What should I do if I miss a dose of my cat’s heartworm prevention? - Animal Hospital of Statesville
If you miss a dose, keep going. As soon as you realize you've missed it, go ahead and give the cat another one and just go on their regular monthly routine. If they are not showing any symptoms, great, we don't go any further with that.
If a cat does get an adult heartworm or two, there is no treatment available. They can't get the conventional treatment that dogs do. The medication is somewhat toxic in a cat. Then as the worm dies, the cat often dies as well, which is why prevention is so vital. If infected, the cat develops chronic asthma-like symptoms once infected with heartworm, and that lasts for life. That's treated with things like corticosteroids and bronchodilators as needed, but there's permanent damage done to the lungs.
What should I do if I miss a dose of my cat’s heartworm prevention? - The Drake Center
You should just go ahead and give the next dose. Alert your veterinarian that you missed a dose. If you live in an area with high incidence of heartworm disease, they might recommend a test. But you should just keep giving the dose. In fact, even if a cat has heartworm disease, we still keep them on the preventative because we're preventing more worms from developing. It's not wrong to give the next dose, but talk to your vet about it.