Most pet parents know that it’s important to give heartworm prevention each month. Still, there are cases reported in all fifty states. Though it’s not likely, it is possible for pets to get heartworm disease even while on a preventative. Here’s what you should know about the efficacy of heartworm preventatives, and how you can work with your veterinarian to keep your pet healthy.

How Heartworm Preventative Works
Your cat or dog can only contract heartworm disease from a mosquito bite. They cannot get it from another animal with heartworm because the microfilaria (baby larvae) can only mature into infective larvae inside mosquitoes. When your pet is bitten, the mosquito passes larvae into their bloodstream.
When your pet takes their heartworm prevention tablet or gets their topical spot-on treatment, the active ingredient enters their bloodstream and kills any larvae that your pet may have contracted in the prior 30 days. Heartworm preventatives do not kill adult heartworms. It takes as little as 51 days for larvae to mature into adults, and about six months for pets to show symptoms of an infection.
By giving your pet their heartworm prevention each month, or by using another approved preventive such as an injection from your vet, you eliminate any larvae your pet may have picked up in the previous month.

Why Pets Get Heartworms While On A Preventative
It’s possible, but not likely for your pet to develop heartworm disease if you give them a preventative. The most common reasons pets get heartworm is because of a skipped or late dose.
Remember, heartworm preventative only kills larvae in your pet’s bloodstream. Since it takes just a little over a month for heartworm larvae to mature into adults, missing a dose by even a few weeks can leave your pet vulnerable to infection.
Some pets get heartworms when their preventative when they do not get the full dose each month. Your pet may vomit outside without your knowledge, or they may spit out their tablet if you’re not watching them closely after giving it to them. A topical spot-on treatment can rub off if you do not administer the whole dose directly onto your pet’s skin. Or, if your pet gets wet less than 24 hours afterwards, perhaps by getting a bath, taking a swim, or getting caught in the rain, the treatment can rinse off before it’s absorbed into the bloodstream.
Heartworm preventatives come with a guarantee. If your pet gets heartworm disease while they are on it, and you’ve been using it as directed, the manufacturer most likely has a guarantee program and will pay for their treatment.

Are Heartworms Becoming Resistant To Preventatives?
There have been a growing number of cases of pets getting heartworm disease even while taking their preventative as directed. It’s possible that a very small percentage of heartworm larvae are resistant to the active ingredients in preventatives.
The majority of breakthrough cases involve large, outdoor-only dogs in areas with high mosquito populations. Those cases have been mild, and it seems that the preventative still successfully kept the numbers low. So while preventatives are not 100% effective, they are still incredibly reliable for protecting your pets against fatal infection.
A heartworm infection takes at least six months to show up on a blood test. If you are taking your pet to the veterinarian each year for their wellness exam and heartworm test, and using prescribed prevention as directed, you can rest assured that your pets are safe.