Heartworm disease is serious, potentially fatal and almost completely preventable. Spread by mosquitoes, it has been found in all 50 U.S. states. Treatment is long, costly and hard on dogs, which makes prevention far better. Here’s what to know about heartworm prevention for dogs.
Quick Answer: Heartworms are spread by mosquito bites. The American Heartworm Society recommends year-round prevention for dogs and yearly heartworm testing. Prevention options include monthly chews or tablets, monthly topicals and injectable products that last 6 or 12 months. All require a prescription from your vet.
How Heartworm Disease Works
- A mosquito bites an infected animal and picks up baby heartworms (microfilariae).
- The larvae develop inside the mosquito.
- The mosquito bites a dog and passes infective larvae into the skin.
- Over about 6 months, larvae grow into adult worms in the heart and lungs.
- Adult worms can reach 12 inches long and live for years.
The American Heartworm Society notes that heartworms can cause lasting damage to the heart, lungs and arteries.
Signs of Heartworm Disease
Early disease often has no signs. As it progresses:
- Mild, persistent cough
- Tiring easily with exercise
- Decreased appetite
- Weight loss
- Swollen belly from fluid buildup
- Fainting or collapse in severe cases
Sudden breathing trouble, pale gums or dark urine in a dog with heartworms can signal caval syndrome, a life-threatening emergency.
Why Year-Round Prevention?
- Mosquito seasons vary and can be unpredictable.
- Indoor mosquitoes can bite dogs in winter.
- Many products also protect against intestinal worms.
- Missing doses leaves gaps in protection.
Prevention Options
| Type | How often | Notes |
|---|---|---|
| Oral chews or tablets | Monthly | Many also treat intestinal worms; some combine flea and tick protection |
| Topical (spot-on) | Monthly | Some also cover fleas and other parasites |
| Injectable | Every 6 or 12 months | Given by your vet; no missed doses |
Your vet will recommend a product based on your dog’s age, health and lifestyle.
Why Testing Matters
- Dogs should be tested before starting prevention (after about 7 months old).
- Annual testing is recommended even for dogs on year-round prevention, because no product is 100% effective and doses can be missed or spit out.
- Testing is a quick blood test.
Giving some preventives to dogs that already have heartworms can cause reactions.
Heartworm Treatment
If your dog tests positive, treatment usually involves:
- Confirming the diagnosis and assessing severity
- Strict exercise restriction for months
- Antibiotics and preventive medication
- A series of injections to kill adult worms
- Follow-up testing
Treatment is expensive and carries risks, especially in severe cases.
Heartworm Prevention for Puppies
Puppies can usually start heartworm prevention by about 8 weeks of age. Ask your vet. See our puppy vaccination schedule.
Reduce Mosquito Exposure
- Remove standing water around your home.
- Keep dogs indoors at dawn and dusk when mosquitoes are most active.
- Use window and door screens.
Pet Health Man also compares popular heartworm medications.
When to Call Your Vet
Call your vet if your dog:
- Has missed a dose of heartworm prevention
- Has a cough, tires easily or is losing weight
- Is due for an annual heartworm test
Common Myths
| Myth | Fact |
|---|---|
| “My dog only needs prevention in summer.” | Mosquitoes can be active much of the year, and gaps leave dogs unprotected. |
| “Heartworm only happens in the South.” | Cases have been reported in every U.S. state. |
| “If my dog is on prevention, testing isn’t needed.” | Annual testing catches missed or failed doses. |
| “Prevention kills adult worms.” | Most preventives kill larvae, not adult worms. |
Frequently Asked Questions
Can dogs get heartworms from other dogs?
Not directly. Heartworms are only spread by mosquitoes.
Do indoor dogs need heartworm prevention?
Yes. Mosquitoes get indoors.
What if I missed a dose?
Give the dose as soon as you remember and call your vet. They may recommend testing later.
Can cats get heartworms?
Yes. Cats can get heartworm disease, and there’s no approved treatment for cats, so prevention is important. See flea and tick prevention for indoor cats.
Are natural heartworm preventives effective?
There’s no scientific evidence that herbal or “natural” products prevent heartworm.
How much does prevention cost compared with treatment?
Prevention costs far less than treatment, which can run into hundreds or thousands of dollars.
Can puppies get heartworm from their mother?
Not in a way that causes adult heartworm disease. Puppies become infected through mosquito bites.
Do heartworm preventives have side effects?
Most dogs tolerate them well. Some may have mild stomach upset. Certain herding breeds with the MDR1 gene can be sensitive to some drugs at high doses, so tell your vet your dog’s breed.
This article is for general information only and isn’t a substitute for professional veterinary advice. If you’re worried about your pet’s health, contact your veterinarian or an emergency animal hospital.
