Can a dog on monthly heartworm preventative still get heartworms? If so, how likely is this to happen?

Answers:
I work in a vet's office. We have had 2 dogs this year come up positive for heartworm despite being on preventative. That does not include the dogs who are not being given any type of preventative.

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It is possible, but extremely rare. The most common cause for this happening is either a late/missed dose or the dog goes off after you give him the preventative and throws it up. I don't know what type of prevention you use, but they all have websites as well as hotlines that you can call.
Most of them will guarantee their product and will pay for treatment if your dog does become heartworm positive. As long as you bought the product directly from a veterinarian. Talk to your vet about this guarantee if you have any questions.

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Yes, but if your vet is worth anything, it's not bloody likely.

Heartworm meds are actually a small amount of poison meant to kill the worms before they become adults and the dose is dependent upon your dog's weight. If your dog is under-dosed then the poison in his system isn't strong enough to kill the critters before they can reach maturity.

On rare occasion, your dog can get a strain of heartworms that is resistent to the formula given. Also, if you miss a dose and then start back up, all of the worms that grow inside his system will have some resistance to the regular formula that you give your dog.