Heartworm is a parasite spread by mosquitoes, not from dog to dog. Larvae passed in a mosquito bite mature into worms that live in the heart and the vessels of the lungs. Risk in British Columbia is low and regionally variable, but travel to endemic areas changes the picture entirely, and prevention is far simpler than treatment.
Why an Okanagan clinic talks about this at all
Heartworm is often filed as somebody else’s problem, something that concerns Ontario or the southern United States. That framing is broadly right for coastal British Columbia and it is not a reason to stop thinking about it here.
Two things keep it on the agenda in the Okanagan. First, this is a valley with hot summers, irrigation, orchards, wetlands and lakes, which is to say a landscape with mosquitoes. Second, and more importantly, dogs move. Kelowna households travel, and dogs come here from rescues and breeders across the country and across the border, arriving from places where heartworm is genuinely common.
Where local risk in BC has been discussed at all, it is the southern Interior rather than the coast that comes up. That is a reason to have an informed conversation with your veterinary team about your individual dog, not a reason for alarm. What that conversation should not be is an assumption in either direction.
How the parasite actually works
An infected dog carries microscopic larvae in its bloodstream. A mosquito feeding on that dog takes up the larvae, which develop inside the mosquito over a couple of weeks, and are then deposited on the skin of the next dog it bites, entering through the bite wound.
From there the larvae migrate through the tissues for months before reaching the heart and the pulmonary arteries, where they mature into adult worms up to about thirty centimetres long. The whole process from bite to detectable adult worms takes roughly six months, which is a crucial detail for testing.
Adults damage the vessel walls and obstruct blood flow, and the immune response to them causes further inflammation in the lungs. That is why the early signs are respiratory rather than obviously cardiac.
What an affected dog looks like
- A persistent soft cough, often the first thing owners notice
- Tiring easily, or being reluctant on walks that used to be routine
- Reduced appetite and gradual weight loss
- Faster breathing, or breathing that looks like more effort
- A swollen abdomen in advanced disease
- Fainting or collapse in severe cases
The important point is that a dog can carry adult worms for a long time with no visible signs at all, and that early infection produces nothing. Waiting for symptoms is not a screening strategy. That is exactly what the annual blood test is for, and it runs through our diagnostic services.
Cats are a different story entirely
Cats are not the parasite’s preferred host, so most worms die before maturing. That sounds like good news and largely is, but it comes with a sting: the death of even a small number of immature worms in the lungs provokes a severe inflammatory reaction.
The result is a respiratory disease that closely resembles feline asthma, with coughing, wheezing and increased breathing effort, and it is frequently misattributed. In some cats the first sign is sudden collapse or death without warning.
Two consequences follow. Testing in cats is less straightforward than in dogs and usually needs more than one type of test alongside imaging. And there is no approved treatment to kill adult worms in cats, so management is supportive only. For cats, prevention is not the easier option, it is the only option, and indoor cats are not exempt because mosquitoes come indoors.
Testing before prevention, and why the order matters
A dog should be tested before starting or restarting preventive medicine, and this is not a formality. Giving a preventive to a dog who already carries adult worms and circulating larvae can cause a serious reaction as large numbers of larvae die at once.
Timing matters too. Because it takes roughly six months from infection to a detectable result, a dog tested too soon after possible exposure can test negative while infected. Your veterinary team will advise when to test based on when the potential exposure occurred, and may recommend retesting later.
Annual testing is recommended for dogs on prevention as well, because doses get missed, medicine is sometimes vomited back up without anyone noticing, and no product is absolute.
Travel is the conversation that matters most
This is where an Okanagan dog’s real risk usually lies. Taking your dog to Ontario, Quebec, the Prairies in summer, or almost anywhere in the United States, particularly the south, moves them into a genuinely endemic area.
Prevention needs to start before you travel rather than on arrival, and it needs to continue for a defined period after you return, because larvae acquired on the last day of a trip are still developing when you get home. Those intervals are specific, and they are worth getting right rather than approximating. Plan this well ahead as part of the wider trip preparation covered by our pet travel services.
The same applies in reverse for adopted dogs. A rescue arriving from an endemic province or from overseas should be tested, and retested at the appropriate interval, regardless of what paperwork accompanied them. This is one of the more common ways an infection turns up in a low-risk region.
Prevention in practice
Preventive medicines work by killing the immature larvae acquired during the preceding period, which is why they are given on schedule rather than when convenient, and why a missed dose matters more than it feels like it should. Several formats exist and many also cover other parasites, so the right choice depends on your dog’s lifestyle, travel and what else you need covered. That is a discussion for a medical consultation rather than a shelf.
Reducing mosquito exposure helps at the margins: emptying standing water around the property, avoiding dusk and dawn walks in the worst of mosquito season, and keeping screens intact. It is a useful supplement and not a substitute.
Whether your dog needs year-round or seasonal prevention in this region is genuinely an individual decision based on travel, lifestyle and current guidance, and it is the sort of thing worth reviewing annually rather than setting once. That review fits naturally into our wellness services, alongside the vaccination and parasite planning covered in our post on pet vaccinations in Kelowna.
Because the outdoor season here brings several risks at once, our guides to trail safety in Kelowna and outdoor safety on Kelowna trails and lakes are useful companion reads.
Frequently asked questions
Is heartworm a risk in Kelowna?
Risk in British Columbia is low and varies by region, with the southern Interior rather than the coast being where local risk has been raised. For most Okanagan dogs the larger exposure comes from travel to endemic areas or from being adopted out of one. That makes it an individual conversation with your veterinary team rather than a blanket yes or no.
Why does my dog need a test before starting prevention?
Because giving a preventive to a dog already carrying adult worms and circulating larvae can trigger a serious reaction when large numbers of larvae die at once. Testing first rules that out. Timing matters as well, since it takes roughly six months from infection for a test to become reliably positive, so your team may advise retesting later.
Can my cat get heartworm?
Yes, and it behaves differently. Cats are not the preferred host so most worms die before maturing, but that die-off provokes severe lung inflammation that closely resembles feline asthma. Testing is more complex and there is no approved treatment to kill adult worms in cats, so prevention is the only real option. Indoor cats are not exempt, since mosquitoes come indoors.
Do I need to give prevention year round?
It depends on your dog and this region, and it is worth reviewing annually rather than deciding once. Travel, lifestyle and current guidance all feed into it. What is not negotiable is consistency during whatever period is chosen, because these medicines work retrospectively on larvae acquired in the preceding weeks and a missed dose leaves a genuine gap.
We are taking our dog to Ontario. What should we do?
Speak to your veterinary team well before you go. Prevention needs to begin before travel and continue for a defined period after you return, because larvae picked up on the last day of a trip are still developing when you get home. Testing before and at an appropriate interval afterward is usually recommended too.
Can heartworm spread from my dog to my other pets?
Not directly. It requires a mosquito to take up larvae from an infected animal, incubate them for a couple of weeks, and then bite another animal. Dogs cannot pass it to each other, or to people, by contact. That does mean an untreated infected dog in a mosquito-rich area acts as a source for the local mosquito population.