As a lifelong dog lover, my pups are my world. I‘d do anything to keep them healthy and happy. But I‘m only human, and sometimes, despite my best intentions, I make mistakes. Last year, in the chaos of a cross-country move, I realized to my horror that my dog Max had gone two months without his heartworm prevention pill. I felt terrible and terrified that my slipup would have devastating consequences.
If you‘ve ever forgotten a dose (or two) of your dog‘s heartworm med, you know the feeling. That sinking sense of guilt, the fear that you‘ve let your best friend down and put their health at risk. It‘s an awful situation, but the good news is, it doesn‘t have to be a catastrophe. With quick action and guidance from your vet, you can get your pup protected and back on track.
Understanding the Enemy: Heartworm 101
Heartworms are foot-long parasites that live in the heart, lungs, and blood vessels of infected animals. They‘re spread by mosquitos and can affect dogs, cats, ferrets, and even wild mammals like foxes and coyotes. Heartworm disease is serious, progressive, and potentially fatal if untreated.
Here‘s a quick breakdown of the heartworm life cycle:
- A mosquito bites an infected animal and ingests baby worms (microfilariae) circulating in the bloodstream.
- The microfilariae develop into infective larvae inside the mosquito.
- When the mosquito bites another animal, it deposits the larvae into the skin.
- The larvae migrate through the tissue and enter the bloodstream, eventually reaching the heart and lungs.
- Over 6-7 months, the larvae mature into adult worms up to 12 inches long.
- The worms mate and produce new microfilariae that circulate in the blood, restarting the cycle.
As the worms clog the heart and pulmonary arteries, they cause inflammation, scarring, and decreased blood flow. The dog‘s heart has to work harder and harder to pump blood, leading to fatigue, coughing, and eventually, congestive heart failure.
Symptoms usually don‘t appear until the disease is quite advanced, and can include:
- Mild, persistent cough
- Reluctance to exercise
- Fatigue after moderate activity
- Decreased appetite
- Weight loss
- Swollen belly (from fluid buildup)
- Labored breathing
- Pale gums
- Dark, bloody stool
- Fainting or collapse
If caught early, heartworm disease can be treated, but it‘s a long, difficult process. The dog must receive a series of injections of an arsenic-based drug to kill the adult worms, then be kept on strict crate rest for 4-6 weeks to minimize the risk of dying worms causing blockages. They need frequent vet checks and may require additional medications to manage complications. It‘s very hard on the dog – physically and emotionally – and there‘s always a chance that the treatment itself could be fatal.
How Prevention Works (When You Remember It)
The good news is, heartworm disease is almost 100% preventable with the consistent use of preventative medications. These meds don‘t actually stop mosquitos from biting or injecting larvae, but they do kill off any immature worms that have been introduced in the past month, before they can develop into adults.
Most preventatives contain macrocyclic lactones (usually ivermectin, milbemycin, or moxidectin) that paralyze and kill microfilariae and larvae by disrupting their nervous system. The meds are dosed according to weight and are available as monthly chewable pills, topical liquids applied to the skin, or injections given every 6-12 months.
The key is to give the preventative on a strict 30-day schedule to make sure there‘s no lapse in protection. Even being a few days late can open a window for larvae to mature past the stage where the medication can effectively kill them.
According to the American Heartworm Society, heartworm preventatives are "highly effective, safe, easy to use, and inexpensive compared to treating a dog or cat after worms have matured." The organization recommends year-round prevention for all dogs and cats, even those who spend most of their time indoors.
What Happens When You Miss a Dose (or Two)
I can tell you from personal experience, realizing you‘ve forgotten to give your dog their heartworm pill is a terrible feeling. Your mind immediately goes to the worst case scenario – your beloved pup sick, suffering, dying from a totally preventable disease. The guilt is overwhelming.
But here‘s what you need to know: While heartworm disease is very serious, one or two missed doses doesn‘t mean your dog is doomed. The risk of infection depends on several factors, including:
- How long it‘s been since the last dose
- The time of year (mosquitos are more active in warm months)
- Your geographic location (heartworm is more prevalent in humid areas)
- Your dog‘s individual risk (dogs who spend a lot of time outdoors are more likely to be bitten)
In general, if you realize you‘ve missed a dose within 30-45 days of when it was due, you can simply give the missed dose immediately and resume the regular schedule. The American Heartworm Society states that "heartworm preventives will protect a dog even if the owner forgets and is a week or two late."
