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What To Do If You Miss a Heartworm Preventative Dose

By JoAnna Pendergrass, DVM
published: February 12, 2019 - updated: February 28, 2022 • 3 min. read
heartworm and dogs

Key Takeaways

  • Many pet parents accidently miss a dose of heartworm medication, but don’t panic!
  • Your dog’s risk of developing heartworm depends on when the dose was missed, how many doses were missed, the medication used and how common heartworm is in your area.
  • If it has been less than two weeks since the missed dose, give your dog the dose immediately.
  • If it has been over two weeks since the missed dose, contact your vet, who will likely advise you to resume normal dosing next month and have the dog tested for heartworm in six months.
  • Heartworm has been found in every U.S. state.

”Oh no! I missed my dog’s heartworm preventative dose!” Many pet parents have felt this sense of panic at one point or another. Despite their best intentions, even the most attentive and diligent pet parent can occasionally forget to give their dog their monthly heartworm preventative. If this has happened to you, realize that you are not alone.

Fortunately, missing a dose of heartworm preventative does not automatically mean that your dog is going to get heartworm disease. Several factors, which will be discussed later in the article, influence a dog’s risk of getting heartworm disease if a heartworm preventative dose is missed. First, let’s review what heartworm disease is all about and the rationale for monthly doses of heartworm preventative.

Heartworm Life Cycle

The dog heartworm, Dirofilaria immitis, is a parasite that is transmitted by mosquitoes. If a mosquito bites a dog that has heartworms, the mosquito will suck up microfilariae, which are baby heartworms produced by female adult heartworms. The microfilariae develop into larvae in the mosquito within 10 to 14 days. The mosquito will then inject the larvae when it bites another dog.

Heartworm larvae take about 6 months to fully mature into adults. Adult heartworms, which grow to nearly 12 inches long and live for several years, take up residence in the heart, lungs, and blood vessels that run between these two organs. Heartworms cause serious respiratory and cardiac problems and can damage other organs in the body. Heartworm disease is very expensive to treat and is often fatal if left untreated.


Why Give a Heartworm Preventative Every 30 Days?

Heartworm larvae are initially susceptible to heartworm preventatives. Once the larvae start maturing into adult heartworms, however, the preventatives are no longer effective. The common consensus is that it takes about 40‒50 days for heartworm larvae to mature to the point where heartworm preventatives no longer work.

So, from the initial mosquito bite to the beginning of maturation, there is an approximate 45-day grace period that a heartworm preventative can provide protection. Because administering a preventative every 45 days is hard to remember, the easy-to-follow recommendation is to give the heartworm preventative every 30 days.

What To Do If You Miss a Dose

Now, back to the issue of missing a heartworm preventative dose. The consequences of missing a dose depend on several factors:

  • When the dose was missed
  • How many doses were missed
  • Which heartworm preventative is being used
  • The prevalence of heartworm disease where a dog lives or has recently traveled to

As you can see, several factors need to be considered to determine whether your dog is at risk of developing heartworm disease after missing a dose of preventative. Your veterinarian can help you determine your dog’s individual risk.

Below are guidelines on what to do you if you miss a dose, based on how long it’s been since the dose was missed:

1.) Less than 2 weeks: Give your dog the missed dose immediately. Your dog will be protected from heartworm disease because you will still be within the 45-day grace period in which the preventative is effective. Resume your normal monthly dosing schedule.

2.) More than 2 weeks: Contact your veterinarian because you are likely outside of the grace period. Your veterinarian will advise that you give your dog the missed dose right away, then resume your normal monthly dosing schedule. They will also advise that you get your dog heartworm tested in 6 months; because it takes about 6 months for heartworm larvae to mature into adults, the test will not be effective until at least 6 months after your dog has been infected with heartworms.

Bringing it Together

Heartworm is a serious but preventable disease. Giving your dog a monthly heartworm preventative year-round (yes, even in the winter!) is one of the most effective ways to prevent heartworm disease. If you miss a dose, be proactive about addressing the problem. Whether the missed dose is less or more than 2 weeks late, contacting your veterinarian is always advisable to be sure of what to do next. Remember: heartworm has been found in every state in the US. Find out more about the disease here.

The content is not intended to be a substitute for professional veterinarian advice, diagnosis, or treatment. Always seek the advice of your veterinarian or other qualified health provider with any questions you may have regarding a medical diagnosis, condition, or treatment options.

joanna pendergrass
By JoAnna Pendergrass, DVM

JoAnna Pendergrass, DVM, is a veterinarian and freelance medical writer in Atlanta, GA. After graduating from the Virginia-Maryland College of Veterinary Medicine with her veterinary degree, JoAnna completed a 2-year research fellowship in neuroscience at Emory University. During this fellowship, she learned that she could make a career out of combining her loves of science and writing. As a medical writer, JoAnna is passionate about providing pet parents at Healthy Paws with clear, concise, and engaging information about pet care. Through her writing, she strives not only to educate pet parents, but also empower them to make good health decisions for their pets. JoAnna is a member of the American Medical Writers Association and Dog Writers Association of America.

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