Summer evenings, backyard play and long walks are some of the best parts of life with a dog, and mosquitoes are part of that picture too. Heartworm prevention for dogs is a simple habit that helps keep those happy days going all year.
Heartworm disease can build quietly before any sign appears. This guide explains how the parasite develops, which risk factors raise a dog’s chances of infection and which heartworm symptoms to watch for. It also covers the types of prevention, what a heartworm test checks, what happens after a missed dose and what a positive test means.
Understanding Heartworm Disease
Heartworm disease is spread by mosquitoes. An infected mosquito passes larvae to the dog, and the larvae develop into adult worms that live in the heart and the blood vessels of the lungs. The AVMA reports that adult female worms can grow up to 12 inches long and that males are about half that length. Adult worms typically live 5 to 7 years, and untreated dogs can carry hundreds of them.
Because the worms live inside the heart and lungs, the damage is internal. Many dogs show nothing for a long time. By the time a cough or tiredness appears the infection has usually been present for months. That is why prevention and testing matter more than watching for signs.
How Dogs Contract Heartworm
Mosquitoes carry heartworm, so any dog in an area with mosquitoes is at risk. A dog does not have to be outside for long. The AVMA recommends year round protection for all dogs, and the American Heartworm Society, or AHS, agrees.
How the Heartworm Life Cycle Works
The life cycle explains the testing and prevention schedule. Larvae enter the dog through a mosquito bite and mature over several months. Once adult worms are present they produce microfilariae, which circulate in the blood and can infect new mosquitoes that feed on the dog.
The AVMA notes that the 7 month point represents the time when a newly infected dog will have detectable adult worms and microfilaria. This is the reason testing starts for dogs 7 months and older. A test given earlier can miss a recent infection.
Preventives work on the early stage. The AVMA explains that they kill immature larvae before they grow into adult worms. They are not a treatment for adult worms, which is why a dog must test negative before prevention starts.
Identifying Heartworm Risk Factors
The AVMA states that heartworm has been found in all 50 states, so there is no safe region. Risk rises with:
- Living in or traveling to areas with many mosquitoes
- Spending time outdoors during peak mosquito hours
- Missed or late doses of prevention
- Gaps between preventive products
- Skipped annual testing
The AVMA notes that the most reliable protection is continuous year round prevention. A dog that is protected eleven months out of twelve still has a month with no defense.
Recognizing Heartworm Symptoms
Many dogs show no signs early. When signs appear the American Heartworm Society’s canine guidelines list cough and exercise intolerance and abnormal lung sounds in moderate disease. In severe disease they add difficulty breathing and abnormal heart and lung sounds. Owners often notice a mild cough or less interest in exercise first.
The AHS also describes caval syndrome, an acute event in heavily infected dogs. It causes sudden severe lethargy and weakness and dark urine. Caval syndrome is an emergency, so call your veterinarian or an emergency clinic at once. For more on heartworm and its effects, see this guide on how to protect your pet from heartworms.
What a Heartworm Test Checks
The AHS recommends annual screening for all dogs over 7 months. The guidelines describe an antigen test, which detects proteins from adult female worms, and a microfilaria test, which looks for the larvae in the blood. A small blood sample is enough, and results are often available during the visit.
The AVMA adds three testing points. Dogs should be tested immediately before starting prevention. Dogs should be tested again before restarting prevention after missed doses. Annual testing is recommended even for dogs on year round prevention. Giving a preventive to a dog that already has adult worms is not a safe plan, which is why the test comes first.
Year Round Heartworm Prevention for Dogs
The AHS recommends year round prevention, and the AVMA agrees. Prevention only works when it is given on schedule. A missed or late dose leaves a gap that larvae can use.
Types of Prevention
| Type | How it is given | What the sources say |
| Oral | Monthly chew or tablet | Prescription medicine started after a negative test |
| Topical | Monthly, applied to the skin | Some products also treat fleas and ticks. Ask your veterinarian |
| Injectable | One injection that lasts 6 or 12 months | The FDA describes ProHeart 12 as an extended release injection for dogs 12 months and older |
Many monthly products also treat other parasites, so ask which ones your dog is covered for. You can learn more about flea and tick prevention as part of a full parasite plan.
What to Know About the Injection
The FDA states that ProHeart 12 is an extended release injectable of moxidectin for dogs 12 months of age and older and that it is the same formulation as ProHeart 6 with three times the moxidectin. The labeling says it should not be used more often than every 12 months and that dogs should be tested for existing heartworm infection first. It also warns that anaphylactic reactions, which are severe allergic reactions, can occur alone or with vaccines and that some have been fatal. In the field study of 593 dogs two treated dogs had hypersensitivity reactions within the first 24 hours.
