What Is Heartworm and Why Does It Matter?
Heartworm disease is caused by a parasitic worm called Dirofilaria immitis, transmitted through the bite of an infected mosquito. Once inside your dog's body, these larvae travel through the bloodstream and eventually settle in the heart, lungs, and surrounding blood vessels. Adult heartworms can reach up to 30 centimetres in length, and a single dog may harbour hundreds of them.
The damage they cause is cumulative and often irreversible. By the time clinical signs appear — coughing, exercise intolerance, lethargy, and eventually heart failure — the disease is already well advanced. Treatment exists, but it is expensive, stressful for the dog, and carries real risks. Prevention, by every measure, is the far smarter approach.
How Heartworm Prevention Works
Preventative medications do not stop your dog from being bitten by an infected mosquito. What they do is kill the larval stages of the parasite before they can mature into adults. Most preventatives target the L3 and L4 larval stages, which are present in the bloodstream for a window of roughly 30 to 60 days after infection.
This is why timing matters enormously. Miss a dose or give it late, and larvae that have progressed beyond that window will survive and continue developing. Consistency is the entire basis of how these medicines work.
Monthly Preventatives: The Traditional Standard
Monthly oral or topical preventatives have been the backbone of heartworm prevention for decades. The most commonly used active ingredients include ivermectin, milbemycin oxime, and selamectin. Many monthly products are combination treatments that also address intestinal worms, fleas, or ear mites alongside heartworm.
Advantages of Monthly Options
- Widely available and familiar to most vets and owners
- Often bundled with protection against other parasites
- Lower upfront cost per dose
- Flexible — if your dog has a reaction, you can stop immediately
- Well-documented safety profiles built over many years
Disadvantages of Monthly Options
- Require consistent owner compliance — missed doses are common
- Easy to forget, especially during colder months when mosquitoes seem less present
- Some dogs resist taking tablets, making administration a monthly battle
Research consistently shows that owner non-compliance is the primary reason dogs on monthly prevention still develop heartworm disease. It is not a flaw in the medication — it is a human behaviour problem.
Annual and Semi-Annual Injectables: A Newer Approach
Moxidectin, marketed under the brand name ProHeart, is available as an injectable preventative that lasts either six months or twelve months depending on the formulation. ProHeart 6 is approved in many countries, whilst ProHeart 12, offering a full year of protection from a single injection, is available in the United States and select other markets.
The injectable is administered by a veterinarian, which means compliance is no longer the owner's responsibility. Once the injection is given, protection is continuous for the designated period without any further action required.
Advantages of Injectable Options
- Eliminates the compliance problem entirely
- Ideal for dogs who resist oral medications
- Convenient for busy owners or those who travel frequently with their pets
- Provides uninterrupted protection with no gaps
Disadvantages of Injectable Options
- Requires a vet visit for administration
- Higher upfront cost compared to a single monthly dose
- If a reaction occurs, the drug cannot be removed — effects must be managed supportively
- Does not cover other parasites; additional products may still be needed
- Not yet universally available in all countries
Which Option Is Safer?
Both monthly and annual options have strong safety records when used as directed. ProHeart 12 was actually voluntarily withdrawn from the US market in 2004 following reports of adverse reactions, but after reformulation and extensive re-evaluation, it was re-approved by the FDA in 2019. The current formulation is considered safe for healthy adult dogs.
It is worth noting that neither option should be given to a dog that has not been tested for existing heartworm infection. Administering a preventative to a dog that already carries adult heartworms can trigger a serious reaction. Annual heartworm testing before prescribing or refilling prevention is standard veterinary practice for this reason.
Do Breed Genetics Affect Prevention Choice?
Yes — this is an important consideration that is often overlooked. Collies, Shetland Sheepdogs, Australian Shepherds, and several other herding breeds carry a genetic mutation in the MDR1 (ABCB1) gene that affects how certain drugs are processed. Dogs with this mutation can experience neurological toxicity from ivermectin at doses that would be perfectly safe in other breeds.
Standard heartworm prevention doses of ivermectin are generally considered safe even for MDR1-affected dogs, but it is always worth discussing your dog's breed and genetics with your vet before choosing a product. DNA testing for the MDR1 mutation is inexpensive and widely available if you are uncertain.
Seasonal vs Year-Round Prevention
In regions where mosquitoes are only active for part of the year, some owners opt to give prevention only during mosquito season. Veterinary organisations including the American Heartworm Society recommend year-round prevention regardless of climate, for several reasons.
- Mosquito seasons are becoming longer and less predictable due to climate shifts
- Year-round products also address intestinal parasites that are active in colder months
- Forgetting to restart seasonal prevention at the right time is extremely common
- Year-round use costs less over time than treating heartworm disease
Making the Right Choice for Your Dog
The best heartworm prevention is the one that actually gets used consistently. If you are diligent about monthly administration and your dog cooperates, a monthly oral product may suit your needs perfectly. If compliance has been a struggle, or if your lifestyle makes monthly scheduling difficult, an injectable option — where available — removes that variable entirely.
Speak openly with your vet about your dog's lifestyle, travel habits, breed background, and your own capacity to maintain a prevention schedule. There is no universally superior option, but there is almost certainly a right option for your specific dog and circumstances.