Ask two Canadian owners what parasite prevention their dog is on and when, and you will often get two confident and completely different answers. Both may be right. This is one of the few areas of pet care where the correct answer genuinely changes between Winnipeg and Vancouver, and between May and November in the same city.
What follows is the shape of the problem — which parasites matter where, what actually drives the timing, and the questions that get you a plan for your own postal code rather than a plan copied from an American website.
Heartworm: the one with a date on it
Heartworm is spread by mosquitoes. The larvae a mosquito picks up have to develop inside the mosquito before that mosquito can infect a dog, and that development is temperature-dependent: it stops below roughly 14 °C. Enough warm days have to accumulate before any mosquito in your area is capable of transmitting anything at all.
That is why Canada has a start date rather than a season of vigilance. The convention here has long been to begin prevention on 1 June, and work published in 2025 by the Ontario Veterinary College's Centre for Public Health and Zoonoses looked at whether a warming climate had moved that date. Across 1996 to 2023 it found no significant overall trend toward earlier transmission, though individual locations varied. The conclusion was that the existing approach still holds.
Two things follow, and they are the parts owners get wrong.
Starting on time is not the same as starting early. Preventives work retroactively within a window — they kill the larval stages an animal has already picked up in the preceding weeks rather than repelling mosquitoes. That is precisely why a June start is adequate rather than reckless.
Heartworm is uncommon in Canada, and that is not a reason to skip it. McGill and colleagues, working from clinic surveys running to 2010 and laboratory antigen-test submissions from 2007 to 2016, found prevalence falling nationally through the late twentieth century, with an increase over 2007–2016 in the tested dog populations of Manitoba and Quebec while Ontario continued to fall. That qualifier matters and the authors make it themselves: only dogs that see a veterinarian and get tested appear in the data, so it is not a measurement of every dog in those provinces. The disease is rare and severe, which is the risk profile where prevention is most worth its cost — and the calculation changes entirely if your dog travels. A dog that summers in a region with real transmission, or one adopted from outside the country, is a different animal from a lifelong resident of a cold city.
Ticks: where they actually are
This is where a US-written article will mislead you, because the Canadian map is specific and it is moving.
The Public Health Agency of Canada maps where blacklegged ticks are a known risk, and the map is wider than most people expect. PHAC calls these risk areas rather than established populations, and its current cross-sections cover: Manitoba and western Ontario, wherever there is suitable habitat below the 53rd parallel; southern Ontario along the north shores of Lake Erie and Lake Ontario and the south shore of Lake Huron; southeastern Ontario and southern Quebec, from around Kingston up the St Lawrence toward Ottawa and across Montérégie, Montréal, Laval and large parts of Estrie and Mauricie–Centre-du-Québec; twelve counties in New Brunswick; all of Nova Scotia; and in British Columbia much of Vancouver Island, the facing coast, and river valleys across the south.
Two things PHAC says are worth carrying with that list. Blacklegged ticks are spreading to new areas in Canada, partly because of climate change and partly because birds and deer carry them, so the map moves. And you can sometimes find them outside the areas where they are known to live — which is why “I am not in a risk area” is a reason to ask your veterinarian rather than a reason to skip the conversation.
The important qualifier: established populations in central and eastern Canada have been expanding in both range and number, and climate is the driver. A region that carried no meaningful risk fifteen years ago may carry it now. This is the single strongest argument for asking a local clinic rather than relying on anything you read once and remembered.
Ticks are not a summer problem. They are active whenever it is above freezing, which in much of the country includes long stretches of spring and autumn and some of winter. The mistake is not failing to use prevention in July. It is stopping in September.
If your dog walks in long grass, leaf litter, brush or woodland edges anywhere in the established zones, a tick check after every walk is the habit worth building — run your hands over the whole body, and pay attention to ears, neck, armpits, groin and between the toes.
Fleas: the indoor parasite
Fleas do not care what the weather is doing, because the part of the life cycle that matters is happening in your house. Eggs and larvae persist in carpet, bedding and floor cracks at indoor temperatures all year, which is why a flea problem that appears in February is not mysterious.
