The American Heartworm Society recommends every dog and cat in the US receive heartworm prevention every month, every month of the year. Below: the cadence, what changes for puppies, cats, and the 6-month injection — and how to keep the schedule from slipping.
Most owners just need to know two things: how often, and starting when. Here's the short version, broken out by situation.
| Situation | Cadence | Starts when |
|---|---|---|
| Adult dog (any breed) | Monthly, year-round | After negative heartworm test, then same date every month |
| Puppy under 7 months | Monthly, year-round | As early as 6–8 weeks (per product label) |
| Adult cat (indoor or outdoor) | Monthly, year-round | Most products labeled from 8 weeks |
| Dog on ProHeart 6 | Every 6 months, year-round | Vet-administered injection |
| Dog on ProHeart 12 | Every 12 months, year-round | Vet-administered injection |
For decades, the conventional advice in northern states was to dose only during mosquito season. The American Heartworm Society now recommends year-round prevention everywhere, and the reasoning is mostly about how owners actually behave.
Mosquitoes are not a binary on-off. They survive indoors year-round in warm garages, basements, and homes. They emerge during winter warm spells, particularly in late January and February in the south, but increasingly further north too. Climate trends have stretched the active season at both ends.
More importantly, seasonal dosing creates two predictable failure points: people forget to restart in spring, and stop too early in fall. The AHS data on missed-dose infections skews heavily to those edge months. A single 12-month schedule eliminates both holes.
The American Heartworm Society's "Think 12" mnemonic captures the full guideline in one line: test every 12 months, dose every 12 months. The annual test catches any breakthrough infection while monthly dosing prevents new ones. Both halves matter — prevention without testing leaves you blind, and testing without prevention is just early detection.
The label on Heartgard, Sentinel, Simparica Trio, and every other monthly preventative says "monthly." It does not say "every 30 days." The difference matters for memory.
If you try to count 30 days, you'll drift two days earlier each year and quickly lose track of which date is "right." Picking a fixed calendar date — the 1st, the 15th, payday, the first Saturday — is what every vet and the AHS guidelines recommend. The same day every month is easier to remember and just as effective.
The default for a reason. It lines up with calendars, billing, and most other monthly habits. Hard to lose track of the start of a month.
If you already check accounts on those days, attach the heartworm dose to the same routine. The trigger does the work for you.
Whatever date you started the medication on, just keep using it. It's already part of your dog's history with the household.
Most monthly preventatives are labeled for puppies starting at 6 to 8 weeks of age. The AHS recommends starting prevention as early as the product label allows. Puppies under 7 months don't need a pre-test before starting, since heartworm larvae take about six months to become detectable on a standard antigen test.
Any dog over 7 months who hasn't been on prevention should be tested before the first dose, and tested again 6 months later. Starting prevention on a dog with an active infection can cause a serious adverse reaction.
Cats need monthly prevention too. Heartworm in cats presents differently — fewer worms, often misdiagnosed as asthma — and there's no approved adulticide treatment. Once a cat is infected, the only options are supportive care and waiting it out. Prevention is the whole strategy.
For dogs, ProHeart 6 and ProHeart 12 are vet-administered injections that replace monthly chews. ProHeart 6 lasts 6 months, ProHeart 12 lasts 12. The cadence is longer, but you still need a reminder — just twice a year (or once a year) for the appointment.
Almost every dog owner knows the recommendation: monthly, year-round. The schedule isn't the problem. The problem is the gap between the abstract rule and the specific Friday in July when the dose was supposed to happen.
A persistent email reminder bridges the two. See the full setup at the heartworm prevention reminder page, or set one right now below.
Pick an anchor date and let the schedule run itself.
Done in seconds. No sign-up required.
The American Heartworm Society recommends year-round prevention for every dog in the US, regardless of climate. Mosquitoes survive indoors and emerge during winter warm spells, and most missed-dose infections happen during gaps in coverage at the edges of "mosquito season."
All twelve. Even in northern states, the AHS recommends year-round dosing. Seasonal-only schedules tend to fail when owners forget to restart in spring or stop too early in fall.
Every month on the same date. The label says monthly because giving it on, say, the 1st and the 1st works fine — even though some months are 31 days. Picking the same calendar date every month is easier to remember than counting 30 days.
Most preventatives are labeled for puppies as young as 6 to 8 weeks. The AHS recommends starting prevention as early as the product label allows, with no pre-test needed for puppies under 7 months. Older puppies and adults should be tested before the first dose.
Yes, monthly. Cat preventatives like Revolution Plus, Bravecto Plus, and Advantage Multi for Cats are dosed once a month. There is no approved heartworm treatment for infected cats, which makes prevention even more critical.
Yes. Mosquitoes get inside. AHS data shows infections in dogs that are described as fully indoor pets. Prevention is far cheaper and safer than treatment, which is why the AHS doesn't carve out an indoor exception.
Free email reminder on your anchor date, every month, with follow-ups until the dose is given. Works for any preventative — chews, topicals, or the ProHeart injection schedule.
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