eating. If left untreated, heartworm will nearly always kill the infected dog.


A simple 10 minute blood test can determine whether a dog is infected with heartworm. However, it only detects the microfilaria (baby heartworm) produced by adult heartworm. Therefore it can take between 6-8 months following infection for a test to be positive. It may be necessary to repeat the test in 6 months to determine whether a dog is truly infected.


For dogs who do test positive for heartworm, the treatment regime varies and can be complex. It may extend from 6 weeks to 3 months, may involve drug therapy, hospitalization and sometimes even surgery. The treatment is not without risk and is expensive. There can also be irreversible heart, lung, kidney and liver damage. Therefore, PREVENTION IS BETTER THAN CURE!


Prevention is simple and there are a wide range of products to choose from. Choices include once a month tablets which not only treat heartworm, but also intestinal worms and fleas. Examples of these products are Panoramis,  Advocate and Interceptor. These are highly effective and have a high safety margin meaning they are still safe to give if you forget to give the tablet on the due date.

By far the most convenient product is a yearly heartworm injection called Proheart SR12. This is administered by a veterinarian and takes away the worry of having to remember to give a tablet every month and you even get a reminder in the mail when your dog is due the following year. This product only prevents heartworm so it is still important that you give your dog an intestinal wormer product regularly. (Check out our puppy information for worming frequencies).



Cat can also get heartworm, however it is seen far less commonly compared to dogs. Treatment for heartworm in cats is more difficult and involves surgical extraction of the heartworms as no treatment is registered for use in cats. There are several products that can be used for prevention of heart worm in our feline friends. These include Milbemax monthly tablets, Revolution and Advocate monthly spot ons.

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