Heartworm in dogs: symptoms, treatment and prevention

Heartworm is one of the few serious dog diseases that is almost entirely preventable — and one of the most expensive and difficult to treat once it is established. This guide explains how heartworm works, what the symptoms look like, how dogs are tested and treated, and how to choose a prevention plan with your veterinarian.

What is heartworm disease?

The American Heartworm Society (AHS) describes heartworm disease as a serious and potentially fatal disease caused by foot-long worms that live in the heart, lungs and associated blood vessels. Dogs are natural hosts: the worms mature, mate and produce offspring inside the dog, and dogs can harbor several hundred worms if untreated. The damage to the heart, lungs and arteries can affect a dog’s health long after the worms are gone.

How heartworm spreads

1. An infected animal (dog, fox, coyote or wolf) carries microfilariae in its blood.
2. A mosquito bites and picks them up.
3. Inside the mosquito, larvae become infective over 10 to 14 days.
4. The mosquito bites another dog and deposits larvae on the skin, which enter through the bite wound.
5. It takes about 6 months for larvae to become adult worms.
6. Adults can live 5 to 7 years in dogs, according to the AHS.

The Merck Veterinary Manual adds that at least 70 species of mosquitoes can transmit heartworm, and that most infections are diagnosed in medium- to large-sized dogs aged 3–8 years.

Heartworm symptoms in dogs

Stage	What you may see
Early	Often no signs
Mild to moderate	Mild persistent cough, reluctance to exercise, fatigue after moderate activity
Advanced	Decreased appetite, weight loss, labored breathing, fainting
Severe	Heart failure, swollen belly from fluid
Caval syndrome (emergency)	Sudden labored breathing, pale gums, dark bloody or coffee-colored urine

The AHS warns that dogs with large numbers of worms can develop caval syndrome, a sudden blockage of blood flow in the heart; without prompt surgical removal of the worms, few dogs survive. Merck notes that active dogs tend to show more dramatic signs than sedentary dogs with similar worm numbers.

Testing: when and how

Heartworm tests use a small blood sample to detect heartworm proteins. The AHS testing guidance:

Dog	Testing schedule
Puppies under 7 months	Start prevention without a test; test 6 months after the first visit, again 6 months later, then yearly
Adults over 7 months not on prevention	Test before starting prevention, again at 6 and 12 months, then yearly
Dogs on prevention	Test every year
Missed or late doses	Talk to your vet, restart prevention, retest in about 6 months

The reason for the waiting periods: worms must be about 7 months old before a test can detect them.

How heartworm treatment works

Treatment is planned by your veterinarian after staging the disease with tests such as chest X-rays and bloodwork. Merck describes key elements:

• Medication against Wolbachia, bacteria that live inside heartworms and contribute to disease; your vet will likely include a drug to kill them.
• Injections to kill adult worms, usually given in a series.
• Strict exercise restriction during and after treatment, because dying worms can block blood vessels in the lungs.
• Prevention to stop new infections and kill circulating larvae.
• Follow-up testing to confirm success.

Small dogs tolerate infection and treatment less well than large dogs, according to Merck, because their blood vessels and heart chambers are smaller.

Heartworm prevention for dogs

Preventives are prescription medications that kill immature larvae before they become adults. Options include monthly chewable tablets, monthly topical products and long-acting injections. Many products also control certain intestinal worms, fleas or ticks. Your veterinarian chooses based on your dog’s age, weight, health, other parasite risks and your routine.

The AHS recommends “think 12”: test every 12 months and give prevention 12 months a year. Heartworm has been diagnosed in all 50 states, risk changes from year to year, and infected mosquitoes can come indoors. The Companion Animal Parasite Council (CAPC) also publishes heartworm guidelines and prevalence maps for veterinarians.

Heartworm in cats

Cats are atypical hosts: most worms do not reach adulthood, but even immature worms cause a lung condition known as heartworm associated respiratory disease (HARD). The AHS notes that the drug used to treat dogs cannot be used in cats, so prevention is the only protection.

Prevention checklist

• Heartworm test done as scheduled
• Prescription preventive chosen with your veterinarian
• Dose given on the same date every month (or injection schedule recorded)
• Reminder set on your phone or calendar
• Doses given year-round, including winter
• Missed doses reported to your vet
• Annual retest booked

Common mistakes

Mistake	Risk
Stopping prevention in winter	Gaps in protection
Buying preventives without a vet prescription or test	Unsafe use in infected dogs; counterfeit products
Skipping the annual test	Late diagnosis
Using “natural” heartworm remedies	No proven protection
Relying only on mosquito repellents	Reduces bites, does not replace preventives

Life after treatment

Successful treatment kills the worms, but the AHS notes that damage to the heart, lungs and arteries can affect a dog’s health and quality of life long after the parasites are gone. After treatment:

• follow the exercise restriction for the full period your veterinarian sets;
• give all medications exactly as prescribed;
• keep follow-up appointments and tests;
• continue heartworm prevention every month, without gaps;
• watch for coughing, breathing changes or fatigue and report them.

Traveling and heartworm risk

The AHS points out that you may unknowingly travel with your pet to areas where heartworm is more common, and that relocated pets have spread the disease to new areas. If you travel or adopt a dog from another region, talk to your veterinarian about testing and make sure prevention is current before you go.

Questions to ask at your dog’s heartworm visit

• When is my dog’s next heartworm test due?
• Which preventive fits my dog best, and what else does it cover?
• What should I do if I give a dose late or my dog vomits it?
• Does my dog’s travel or lifestyle change its risk?
• If my dog tests positive, what are the next steps and costs?

Write the answers down and set reminders for every dose and the next test.

Key takeaways

Heartworm is spread by mosquitoes, silent early, hard to treat late and easy to prevent. Test every year, give prevention every month of the year, and talk to your veterinarian about any missed dose.

Cost of prevention vs. treatment

Monthly prevention costs far less than treatment, which involves diagnostics, a series of injections, hospital visits, medications and weeks of restricted activity. Ask your veterinarian for a written estimate of both if you are deciding.

Related: flea and tick prevention for dogs, kennel cough and the dog health guide.