Heartworms are visible via ultrasound in some cases. You will not see them because they live inside the heart and the pulmonary arteries, so you’d need some advanced imaging to be able to see them.

It’s not painful, per se, but they feel sick, uncomfortable, and they're likely having difficulty breathing. They're not perfusing very well, so they don't feel well. Is it overt pain? I'm not sure.

A dog can show signs of heartworm as early as about six months. They typically don’t show any symptoms before that, as the infestation isn't mature enough at that point.

In a dog, typically, you're going to see coughing. And that's the most common sign. Sometimes, weight loss, you'll see that as well. Those are the two most common signs. Labored breathing, sometimes, as well. And then, as it advances, you can get heart failure and some other things, but those are the big signs. Anytime we have a dog coming in, coughing, gagging, and that's not on heartworm preventative, we highly recommend a test.

Dogs that are outside or in pens all of the time can be inundated with mosquito bites compared to a dog that's more indoors. Of course, your field dogs, your hunting dogs, things like that probably have a higher risk. All dogs have risks despite their lifestyle, but certain dog lifestyles are higher risk, such as predominantly outside and athletic dogs.

If your neighbor doesn't treat for heartworms, you're also at a bit more risk than if everybody around you treats for heartworms, which you don't know.

Heartworms are transmitted via a mosquito bite. And then, that mosquito injects baby worms into the dog's skin. And they migrate into the dog's skin for about two months till they hit the bloodstream. And then, they end up in the pulmonary arteries or arteries of the lungs seven months later. That tissue stage, or those first two months, is where the heartworm preventatives destroy that infection before it gets any further. That's the only time that those are an effective heartworm control or prevention.

Give the next regularly-scheduled dose. Once a dog gets infected, it takes seven months for that infestation to mature. Keep giving your regular heartworm preventative as directed, and then, seven months later, get another test. And if it's negative, great, your dog is probably fine.