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.