An injection is convenient for owners who forget monthly doses, but it is not right for every dog. Your veterinarian reviews your dog’s health and history before recommending it and explains what to watch for on the first day afterward.
Annual Testing
Annual testing continues even when prevention is perfect. A missed dose or an unusual circumstance can leave a gap, and a test catches an infection while it can be managed. The AVMA recommends the test for dogs on year round prevention for this reason.
What Happens After a Missed Dose
If a dose is late or missed, call your veterinarian. The AVMA recommends testing immediately before restarting prevention after missed doses. Your veterinarian may also ask for a follow up test later, since a dog infected during a gap will not show a positive result right away. This is the reason the 7 month rule exists. Do not simply double the next dose without veterinary advice.
If Your Dog Tests Positive
A positive test is serious but it is not a reason to panic. The AHS guidelines describe a treatment plan chosen by the veterinarian. They state that exercise restriction begins early and becomes stricter as signs become more pronounced. They also state that a year round prevention program continues alongside treatment. Your veterinarian explains the steps, the timeline and the recheck tests. Because the plan is complex, prevention is far easier than treatment.
Reducing Mosquito Exposure
The AVMA suggests limiting outdoor time during peak mosquito hours and using veterinarian approved mosquito repellents. These steps add a layer of protection but do not replace medicine. Ask your veterinarian before using any repellent on your dog, since some products made for people or other pets can harm dogs.
The Veterinary Role in Pet Protection
Your veterinarian does more than write a prescription. The team runs the annual test and chooses a preventive that fits your dog’s age and health and lifestyle. They explain how to give it and what to do about missed doses. They also check your dog for related signs at each visit. For why those routine visits matter, see this piece on why regular vet checkups are the best gift for your pet’s health.
What the Sources Cannot Tell You
The AHS guidelines are from 2018 and the AVMA page is not dated, so check with your veterinarian for current advice. The sources do not rank preventive products against each other or state a cost. The FDA labeling describes ProHeart 12 only, so ask your veterinarian about other injections. The sources also do not tell you the risk level for your own neighborhood.
Keeping Your Dog Protected Every Month of the Year
Heartworm disease is preventable. Year round prevention and a yearly test give your dog the best protection, and the habit takes only a few minutes each month.
At Kainer Veterinary Hospital, our team provides heartworm testing and personalized prevention plans for dogs of every age. Contact us today to schedule your dog’s heartworm test and choose the right prevention.
Sources
American Veterinary Medical Association, Heartworm Disease (accessed 2026).
Professional veterinary association.
Supports the 50 state statement, worm size and lifespan, how preventives work, the 7 month testing point, testing before and after missed doses, annual testing and the mosquito steps.
American Heartworm Society, Current Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm Infection in Dogs (2018).
Professional society guidelines.
Supports annual screening over 7 months, year round prevention, the antigen and microfilaria tests, the signs by severity, caval syndrome and the exercise restriction and continued prevention during treatment.
US Food and Drug Administration, FDA Approves ProHeart 12 (moxidectin) for Prevention of Heartworm Disease in Dogs (accessed 2026).
US government.
Supports the injectable description, the age limit, the testing requirement, the anaphylaxis warning and the field study figure.
FAQs
Q: Is heartworm prevention really necessary for all dogs
A: Yes. Heartworm occurs in all 50 states, and the AHS recommends year round prevention for all dogs. Indoor dogs can still be bitten by mosquitoes.
Q: What are the common types of heartworm prevention for dogs?
A: Oral chews and tablets and topical products and injections. Your veterinarian chooses based on health and lifestyle, and the FDA labeling for ProHeart 12 requires a negative test first.
Q: Why does my dog need a heartworm test if it takes prevention every month?
A: The AVMA recommends annual testing even for dogs on year round prevention. A missed dose or an unusual circumstance can leave a gap, and testing finds an infection early.
Q: At what age should my dog start heartworm testing?
A: The AHS recommends annual screening for dogs over 7 months, because a recent infection may not show on a test sooner. Your veterinarian will plan prevention for a puppy and explain when the first test is due.
Q: What should I do if my dog misses a heartworm dose?
A: Call your veterinarian. The AVMA recommends a test immediately before restarting prevention after missed doses. Do not double the next dose without veterinary advice.
