**Heartworm in dogs: the six-month window and why prevention wins**

> How a mosquito bite becomes an adult worm, why preventives only work on larval stages, what treatment involves, and why testing comes first.

Source: https://galen.vet/journal/heartworm-in-dogs/ · updated: 2026-09-05

[Journal](https://galen.vet/) /Prevention

Prevention

# Heartworm in dogs: the life cycle, the six-month window and prevention

Everything about heartworm turns on one interval: preventives act on larvae, and larvae become untouchable adults in about six months.

Written by the [Galen editorial team](https://galen.vet/about/). Reference material — not a diagnosis, a prescription or a dosing instruction.

Published 5 September 2026

![A dog resting on a porch at dusk with long grass and still water in the background](https://galen.vet/img/journal/heartworm-in-dogs.webp)

*A dog resting on a porch at dusk with long grass and still water in the background*

## The short answer

- Heartworm is transmitted by mosquitoes, not between dogs. The larvae enter through the bite wound and take around six months to mature into adults.
- Preventive medication kills larval stages from the preceding weeks. It has no effect on established adult worms, which is the entire reason a lapse matters.
- A dog must be tested before a preventive is started or restarted, because giving one to a dog carrying microfilariae can provoke a serious reaction.
- Treatment of established infection is possible but demanding: it is expensive, carries real risk from dying worms embolising, and requires months of strict exercise restriction.
- Whether prevention is needed at all is geographic. Where heartworm occurs, indoor dogs and cats are not exempt, because the vector flies.

On this page

1. The cycle, and the window inside it
2. Why testing comes first
3. What the disease does
4. What treatment asks of an owner
5. Where it matters
6. Keeping prevention working
7. Bottom line

Heartworm is unusual among parasitic diseases in how completely the arithmetic favours prevention. A monthly preventive is one of the cheapest interventions in veterinary medicine. Treating an established infection is one of the more expensive and one of the few where the treatment itself carries a meaningful risk of killing the patient.

The reason the gap is so wide sits in the life cycle, and specifically in one interval of about six months.

## The cycle, and the window inside it

***Prevention is retroactive and adults are permanent — which is the whole strategy in one line.** A monthly dose clears what was picked up in the weeks before it; once larvae cross into maturity, nothing in the preventive category reaches them. Free to reuse with a link to this page.*

A mosquito feeding on an infected animal takes up microfilariae. Inside the mosquito they develop into infective larvae, and at a subsequent blood meal those larvae are deposited on the skin and enter through the bite wound. They migrate through tissue for weeks, then travel via the bloodstream to the pulmonary arteries and the right heart, where they mature. Adults produce microfilariae, and a mosquito completes the loop.

Two features of that cycle do all the work.

**Preventives act on larval stages.** They are given monthly not because they last a month, but because a dose clears the larvae acquired during the preceding weeks. The schedule is a rolling clean-up.

**Adults are outside their reach.** Once worms have matured, a preventive is irrelevant to them. That asymmetry is why a forgotten summer is not fixed by starting again in September, and why the response to a lapse is a test rather than a resumption.

## Why testing comes first

Testing before starting or restarting is not administrative caution.

If adult worms are present, a preventive will not remove them, and starting one produces exactly the wrong belief — that the dog is now protected — while pulmonary arterial disease progresses quietly. And in a dog carrying circulating microfilariae, the rapid microfilarial kill produced by some products can provoke a serious adverse reaction.

The tests answer different questions: an antigen test detects a protein produced by adult female worms, and a microfilaria test looks for larvae in circulation. Because it takes months from infection for an antigen test to turn positive, the timing of a post-lapse test is not arbitrary and the practice will specify it.

## What the disease does

Adult worms living in the pulmonary arteries damage the arterial lining and provoke inflammation. The vessels thicken and stiffen, resistance rises, and the right side of the heart works against a load it was not built for.

