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I Forgot to Give My Dog Their Heartworm Pill, What Should I Do? Our Vet Answers

Written by: Dr. Rachel Ellison DVM (Veterinarian)

Last Updated on June 19, 2024 by Dogster Team

Owner giving medicine in a pill to his dog

I Forgot to Give My Dog Their Heartworm Pill, What Should I Do? Our Vet Answers


Dr. Rachel Ellison  Photo


Dr. Rachel Ellison

DVM (Veterinarian)

The information is current and up-to-date in accordance with the latest veterinarian research.

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Heartworm disease has been found in dogs in all 50 of the United States as well as in many other countries throughout the world. Thankfully, this disease is preventable with routine medications, but sometimes, even the most conscientious and responsible dog owner may forget their dog’s heartworm preventative at one point.

If this happens to you, what should you do? In this article, we’ll explore this concept as well as a little about prevention and heartworm disease in general.

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I Missed a Dose, What’s Next?

If it has been 2 weeks or less, give the single dose of preventative right away. We’ll discuss more of the specifics of why later in this article. Contact your veterinary office to determine if any additional steps will need to be taken and what to do going forward. If it has been more than two weeks, this lapse increases the likelihood that an infection can occur. And the longer a dog is off their routine preventative, the higher the risk of them getting heartworm disease. At this point, you’ll want to contact your veterinarian promptly.

As far as the next steps, they’ll consider several factors, such as the preventative, its historical dosing, how long it has been since the last dose, the area and seasonality in which you live, and the prevalence of heartworm where your dog is located or has traveled.

Depending on the timing of how late the dose is (e.g., is it 3 weeks late versus 7 months late?), the veterinarian may want to run a heartworm test prior to starting preventative again. Regardless, you can expect your veterinarian to schedule a heartworm test 6–7 months out from this missed dose. This is because standard veterinary heartworm tests can only detect infection about 6 months after it has already started.

If the dog did develop an infection during the missed dose of preventative, the purpose of this test is to catch that infection before it continues to grow and causes more harm to the dog. There may also be some circumstances, depending on various factors, in which a veterinarian may consider adding an additional temporary medication to the routine preventative, such as the antibiotic doxycycline, which can help kill young heartworms.

Young veterinarian with medical document touching dog neck and cuddling it during appointment
Image Credit: Pressmaster, Shutterstock

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What Is Heartworm Disease?

Heartworm disease is a serious and potentially life-threatening disease for dogs. The parasite that causes heartworm disease is Dirofilaria immitis. They spread via mosquitoes to their definitive host, the domestic dog. Other species of animals can also be infected, such as cats and ferrets.

Adult heartworms typically like to live in a dog’s heart, lungs, and the blood vessels near those areas. Full-grown male worms can be 5–6 inches in length, while a female worm can be 10-12 inches long! The presence of heartworms can result in an affected dog having heart failure, lung disease, and other associated health problems, including death.

Heartworm Transmission

The transmission of heartworm disease is a bit of a merry-go-round situation. Ultimately, heartworm disease is spread to dogs by an affected mosquito’s bite, which results in heartworms wreaking havoc on that dog’s heart, lungs, and body. The newly infected dog then becomes the source for other mosquitos to infect other dogs.

A mosquito that feeds on a heartworm-infected dog ingests immature heartworms, called microfilaria. Once the microfilariae are in the mosquito, they then mature into larvae. As time goes on, these affected mosquitos then transmit the larvae to a new dog that they feed on, which causes a new cycle of infection.

Once the larvae are in the dog’s body, they move through tissue and eventually make it to the bloodstream. Their next travel destination is to the dog’s heart and lungs where they set up camp and then grow into adult worms. These worms then start to make new microfilaria 6–9 months after initial infection. An adult worm can live for up to 5–7 years in a dog!

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How a Preventative Works

There are several different options for heartworm prevention in dogs. Some examples are monthly chewable tablets, monthly topical treatments, and an injectable preventative that, depending on the type, can be given every 6 or 12 months. Because heartworm prevention is available via prescription only, your veterinarian can discuss the pros and cons of each and give you recommendations based on your and your dog’s situation and lifestyle. Because this article is about monthly heartworm pills, we’ll focus on this preventative method going forward.

