Preventing Heartworm in Dogs: What Actually Works

By Dr Duncan Houston

Quick Answer

Heartworm prevention in dogs works by killing immature larvae before they develop into adult worms. Monthly preventives or long-acting injections are highly effective when used consistently. Prevention should always be started after a negative heartworm test and maintained year-round in most regions.

Heartworm is one of those diseases that’s easy to prevent… but very difficult to treat.

And the frustrating part is, most cases I see could have been avoided with consistent prevention.

If you’re doing it right, heartworm should never become a problem.

What to do to protect your dog from heartworm

If you want to keep things simple:

1. Start prevention after a negative test

2. Give it consistently every month or use long-acting injections

3. Do not skip doses

4. Stay on prevention year-round in most areas

Missed doses are where problems begin.

How heartworm prevention works

Heartworm preventives don’t stop mosquitoes.

They work by killing immature larvae (L3 and L4 stages) that your dog was exposed to in the previous month.

Some products also target slightly older larvae, giving a small buffer if a dose is delayed.

But they do not reliably kill adult worms.

That’s why consistency matters.

Types of heartworm prevention

There are several main options, each with pros and cons.

Ivermectin-based products

Common options:

• Heartgard Plus

• Tri-Heart

• Iverhart

These are typically combined with pyrantel to cover:

• roundworms

• hookworms

Pros

• reliable

• cost-effective

• widely used

Cons

• no whipworm coverage

Milbemycin-based products

Examples include:

• Interceptor

• Sentinel

• Trifexis

• Interceptor Plus

These offer broader parasite coverage, including:

• whipworms

• sometimes tapeworms (depending on product)

Pros

• broader internal parasite control

Cons

• less forgiving if doses are missed

Selamectin (topical option)

Example:

• Revolution

Covers:

• heartworm

• fleas

• mites

• some intestinal worms

Pros

• good option for dogs that won’t take tablets

Cons

• needs consistent application

• slightly different coverage profile

Moxidectin-based products

Options include:

• Advantage Multi (monthly topical)

• Proheart injections (6 or 12 months)

These are useful for owners who struggle with monthly dosing.

Pros

• long-acting options available

• reduces risk of missed doses

Cons

• must be used appropriately and under veterinary guidance

What happens if you miss a dose

This is where things get risky.

Some products, particularly ivermectin-based ones, have a small safety buffer, meaning they can still work if slightly delayed.

Others are less forgiving.

The key point:

• missing doses increases risk

• restarting without advice is not ideal

If you miss a dose, speak to your vet about the next step.

Why testing matters

Heartworm prevention should only be started in heartworm-negative dogs.

Giving preventives to a heartworm-positive dog can increase the risk of reactions.

That’s why:

• annual testing is recommended

• testing is required before starting prevention

When to start prevention

In most regions, the safest approach is:

• year-round prevention

Heartworm transmission depends on temperature and mosquito activity, but in practice, year-round prevention removes the guesswork.

Heartworm resistance

There have been reports of resistant strains, particularly in some regions.

The biggest risk factors for resistance are:

• inconsistent dosing

• missed doses

• incorrect use

Consistency is the best protection.

Additional protection against mosquitoes

Preventives kill larvae after infection.

They do not stop mosquito bites.

Using a flea and tick product with mosquito-repellent properties can add another layer of protection.

This is especially useful in high-risk areas.

Signs of heartworm disease

Prevention is important because once infection develops, signs can include:

• coughing

• exercise intolerance

• weight loss

• breathing difficulty

Advanced cases can become life-threatening.

FAQ

Do all dogs need heartworm prevention

Yes, in most areas. Risk exists wherever mosquitoes are present.

Is heartworm prevention safe

Yes, when used correctly and under veterinary guidance.

What is the best heartworm prevention

It depends on your dog and lifestyle. The best one is the one you use consistently.

Can I stop prevention in winter

In most cases, no. Year-round prevention is recommended.

Final thoughts

Heartworm prevention is simple when done consistently.

The biggest problems come from:

• missed doses

• stopping and starting

• assuming risk is low

Stay consistent, stay on schedule, and you will almost always avoid the problem entirely.

Still unsure what to do next?

Choosing the right prevention can feel confusing.

Which product is best?
What if you miss a dose?
Does your dog actually need it year-round?

If you’re unsure, getting guidance early helps you avoid mistakes and keep your dog protected.

The ASK A VET™ app gives you access to veterinary advice when you need it, so you can make the right choice with confidence.