*How heartworm infection presents as it progresses. The most important row is the first one, because it is the reason the disease is found by testing rather than by noticing.*

| Stage | What is happening | What is visible |
| --- | --- | --- |
| Early | Worms maturing; arterial changes beginning | Nothing at all, commonly for a long time |
| Developing | Pulmonary arterial disease established | Cough; tiredness after modest exercise |
| Advanced | Right heart failing against high resistance | Weight loss, poor appetite, abdominal fluid, marked exercise intolerance |
| Caval syndrome | A mass of worms obstructs blood flow through the heart | Sudden collapse; an emergency requiring surgical removal |

The relationship between worm burden and signs is not neat: a dog with few worms may show little, and a very active dog may show more than a sedentary one with the same burden, because exercise raises pulmonary arterial pressure.

## What treatment asks of an owner

An established infection is treated with a specific adulticide protocol, given by a veterinarian and usually staged over weeks alongside other medication. The pharmacology belongs in the practice. The part that belongs at home is harder than most people expect.

**Exercise restriction is the dangerous part of heartworm treatment, and it is the owner's responsibility.** Worms killed by the adulticide fragment and lodge in the pulmonary arteries. Physical exertion raises pulmonary blood flow and pressure, and that is what converts fragments into a life-threatening embolic event. The restriction is strict, it lasts for months, and it applies to a dog that by then feels well enough to want to run. Cage rest, lead-only toileting and no stairs or play is the standard shape of it.

That requirement is why prevention is not merely cheaper. Treatment demands months of confinement from an animal that does not understand why, in a household that has to enforce it perfectly.

## Where it matters

Heartworm is geographic. It is endemic across much of the United States, present in southern Europe and expanding northward, and established in parts of Asia, South America and Australia. Whether prevention is indicated for a particular dog is therefore a question for a local practice, and it changes when a dog travels.

**Travel is the point most often missed.** A dog taken on holiday from a heartworm-free area into an endemic one needs prevention arranged before it goes, not after it returns — and the plan for a travelling dog usually covers other regional parasites at the same time.

Cats are also susceptible, and feline heartworm behaves differently: fewer worms, a more severe respiratory reaction, and no approved adulticide, which makes prevention the only available strategy. That indoor status does not exempt them is covered in [parasite prevention for indoor cats](https://galen.vet/journal/parasite-prevention-indoor-cats/).

## Keeping prevention working

Three practical points, none of them about the medicine itself.

**Do not leave gaps.** Set a recurring reminder on the same date each month. A missed dose is a real event and the practice would rather hear about it than discover it at the next annual test.

**Check the dose was actually received.** A tablet spat out behind a sofa, or a spot-on applied to fur rather than to skin, is a missed dose that looks like a given one.

**Store the product properly.** Heat and light degrade veterinary medicines like any others, and a product kept in a car glovebox through a summer is not the product on the label — the reasoning is in [storage conditions and the cold chain](https://galen.vet/journal/storing-veterinary-medicines-cold-chain/).

## Bottom line

Heartworm reaches a dog through a mosquito bite, not from other dogs, and the larvae take about six months to become adult worms in the pulmonary arteries. Preventives kill larval stages and act retroactively over the preceding weeks; they do nothing to established adults, which is why a lapse cannot be corrected by resuming and is followed by a test instead. A dog must be tested before prevention is started, both because a preventive will not clear existing adults and because a rapid microfilarial kill can provoke a serious reaction. Infection is often silent until pulmonary arterial disease is well advanced. Treatment exists, is expensive, carries real risk from dying worms embolising, and requires months of strict exercise restriction enforced at home. Whether any of this applies depends on where the dog lives and where it travels — ask locally, and set a monthly reminder.

## Questions readers ask

How do dogs get heartworm?

From a mosquito. A mosquito feeding on an infected animal takes up microfilariae, which develop inside it into infective larvae; when that mosquito next feeds, the larvae are deposited on the skin and enter through the bite wound. They then migrate through tissue and eventually reach the pulmonary arteries and the right side of the heart, maturing into adult worms over roughly six months. Dogs do not transmit it to each other directly, and it is not caught from faeces or contact.

Why does missing a dose of preventive matter so much?

Because preventives work on larval stages, not on adults. A monthly product acts retroactively, clearing the larvae acquired over the preceding weeks. If a gap allows larvae to survive long enough to develop past that susceptible stage, no amount of subsequent prevention will affect them; they continue to adulthood and the dog becomes infected. This is also why a lapse is not fixed by simply resuming — a test is needed after the appropriate interval.