The practical consequences: treating one animal in a multi-pet household usually fails, treating the animal without dealing with the environment usually fails, and an indoor-only cat is not automatically exempt — fleas arrive on people, on other pets, and in second-hand furniture.
Cats are not small dogs here
Two points that matter more than owners expect.
Never put a dog product on a cat unless the label says it is for cats. The compound to know about is permethrin. Merck records that some pyrethroids, permethrin among them, can be highly toxic to cats, producing muscle fasciculations and seizures; cats lack an enzyme dogs have for clearing it and cannot metabolise it at the concentrations used in dog formulations.
The part owners miss is that the cat does not have to be the one treated. Reviewing the evidence, Pfister and Armstrong conclude that any contact between cats and permethrin-containing products — including contact with a permethrin-treated dog — must be avoided. If you have both species in one house, say so at the appointment and let the plan be built around it. A cat showing tremors or seizures after any such exposure is an emergency: phone your veterinary clinic or the nearest emergency hospital straight away rather than waiting to see how it goes.
Indoor-only lowers exposure, it does not zero it. The same reasoning as with vaccination: cats get out, other animals come in, households change. What an indoor cat needs is a real conversation rather than an assumption, and it is covered in the kitten vaccination guide from the other direction.
The shape of a year
Written as a shape, not a schedule. Your own clinic's version will differ, and should.
| Time of year | What is usually in play |
|---|---|
| Late winter to spring | Ticks become active as soon as temperatures rise above freezing, often well before anyone is thinking about parasites. Fleas continue indoors regardless. |
| 1 June | The conventional start date for heartworm prevention in Canada, set by how long mosquitoes need above about 14 °C before they can transmit anything. |
| Summer | Peak mosquito activity and continued tick activity. The season most people already cover. |
| Autumn | The commonly missed window. Tick activity continues, and often intensifies, long after the weather has stopped feeling like tick weather. |
| Winter | Fleas persist indoors. Ticks remain possible during thaws in established areas. Whether prevention continues is a regional and clinical decision, not a default. |
What to ask, and why the answer is local
Bring specifics to the appointment. The difference between a useful recommendation and a generic one is entirely in what you tell them.
Worth asking at your clinic
- Are blacklegged ticks established where I live, and has that changed recently?
- Which months should this animal be covered for ticks, and when should I stop — if I should?
- When do you want heartworm prevention started, and does it run to a fixed end date?
- Does my dog need a heartworm test first, and how often after that?
- We have a cat and a dog in the same house — which products are safe with both?
- My animal swims, hikes, goes to a cottage, or travels to another province. Does any of that change the plan?
- Is this animal's vaccination profile affected by where we walk — Lyme, for instance?
- What do I do if I find an attached tick, and do you want to see it?
If you find a tick
Remove it promptly rather than perfectly, and follow the public health guidance rather than a folk method. Use fine-tipped tweezers or a tick remover, grasp the tick as close to the skin as you can, and pull it straight out gently and steadily without twisting, jerking or crushing the body. Clean the bite afterwards. Do not apply heat, a lit match, petroleum jelly, alcohol or anything else meant to make the tick back out on its own — those methods do not work and can make matters worse.
Note the date and where you were. If your animal becomes unwell in the following weeks, that detail turns a vague history into a useful one.
Where this fits with everything else
Parasite prevention is a running cost rather than a one-off, and it scales with body weight because most products are dosed by size — which is one reason a large dog costs multiples of a small one to keep. Both the dog and cat cost guides put it in the recurring column.
It is also one of the questions worth settling at the very first appointment, alongside the vaccination plan — for a new puppy or a new kitten. And if you have not yet chosen a practice, that decision comes first, because the local knowledge is the product you are buying.
The short version
Heartworm has a date: 1 June is the Canadian convention, and the reason is arithmetic about mosquitoes and temperature rather than tradition. Ticks have a map, PHAC publishes it, and it is spreading — the current risk areas run from Manitoba and western Ontario through southern and southeastern Ontario and southern Quebec to New Brunswick, all of Nova Scotia, and parts of southern British Columbia. Fleas have neither, because they live in your house.
The mistake almost nobody makes is forgetting prevention in July. The mistake a great many people make is stopping in September.
