 Wow. But it can be prevented? Yes. Talk to your veterinarian. Heartworm disease does occur in cats, even indoor cats. You say, well, how do cats or dogs get heartworm through mosquitoes? How often have you seen a mosquito in your house? It does happen. And it happens depending on where you live. Likely all the time the mosquitoes get in. And all it takes in cats is one individual heartworm.