If my dog lives only indoors, does it really need prophylaxis?

The risk is reduced but not zero, because mosquitoes come in through windows and balconies. In areas where heartworm is present, a dog that almost never goes out is still exposed, just less than one that spends many hours outside. The decision should be made with your local vet, who knows the regional spread: in an endemic area prevention is usually advised even for sedentary animals.

I missed a month of prophylaxis: do I have to start over?

You do not start over, but do not ignore the gap either. A missed dose opens a window during which any larvae can mature beyond the stage the product reaches. Resume dosing as soon as you notice and tell your vet about the delay: they can advise whether an earlier check is worthwhile. The most useful thing is to fix your reminder system so the gap does not happen again.

Should prophylaxis be given all year or only in summer?

It depends on the area. Where the climate is mild and mosquitoes are active almost constantly, many vets advise year-round coverage. Where the season is marked, prophylaxis covers the biting period with a margin at both ends, because larvae deposited at the start and finish of the season still need to be reached. It is a local decision, not a universal rule: let it be set by someone who knows your area's pattern.

Does the tick and flea product also protect against heartworm?

Not always. Some combined products cover several parasites, others do not. Do not assume that the external product you use against ticks and fleas also includes heartworm prevention, which acts on a different parasite through a different mechanism. Check with your vet exactly what your product covers, so you do not leave uncovered precisely the most insidious part.

Health and prevention8 min readUpdated on August 20, 2026

Preventing heartworm disease: how to actually protect your dog

How heartworm prophylaxis works, when to start it, the test you need before beginning and the calendar mistakes that leave a dog unprotected for months.

Audience: Pet owners, Veterinarians
Species: Dog, Cat
Scope: Valid everywhere, European Union, Italy

Heartworm disease is a mosquito-borne illness in which a worm several centimetres long settles in the right side of the heart and the pulmonary arteries, slowly damaging organs that give no warning until the picture is already advanced. It is one of the few serious canine diseases that can be almost completely prevented with simple, regular management, and yet every year many animals become infected because prevention is started late, stopped too soon, or given inconsistently. Understanding how prevention works, and above all why the calendar matters more than the individual product, is what separates a genuinely protected dog from one that is only apparently covered. This page explains the logic of prophylaxis, the role of the test and the mistakes that undo it.

What happens in the body and why it is deceptive

When an infected mosquito bites a dog, it deposits microscopic larvae that mature over the following months and migrate towards the heart and lungs. There the adult worms grow and obstruct blood flow, causing inflammation of the pulmonary arteries, coughing, exercise intolerance and, in advanced cases, heart failure. The trouble is that all of this happens silently: six or seven months can pass from the bite before a test can detect the infection, and longer still before symptoms appear.

That slowness is what makes the disease deceptive. A dog can look perfectly healthy while the infection is already under way, and treating the adult form is long, costly and not free of risk, because the death of the worms can cause lung complications. Prevention, by contrast, acts on the immature larvae before they become a problem, which makes it vastly simpler and safer than the cure.

Where and when the risk is real

Heartworm follows the mosquitoes, so risk depends on climate and season. In some areas it is endemic and present almost all year, in others it is tied to the warm season. A frequent mistake is thinking in terms of administrative borders rather than actual conditions: a patch of standing water, a humid microclimate and mild temperatures are enough for mosquitoes to be active.

Situations and the level of attention prophylaxis requires
Situation	Risk level	What it implies
Endemic area, mild climate all year	High and continuous	Prophylaxis considered on a year-round basis, no casual pauses
Area with a marked season	High in summer	Coverage extended beyond the biting period, with a margin at both ends
Travel to an endemic area with an untreated dog	Sudden and underestimated	Prevention set up before departure and continued on return
Dog that lives only indoors	Reduced but not zero	Mosquitoes come inside: the risk remains and is worth discussing with the vet

The practical rule is simple: the decision on the duration and type of prophylaxis is not made on the basis of what you did last year, but on the basis of where the dog lives or travels. Your local vet knows the regional pattern and the season in which mosquitoes become active, and is the right source for setting the calendar.

How prophylaxis works, step by step

Prevention does not kill the adult worms: it eliminates the immature larvae accumulated over the preceding weeks. That is why the logic of the calendar is central. Each dose covers the time that has passed, not the time to come, and a missed dose opens a window during which larvae can mature beyond the stage at which the product reaches them.

