Heartworm Disease in Dogs: Prevention, Signs and Treatment

By Tailwisp Editorial TeamUpdated October 17, 20263 min read

Heartworm disease is one of the most preventable serious illnesses in dogs, and one of the hardest to treat once established.

How it spreads

An infected mosquito bites a dog and deposits larvae. Over about six months the larvae mature into adult worms that live in the heart and the arteries of the lungs, growing up to roughly a foot long. Adult worms produce offspring that circulate in the blood and infect new mosquitoes. Dogs do not pass it directly to each other.

Where it occurs

Heartworm has been reported in all US states and in many warm and temperate regions worldwide. Risk varies with climate and mosquito activity, but your vet can advise on local risk. Do not assume your area is safe because winters are cold, since mosquitoes can persist indoors.

Signs

Early infection often causes no signs. As disease progresses you may see:

• Mild persistent cough
• Reluctance to exercise and tiring easily
• Weight loss and reduced appetite
• Fast or labored breathing
• Swollen belly from heart failure
• Collapse in severe cases, sometimes called caval syndrome, which is an emergency

See dog coughing causes and congestive heart failure in dogs.

Testing

Vets use blood tests to detect antigen from adult female worms and sometimes microfilariae. Tests are generally recommended annually and before starting prevention in older puppies or adults. Positive results are confirmed, and X-rays, bloodwork and echocardiography may be used to assess damage.

Treatment

Treatment usually involves:

1. Stabilizing and stopping prevention-related problems under your vet's direction
2. Medication to kill adult worms, administered under strict protocols
3. Doxycycline to reduce a bacterium called Wolbachia that lives in worms
4. Strict exercise restriction for weeks to months, since dying worms can cause blockages
5. Follow-up testing to confirm clearance

Treatment is hard on the dog and expensive, with possible complications. Prevention is far easier.

Prevention

• Give a veterinarian-prescribed heartworm preventive, monthly or as an injectable schedule
• Keep to the schedule year-round if advised
• Test annually, even if you have not missed doses
• Reduce mosquito exposure where practical

See dog flea, tick and heartworm medications and puppy flea, tick and heartworm prevention.

What if you miss a dose?

Contact your vet. Depending on how long it has been, they may recommend testing before restarting. Set reminders so doses are never forgotten.

Choosing a preventive

Options include chewables, topical products and injections. Some also protect against intestinal worms, fleas or ticks. Ask about your dog's weight, health and sensitivity, including certain herding breeds with the MDR1 mutation. See MDR1 gene mutation in dogs.

Common mistakes

• Skipping prevention in winter
• Assuming indoor dogs are safe
• Not testing annually
• Using leftover products from other animals

Troubleshooting

• My dog threw up after a dose: ask your vet whether to repeat.
• I do not remember the last dose: call your vet.
• My rescue dog tested positive: ask for a treatment plan and strict rest guidance.

A simple plan

1. Pick a vet-prescribed preventive.
2. Set monthly reminders.
3. Test annually.
4. Report coughing or lethargy.

Real-life examples

A retriever on monthly prevention tests negative every year. A rescue dog with a positive test completes treatment and strict rest and recovers. A terrier who missed several doses has early disease detected on routine testing. Prevention makes a difference.

Why it matters

A small monthly step prevents a disease that can damage organs for life.

The takeaway

Heartworm is preventable. Use year-round prevention, test yearly and call your vet if your dog coughs or tires.

This article is general educational information, not veterinary advice. Every dog is different. If your dog is unwell, or you are unsure about a symptom, diet change or medication, contact your veterinarian.