Heartworm in Atlantic Canada: What you need to know

What Causes Heartworm Disease?

Heartworm disease is caused by a blood borne parasite, dirofilaria immitis, which is transmitted through the bite of an infected mosquito. A mosquito first feeds on the blood of an infected cat or dog and can then pass on ‘baby’ heartworms into the blood of their next meal. These babies eventually mature into adult worms found in the right side of the heart and major vessels stemming from the heart.

Where Is Heartworm Disease Found?

Though highly prevalent in the United States, in Canada the more common areas of occurrence seem to be isolated to areas of Ontario surrounding the Great Lakes, southern Quebec, and southern Manitoba. 2010 studies showed 3 confirmed cases in Atlantic Canada, though with climate change we expect this disease to continue spreading.


How Does It Cause Disease In Infected Cats And Dogs?

Adult heartworms: cause disease by clogging the heart and major vessels, causing similar signs as heart disease and can eventually cause congestive heart failure. Furthermore, by obstructing blood flow, these worms can cause damage to major organs such as the lungs, liver, and kidneys.

‘Baby’ heartworms (microfilaria): circulate in the blood through smaller vessels and throughout their travels can cause damage to the liver, lungs, and sometimes even kidneys.


  • Soft, dry cough
  • Shortness of breath
  • Weakness
  • Listlessness
  • Exercise intolerance
  • Fainting
  • Fluid build up in the abdomen or limbs
  • Signs of liver failure such as jaundice
  • Sudden death

How Is It Diagnosed?

In this area, generally diagnosis begins with a complete physical examination performed by your veterinary and a thorough history. Your vet will be suspicious of heartworm disease if your pet has any of the above clinical signs or symptoms and if they had any exposure to regions that are at risk, or if there have been any visiting pets from these areas. It can take up to 6-7 months for a ‘baby’ heartworm to mature into an adult and subsequently cause clinical disease, so exposure does not need to be recent.

In most cases, a simple blood test will be enough to determine if the parasite is present. Further workup (ie full blood work, X-rays, and heart ultrasound) may be necessary to determine the extent of disease and how to proceed with treatment.

Is There A Treatment?

If diagnosed and treated early, the adult and baby heartworms can be killed in 2 stages and the dog return to normal health. Killing the adults, however, does carry the risk of pieces of decomposing worms breaking up and causing strokes or embolisms, thus absolute cage rest for at least 30 days after treatment is a necessary precaution.

If there is already substantial damage to the heart, vessels, or other organs, your pet may need to be treated for these conditions lifelong.

In cats, there are no treatments approved, though some have tried using the dog products (which carry significant adverse effects).

Even though this disease only rarely occurs in this region we should still be vigilant and protect our pets against this serious and potentially fatal disease. There are effective and inexpensive monthly heartworm preventatives available at your veterinarian and should be administered once monthly, spring, summer and fall. Current Canadian heartworm protocol is to continue treatment until the end of November. Just ask your vet or clinic staff which product would be most suitable for your pet.