Have a test done before starting
In a dog over six or seven months that has never been treated, or whose prophylaxis lapsed, the vet checks that there is no existing infection before beginning prevention.
Agree the product and interval with the vet
Options include monthly oral or spot-on forms and long-acting injectable formulations. The choice depends on lifestyle, the owner's reliability and local risk.
Keep to the date, not the approximation
Consistency matters more than the perfection of the product. A fixed monthly reminder is what makes protection real, because the window of efficacy rests on regularity.
Extend coverage to the edges of the season
Starting a little before mosquitoes appear and continuing a little after they are gone covers the larvae deposited at the beginning and the end of the risk period.
Schedule the annual check
Even with regular prophylaxis, a periodic test confirms that everything is working and catches any coverage gaps that slipped past unnoticed.

Why a test comes before you start

The initial test is not bureaucratic box-ticking: starting prophylaxis on an already infected dog can, in some situations, be pointless or create problems. The test checks for adult worms and lets the vet choose the right path. In a very young puppy it is not needed, because not enough time has passed for an infection to be detectable, but it becomes important from six or seven months onward and whenever coverage has lapsed.

When the test makes sense
Dog's situation	Test advised
Puppy starting prophylaxis within the first months of life	Not needed at the start, folded into later checks
Adult never treated before	Yes, before starting
Prophylaxis lapsed for one or more seasons	Yes, before resuming
Regular prophylaxis for years	Periodic check per the agreed plan

The mistakes that leave a dog exposed

Almost every failure of prevention comes not from the product but from how it is used. These are the ones that recur most often.

Starting late, when the mosquito season is already under way and some larvae have already been deposited.
Stopping at the first cool day, before the risk period is genuinely over.
Skipping a dose and catching up days later without realising the window left uncovered.
Changing routine during travel to an endemic area without adjusting prophylaxis before leaving.
Resuming prevention after a long pause without a test, assuming the dog is still healthy.
Confusing tick and flea protection with heartworm prophylaxis: they are different things and do not always sit in the same product.
Relying on memory rather than a fixed reminder, so the dose drifts progressively later month after month.

The common thread in these mistakes is discontinuity. Heartworm is not prevented by one perfect gesture a year, but by a mundane routine repeated without gaps. That is why the real ally of prevention is a reliable reminder system more than the product itself.

And the cat? A different but real risk

The cat is not the parasite's ideal host and develops infections with fewer worms, but that does not make it immune. In cats heartworm can show up as asthma-like respiratory trouble and, in some cases, as sudden and serious events, and it is harder to diagnose and to treat than in the dog. Even an indoor-only cat can be bitten by mosquitoes that come in through the windows.

For that reason, in areas where the disease is present, prevention should be considered for the cat too, with cat-specific products agreed with the vet. The logic is the same as for the dog: prevention is simple, treatment is complicated, and in cats the therapeutic options are even more limited. The practical conclusion does not change: consistency in the calendar and a periodic check.

Frequently asked questions

If my dog lives only indoors, does it really need prophylaxis?: The risk is reduced but not zero, because mosquitoes come in through windows and balconies. In areas where heartworm is present, a dog that almost never goes out is still exposed, just less than one that spends many hours outside. The decision should be made with your local vet, who knows the regional spread: in an endemic area prevention is usually advised even for sedentary animals.
I missed a month of prophylaxis: do I have to start over?: You do not start over, but do not ignore the gap either. A missed dose opens a window during which any larvae can mature beyond the stage the product reaches. Resume dosing as soon as you notice and tell your vet about the delay: they can advise whether an earlier check is worthwhile. The most useful thing is to fix your reminder system so the gap does not happen again.
Should prophylaxis be given all year or only in summer?: It depends on the area. Where the climate is mild and mosquitoes are active almost constantly, many vets advise year-round coverage. Where the season is marked, prophylaxis covers the biting period with a margin at both ends, because larvae deposited at the start and finish of the season still need to be reached. It is a local decision, not a universal rule: let it be set by someone who knows your area's pattern.
Does the tick and flea product also protect against heartworm?: Not always. Some combined products cover several parasites, others do not. Do not assume that the external product you use against ticks and fleas also includes heartworm prevention, which acts on a different parasite through a different mechanism. Check with your vet exactly what your product covers, so you do not leave uncovered precisely the most insidious part.

What to do next

Agree an initial test with your vet if the dog is an adult never treated before or if prophylaxis lapsed, then set up regular prevention suited to your area and the mosquito season. What truly makes the difference is not the product but consistency: set a monthly reminder, log every dose and the date of the next check, and extend coverage to the edges of the risk period. In an endemic area, consider prevention for the cat as well.