Chewable Pills

One thing to keep in mind about the monthly chewable pill preventive is that it actually works retroactively. It unfortunately does not prevent an affected mosquito from biting a dog, but rather, essentially kills any heartworm larvae that were picked up by the dog within the last month. With that being said, it can only work if dosed correctly (by the dog’s weight) as well as if given within a certain window. It has been found that within as little as 51 days, heartworm larvae that are present in the dog become young adult worms that are not as likely to be killed by the preventative.

labrador sniffing the chewable pill
Image Credit: Olya Maximenko, Shutterstock

When to Dose

There is some potential variance in this number, and it could even be a little less. This plays into the recommendation of giving the missed heartworm dose that is 14 days or less days late. Normal prevention is recommended every 30 days both for the best chance of efficacy and the ease with which to build a routine.

But 30 days plus 14 days is less than (around) 51 days. At a certain point, a heartworm infection will progress past the point where prevention alone can kill the parasites as they are too big. Then, an infection would continue unless the treatment plan changes. With that being said, following the manufacturer’s recommended dosing interval allows the chance for them to cover treatment should your dog be on it exactly as directed and still obtain heartworms.

Preventative treatment for our canine friends is recommended to start at 6–8 weeks of age. To limit exposure, the American Heartworm Society recommends year-round prevention after being started. To check that preventative is, in fact, working, routine testing for heartworm disease, even those on routine preventative, is needed. It takes 6 months after starting an infection before testing can pick up a positive result.

In addition, while correctly given preventatives are very successful at eliminating very early infection, they are not always 100% effective. Regardless of how, if a dog does become infected, you’ll want to know as early as possible to help limit the disease from becoming worse.

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Signs of Heartworm Disease

male veterinarian examining labrador retriever dog at vet clinic
Image Credit: SeventyFour, Shutterstock

Most dogs will show little to no signs in the early stages of infection. As the disease worsens, they may be more likely to show signs.

Signs can include:
  • Cough
  • Weakness
  • Exercise intolerance
  • Eating less
  • Weight loss
  • Swollen belly due to excess fluid from heart failure
  • Caval syndrome (worms that cause blood flow blockages that lead to collapse, difficulty breathing, pale gums, etc. )

Diagnosis and Treatment

Heartworm blood tests can be done to determine if a dog has heartworm disease. If a test does show up as positive, a different kind of test will need to be done to confirm there is an infection. If positive, bloodwork, urinalysis, and radiographs of the heart and lungs as well as potential ultrasound of the heart (echocardiogram) are also key tests to help determine the extent of the disease and other associated medical problems.

Treatment recommendations for veterinarians are given by The American Heartworm Society. These protocols contain many steps that can be tailored toward the affected dog.

Some elements of treatment can include:
  • Stabilization, if needed. This may include things like fluid therapy and medications such as steroids, as well as those to decrease fluid buildup in the body (diuretics), open up the blood vessels (vasodilators), and increase heart muscle contractions (positive inotropic drugs).
  • Heartworm preventative to kill immature worms and prevent a new infection from occurring.
  • Doxycycline, which is an antibiotic that kills bacteria that live in the heartworms, helps them live and reproduce.
  • Injections of melarsomine. This is a drug that kills adult heartworms.
  • Strict confinement and exercise restriction during treatment; this is imperative as exercise increases the likelihood of complications and can even cause death as the adult worms are dying and being cleared from the body.

Because treatment is expensive and comes with its own risks, the goal is always to try and prevent a case rather than to have to treat one.

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How Can We Prevent Forgetting Prevention?

dog owner talking to vet
Image Credit: SeventyFour, Shutterstock

In the event of a missed dose of preventative, think about what may work better for next time so it can be given on time. If needed, ask your veterinarian for ideas or help with this endeavor. Some may want or need multiple reminders.

Here are some common ideas that may help with compliance:
  • Reminder stickers to put on a physical calendar.
  • Putting a schedule on the fridge (or highly visible place) to cross off each month once given.
  • Reminders in phone.
  • Calendar reminders.
  • Email reminders (many preventative manufacturers even offer this for free).
  • Some veterinary practices offer phone calls, e-mails, or mail reminders.
  • Switching preventative type to one better suited to your lifestyle.

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A missed dose of heartworm preventative can pose a serious health risk to your dog. Because timing is important if this happens, contacting your veterinarian is never the wrong answer and, in cases of more than 14 days, the definitive answer. How long it has been since the dose was missed will dictate the next steps.

Going forward, ensuring you find a fool-proof method that reminds you of your dog’s timed preventative medication is essential and will be your best bet for preventing heartworm disease in your dog.

See Also: 

Featured Image Credit: Creative Cat Studio, Shutterstock

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