Heartworm prevention for dogs

Heartworm prevention for dogs means a vet-prescribed preventive given 12 months a year, without gaps: a monthly chewable or topical, or an injection that lasts 6 or 12 months. The American Heartworm Society (AHS) also recommends a heartworm test every year for dogs over 7 months old, even when they never miss a dose.

Heartworm disease is serious, slow and silent in its early stages, and much easier to prevent than to treat. Here is how it spreads, how the preventives work, and what to do if a dose slips.

How dogs get heartworm

Heartworm is spread only by mosquitoes:

1. A mosquito bites an infected animal (a dog, coyote, fox or wolf) and picks up tiny baby worms called microfilariae.
2. Over about 10 to 14 days inside the mosquito, they develop into infective larvae.
3. When the mosquito bites your dog, the larvae enter through the bite.
4. Over about 6 months, they grow into adult worms that live in the heart, lungs and the blood vessels between them.

According to the AHS, adult heartworms can reach a foot long and live 5 to 7 years in a dog. Because they live so long, a dog can pick up more worms with each mosquito season, and the damage to the heart and lungs builds up.

Dogs can’t catch heartworm directly from each other. But one infected dog, or local wildlife, is enough to keep mosquitoes passing it around a neighbourhood.

Where heartworm is found

In the US, heartworm has been diagnosed in all 50 states, and the AHS notes that risk can vary a lot even within one community, depending on climate, mosquito species and wildlife. It is also common in parts of Southern and Eastern Europe, where the European parasite experts at ESCCAP advise monthly prevention for dogs living in or travelling to those areas. If you live somewhere heartworm isn’t established, such as the UK, ask your vet about prevention before travelling with your dog.

Signs of heartworm disease

Many dogs show few or no signs early on, which is why testing matters. As the disease progresses, the AHS lists:

• A mild, persistent cough
• Reluctance to exercise, or tiring easily after moderate activity
• Reduced appetite and weight loss
• A swollen belly from fluid build-up

In heavy infections, worms can suddenly block blood flow in the heart (caval syndrome), causing collapse, pale gums and dark urine. That is an emergency: call a vet or emergency clinic now.

Why prevention is year-round

It might seem logical to stop in winter, but the AHS and the FDA both recommend year-round prevention:

• Mosquito seasons are unpredictable. Warm spells, indoor mosquitoes and regional differences make it hard to know when transmission really stops and starts.
• One missed dose can be enough. The AHS notes that a dog can become infected because of a single skipped or late dose.
• Year-round is easier to remember. “First of every month” is a habit; “restart in spring” is a date people forget.
• Many preventives also treat intestinal worms, which are around all year.

How heartworm preventives work

Most preventives (a drug group called macrocyclic lactones) don’t create a shield. They kill the larvae your dog picked up over the previous weeks, before they can grow into adults. That is why timing matters: give a monthly dose late, and some larvae may have grown past the stage the drug can stop. The AHS describes these medicines, used as labelled, as highly effective and among the safest in veterinary medicine.

Preventives do not kill adult heartworms, which is one reason dogs are tested before starting.

Types of heartworm preventive

All heartworm preventives need a prescription from your vet, who will choose the product and dose for your dog’s weight and health.

Some products also repel or kill mosquitoes, which the AHS says can add a layer of protection. If your dog takes flea and tick prevention as well, ask whether one product can cover both: see flea and tick prevention for dogs. If your dog spits out tablets, our guide on how to give a dog a pill can help.

The heartworm testing schedule

The AHS recommends:

Why test a dog that never misses a dose? Doses get spat out, vomited or forgotten, and the AHS notes that some heartworms resistant to preventives have been documented. A yearly test catches an infection early, when it is easiest to treat.

Puppies should start prevention as early as the product label allows, usually at around 8 weeks. Ask about it at your puppy’s first vet visit.

If you missed a dose

It happens. Don’t double up without advice; call your vet. Usually they will suggest:

1. Giving the missed dose as soon as you remember, then carrying on monthly from that date.
2. A heartworm test in about 6 months, because a new infection can’t be detected until the worms are around 7 months old.

The longer the gap, the more important that follow-up test is. If remembering the date is the hard part, PetTrackr can remind you on the day and let you tick it off from the notification, so you always know whether this month’s dose was given.

If your dog tests positive

Heartworm disease is treatable in dogs, but it is not a quick fix. The AHS describes a course of injections at the vet with a drug that kills adult worms, usually after about two months of preventive, followed by weeks of strict exercise restriction to protect the lungs as the worms die. A follow-up test comes about 9 months after treatment ends. Your vet will guide every step.

What about cats?

Cats can get heartworm too, from the same mosquitoes, and there is no approved treatment for cats. If you have a cat in a heartworm area, ask your vet about a cat preventive. Never give a cat a product made for dogs.