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If you're a dog owner, chances are you've heard of heartworm disease. And if you aren't familiar with it, well, brace yourself for some serious nightmare fuel.

It's basically exactly what it sounds like: an infestation of worms, collectively known as Dirofilaria, in a dog's heart. These parasitic nematodes are transmitted by mosquitoes, and different species are found all over the world. The most common in North America is Dirofilaria immitis.

The adult parasites live and reproduce in the dog's pulmonary arteries, eventually clogging the chambers in the right side of the heart. This leads to all kinds of nasty complications like fatigue, coughing up blood, and ultimately, heart failure. And usually, if your dog has enough worms to show symptoms, getting rid of them is really hard.

That's why vets encourage pet owners to use preventative medications that decrease their risk of infection and lessen the spread of disease. And it turns out that lowering the risk of transmission is good for dogs and people, because humans can be infected with heartworm, too! Now, before you freak out too much, heartworm infections in people don't cause symptoms nearly as bad as they do in dogs.

And they're super rare. Fewer than one hundred twenty cases have been reported in the United States since 1941. The main difference is that our immune systems aren't as easily tricked by the worms.

You see, when a mosquito bites an animal with a heartworm infection, they suck up microfilariae, the earliest larval stage of the worm. Those mature through their next larval stages inside the mosquito, and then migrate to its proboscis, the stabby part it uses to suck up blood, and arguably it's least endearing feature. When that mosquito goes for another blood meal, be it from a dog or human, the larvae bust out and get onto the skin.

Then they, no joke, crawl around until they find a way in, like, the tiny hole made by the mosquito. From there, they have to wiggle their way through the skin tissue to get into small blood vessels so they can travel around the bloodstream, eventually making their way to the pulmonary arteries in the lungs. Inside the body, they grow and mature for about 6 months, until finally, the mature worms reproduce and release their little microfilariae back into the bloodstream, starting the whole process over again.

Now, where dogs and humans differ is that, usually, the larvae never make it into the bloodstream. This is probably because the larvae transmitted by the mosquito are in their third stage of development, or the L3 stage. And studies of human infections with related species of nematodes have shown that our immune system is really good at recognizing and mounting a response against parasites when they're at this stage.

In fact, this is probably why human heartworm infections are considered rare to begin with. It's not that we're rarely infected, it's just that we rarely stay infected long enough for anyone to notice. Though sometimes, a rogue larva does find its way into a person's lungs.

But, even when this happens, the worm never gets a chance to grow and reproduce the way it would in dogs. The immune system always spots it and sends cells to destroy it. This destruction process forms nodules in the lung tissue, which is usually how we end up figuring out that someone had heartworm infection.

These nodules, called coin lesions, are rarely harmful. They're mostly just annoying, because other, deadlier conditions also cause them, so doctors have to take them seriously. As for why our immune systems always end up finding and killing those worms: well, it probably has as much to do with bacteria as it does with the parasite itself.

Filarial nematodes like Dirofilaria have an intimate partnership with bacteria called Wolbachia. The bacteria live inside the worm's cells, and it's a mutually beneficial relationship. The worms provide the bacteria with amino acids for growth, while the Wolbachia are essential for the development of the worm's larvae.

And the bacteria also play an important role in the parasite's ability to infect a host animal. Proteins produced by Wolbachia cause the host's immune system to start fighting a bacterial infection, a type of immune response called a Th1 response, even though there isn't a bacterial infection going on! Now, here's why this is important:.

Th1 responses are counterbalanced by immune responses called Th2 responses. They basically do opposite things:. Th1 responses promote inflammation, while Th2 responses dampen it.

So, the presence of Wolbachia can shift the immune system toward more of a Th1 response. But the Th2 responses are what our immune systems use to attack worms! So by inducing the Th1 response, the bacteria seem to essentially distract the immune system, allowing the worms to sneak around and proliferate.

Some scientists even think a strong Th1 response helps the larval worms grow and mature. Also, dogs really get the short end of the stick here, because studies have shown the Th1 response is increased when the host has more microfilariae in their system. So once a few worms have set up shop and started breeding, the dog's immune system gets even worse at fighting them off.

So humans can totally be infected with heartworm, but the reason the disease hits dogs way harder has to do with how our different immune systems react to the worms and their bacterial allies. And that's why it's really important to talk to your vet about heartworm meds for your pets. Your furry friends will breath a little easier; and so, presumably, will you.

Heartworms are pretty nasty, so it's great that our bodies have figured out how to kill them. But you might not want your body to get rid of every worm that wiggles its way in. In fact, having worms may be a good thing for your immune system!

And we have a whole episode that explains why, which you can watch next. [♪ OUTRO].