However, once you hit that 45 day mark, the risk becomes more significant. A 2017 study in the Journal of Veterinary Parasitology found that after about 60 days without prevention, any larvae introduced have time to develop into "late stage" larvae that are resilient to preventative medications.
This is what happened in my case with Max. By the time I realized my error, it had been about 65 days since his last dose. I knew I needed to act fast.
Treating a Prevention Lapse
If you‘ve missed two or more doses of heartworm prevention (around 60 days), here‘s what you should do:
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Give your dog their regular preventative dose immediately. Don‘t double up or try to "catch up" with multiple doses, as this could be dangerous. Just resume the normal schedule.
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Call your vet and explain the situation. Be honest about exactly how long it‘s been since the last dose. Your vet may recommend additional precautions based on your dog‘s individual risk factors. In some cases, they may prescribe a course of doxycycline (an antibiotic) to be given along with the preventative to stop any larvae from developing further.
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Schedule a heartworm test for 6 months from now. Heartworm tests look for proteins released by adult female worms, so they can only detect infections that are at least 6-7 months old. If your dog does have larvae from the missed doses, this test will catch them before they can do too much damage.
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Watch your dog closely for any signs of illness. Coughing, fatigue, weight loss, or changes in behavior could indicate a developing heartworm infection. Alert your vet right away if you notice anything concerning.
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Make a plan to stay on track with prevention going forward. If remembering monthly doses is a struggle, consider switching to an injectable preventative that only needs to be given every 6-12 months. Set reminders in your phone, mark your calendar, or sign up for alerts from your vet or the medication manufacturer.
In my case, Max tested negative at his 6-month follow up, and I was able to breathe a huge sigh of relief. He‘s been happily healthy since, and you can bet I have multiple safeguards in place to make sure I never forget his prevention again!
The Big Picture: Heartworm in 2024
Like many parasitic diseases, heartworm isn‘t going away anytime soon. The latest data map from the Companion Animal Parasite Council shows infections diagnosed in every state, with the highest prevalence in the Southeast and along the Mississippi River. Overall, positive heartworm tests increased by 16.7% between 2022 and 2023.
Several factors are driving this spread, including:
- Warming temperatures expanding the range of mosquito species
- Increased pet travel introducing heartworm to new areas
- Inconsistent use of preventatives, often due to financial constraints
- Growing mosquito resistance to common insecticides
Dr. Chris Rehm, President of the American Heartworm Society, emphasized the importance of prevention in a 2024 statement, saying:
"No matter where you live or how your pet spends their time, heartworm prevention should be a year-round priority. Skipping doses or only giving prevention seasonally leaves pets vulnerable to a devastating disease. The cost of prevention is a fraction of the cost of treatment, and more importantly, it spares pets from significant suffering. We urge all pet owners to work with their veterinarian to find a prevention protocol that works for their lifestyle and budget."
Doing Right By Your Dog
As a dog lover, I know the unique bond we share with our canine companions. They give us their whole hearts, and it‘s our responsibility to protect those hearts however we can. Heartworm prevention is a critical part of that.
I‘ll be honest – heartworm preventatives aren‘t cheap, especially for large dogs. But when I think about how much Max means to me, and how awful it would be to see him go through heartworm treatment (or worse, lose him to the disease), the cost seems minor in comparison. Keeping him on a regular prevention schedule is a small price to pay for his health and happiness.
If you‘ve forgotten a dose, don‘t beat yourself up too much. It doesn‘t make you a bad dog owner. The fact that you‘re reading this article shows how much you care! Just get your pup back on track ASAP and commit to staying consistent in the future.
Life gets hectic sometimes, and it‘s easy for that monthly pill to fall off the radar. If you can build heartworm prevention into your regular care routine, it will soon become second nature. Maybe you pair it with your dog‘s bath time or trip to the dog park. Or you set an alert to go off when you‘re watching your favorite show. Find a system that works for you and stick to it.
At the end of the day, keeping our dogs healthy is one of the most important things we do as pet parents. We may not be perfect, but we can always strive to do better. Committing to consistent heartworm prevention is a surefire way to give your best friend their best chance at a long, fulfilling life. It‘s what every good dog deserves.
