Heartworm Disease in Dogs by Beverly Riley, RVT

Heartworm disease is caused by a blood-borne parasite known as Dirofilaria immitis. It is a very serious and potentially fatal disease, however heartworm disease in dogs is almost 100% preventable with medication!

How prevalent is heartworm in Ontario?
Dr. Slocombe from the Ontario Veterinary College did a study in 2010, which found that 431 dogs were diagnosed with heartworm disease in Ontario. Over 51% of these dogs had never left Ontario.
The major focus of Heartworm endemic in Canada is Southwest Ontario (defined as Toronto west to Guelph, south to Simcoe, east to Fort Erie and north to Toronto). The prevalence of heartworm disease is as high as 5-10% of dogs, in these endemic areas of Ontario.

How can it be prevented?
There are many different choices for heartworm prevention including oral tablets and topical options. Your dog should be tested annually for heartworm disease (through a blood test) and then a discussion with your Veterinary team can help you decide on the best prevention option for your dog. Many prevention medications also come with other parasite control, from internal to external parasites.

How is Heartworm diagnosed?
For the most part heartworm disease is diagnosed through one or more simple blood tests.
Your dog may have no symptoms and test positive for heartworm during annual testing. There are also cases where your dog may be demonstrating symptoms, and then through diagnostic testing are found to have heartworm disease

What happens if my dog is diagnosed with Heartworm Disease?
Once your dog is diagnosed with Heartworm disease a treatment is needed. The treatment consists of a medication that is given by injection deep in the muscles of the back. The treatment protocol will depend on the severity of the infection. In less severe cases the dog may be treated with 4 months of preventative medication to kill any migrating heartworm larvae and to decrease the size of the female worms, followed by an injection to kill the adult heartworms. Then five weeks later, your dog would be treated with two more injections of the adulticide. Retesting through an antigen test should be done 4 months after treatment to make sure they have been fully treated. It may be necessary to have another round of treatment. If the disease is more severe than it may be necessary to use the injection that kills the adult worm before 4 months of preventative medication.
Dogs with severe heartworm disease may require further treatments such as, antibiotics, pain relief, special diets, diuretics to remove fluid from the lungs, or medication to improve the heart function prior to their heartworm treatment.