Canine Heartworm Disease – Is Your Dog Protected?


April is Heartworm Awareness Month – but it’s not only during spring and summer that we need to worry about heartworm infection in our canine companions.  Any dog can be infected with heartworms, no matter how much or how little time they may spend outside.  The picture above is of a actual dog’s heart infected with large numbers of heartworms.

What are heartworms and what is Canine Heartworm Disease?
Heartworms are a type of parasite found primarily in domestic dogs as well as wild canids, such as foxes and coyotes. They are long, thin worms that reside mainly near the heart chambers and in the large blood vessels around the heart and lungs. They can make dogs sick by causing damage and inflammation to these organs and can be fatal if left untreated. Signs of heartworm disease in a dog include coughing, labored breathing and lethargy.

How do dogs get heartworms?
Heartworms are transmitted between dogs by mosquitoes. A mosquito bites an already infected dog, picks up the larval (immature) form of the heartworm, and then transmits them to the next dog it bites. If the dog is not on preventative medication, the larvae go through a series of developmental stages in the dog’s body and within 5 to 7 months, become adult heartworms. The adults produce their own larvae and the cycle repeats itself.

My vet’s office often asks me to bring in a stool sample from my dog to check for “worms” – does that include heartworm?
Stool samples are used to check for intestinal parasites – meaning those that infect the gastrointestinal system, such as roundworms and hookworms. Heartworms need to be tested for by use of a simple blood test that often can be performed right in your veterinarian’s office.

Heartworm testing and prevention seems awfully expensive – is it really necessary?
Only if you don’t want your dog to get heartworms! All dogs are at risk, regardless of how much time they spend outside or how many mosquitoes you think are in your yard. Heartworm disease is fatal if not treated but the cost of treatment if your dog has heartworm can equal 10 YEARS worth of preventative medication! It also takes several months to completely rid a dog of a heartworm infection and they must be strictly confined during this time to minimize the risk of complications, including deadly blood clots. Depending on the size of your dog, preventative medication may only cost you between $7 and $12 per month – a lot less than what you will spend on dog food!

What kind of preventative medication is there and where can I get it?
The most widely used heartworm preventatives are in the form of a chewable tablet that you give to your dog by mouth once a month. Most of these will help protect your dog against intestinal parasites as well, and some contain additional medication to help control fleas. There are also topical spot-on products that are also used monthly. It is important not to get these products confused with other topical spot-ons that only treat for fleas and ticks. It is also imperative that the preventative medication be given every 30 days, to kill any larval heartworms that your dog may have been infected with, at the life stage when they will be killed by the medication.  All heartworm preventatives can be obtained at your vet’s office and they can help you decide what is the best product for your pet. A blood test to check for adult heartworms is needed once a year to make sure the medication is working properly. If your dog has not been on preventative, or has been in the past but not currently, the blood test will need to be done first. Even though it only seem like a priority during the warmer months, heartworm prevention is now recommended all year round. For pet owners that travel down south over the winter with their pets, it is absolutely essential!

Can’t I save money by just skipping the test and getting the medication from another pharmacy or on-line?
Skipping the heartworm test is not recommended. The blood test is to check for adult heartworms and without testing, an adult heartworm infection could be missed – which needs to treated in a much different manner than giving the monthly preventative. It may seem like a cost savings to get the medication from a source other than your vet’s office but be warned – the companies that make and produce these medications only guarantee their products if they are purchased directly from a veterinarian, meaning they will not cover the cost of treatment if your dog happens to be diagnosed with heartworm. Even with a written prescription, you cannot be assured you are getting the real thing, or that it has been stored or handled properly and will still be effective. If you find a website that allows you to purchase heartworm preventative without a prescription, that is a huge red flag! Veterinarians generally try to keep their prices competitive with what can be found on-line and from human pharmacies. In addition, manufacturers often provide rebates and other incentives for products purchased directly from the veterinarian, saving you more money in the long run.

My vet told me cats can get heartworms too – is this true?
Indeed it is! Cats that spend time outside are more at risk, but even indoor cats can be exposed if mosquitoes make their way into the house. Heartworms in cats can cause respiratory symptoms and breathing difficulties similar to asthma and in some cases, even sudden death. Because there is no approved treatment for heartworm infection in cats, this makes prevention even more imperative. There are heartworm preventatives formulated just for cats, usually as topical spot-ons that are applied monthly and can help protect cats against fleas and other parasites as well. Again, your family veterinarian is your best source for information as to what is best for your pet and to answer any questions that you may have! We are here to help you keep your pets happy and healthy, and help them live as long as possible!

For more information on heartworm disease in pets, contact your veterinarian, or visit the website of the American Heartworm Society,, or the website of the Companion Animal Parasite Council,