Why must a dog be tested before starting a preventive?

Two reasons. If adult worms are already present, a preventive will not remove them, and starting it creates a false sense of protection while the infection progresses. More acutely, in a dog carrying circulating microfilariae, the rapid kill produced by some preventives can provoke a serious adverse reaction. Testing first establishes which situation you are in, and it is standard practice before starting and after any significant lapse.

What are the signs of heartworm infection?

Often none at all for a long time, which is why testing exists. As disease develops the picture includes a persistent cough, reduced tolerance of exercise, tiredness after mild activity, weight loss and reduced appetite; later, signs of right-sided heart failure such as a distended abdomen from fluid accumulation. A dog can be carrying adult worms and appear entirely normal, and by the time signs are obvious the damage to the pulmonary arteries has usually been under way for a considerable time.

How is an established infection treated?

With a specific adulticide protocol given by a veterinarian, usually alongside other medication, staged over weeks. What owners most need to know is what the protocol demands of them: strict exercise restriction for a prolonged period, because dying worms fragment and lodge in the pulmonary arteries, and physical exertion is what turns that into a life-threatening embolic event. It is expensive, it carries genuine risk, and it is entirely avoidable by prevention that costs a fraction of it.

Do indoor pets need heartworm prevention?

Where heartworm is present, yes — this is a question for the local veterinary practice rather than a general rule, because the parasite's distribution is geographic. Mosquitoes come indoors through open windows and doors, and a meaningful proportion of feline heartworm cases occur in cats described as indoor-only. In regions where heartworm does not occur, prevention against it is not indicated, and the parasite plan addresses whatever is present locally instead.

## Keep reading

- [Parasite prevention for indoor cats: the routes that bypass a closed door An indoor cat is protected from most of what it would meet outside. The parasites that matter are the ones that do not need the cat to leave.](https://galen.vet/journal/parasite-prevention-indoor-cats/)
- [Diabetes in dogs: what the signs mean and what management involves Four changes precede the diagnosis, and three of them look like a dog being a dog. The fourth is sudden blindness.](https://galen.vet/journal/diabetes-in-dogs/)
- [Dehydration in cats and dogs: the checks, the causes and the point of no delay By the time dehydration is visible, several per cent of body water has already gone. The checks are worth knowing precisely because they are so late.](https://galen.vet/journal/dehydration-in-cats-and-dogs/)
- [Intestinal obstruction in dogs: what it is and which signs cannot wait A dog with a blockage and a dog with a stomach upset look similar for the first few hours. After that they diverge sharply, and only one of them is still improving.](https://galen.vet/journal/intestinal-obstruction-in-dogs/)
- [Why dogs chew furniture and what actually stops it Chewing is not one problem. Three different causes produce the same wrecked chair leg, and only one of them responds to more exercise.](https://galen.vet/journal/why-dogs-chew-furniture/)

## Sources

1. [Canine heartworm guidelines](https://www.heartwormsociety.org/veterinary-resources/american-heartworm-society-guidelines) — American Heartworm Society
2. [Parasite control recommendations](https://capcvet.org/guidelines/) — Companion Animal Parasite Council (CAPC)
3. [Parasite control guidelines for cats and dogs](https://www.esccap.org/guidelines/) — ESCCAP (European Scientific Counsel Companion Animal Parasites)
4. [Dog owners — veterinary manual](https://www.merckvetmanual.com/dog-owners) — MSD/Merck Veterinary Manual
5. [The Merck Veterinary Manual](https://www.merckvetmanual.com/) — MSD/Merck Veterinary Manual

Galen publishes reference material on veterinary medicines. It describes how
products are labelled, stored and classified; it does not tell you what to give an
animal, in what amount, or for how long. Those decisions belong to a veterinarian
who has examined the animal, working from the manufacturer's labelling for the
exact product in front of them. Where national rules apply — to prescribing,
residues in food, or reporting a suspected reaction — the authority in your own
country is the one that governs.

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