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If you're a dog owner, chances are you've heard of heartworm disease.
如果你是一名狗主人，那么你可能听说过犬心丝虫。
And if you aren't familiar with it, well, brace yourself for some serious nightmare fuel.
如果你对它不熟悉，请做好做噩梦的准备吧。
It's basically exactly what it sounds like: an infestation of worms, collectively known as Dirofilaria, in a dog's heart.
这种疾病如其名：狗狗心脏内有蠕虫感染，统称为犬心丝虫。
These parasitic nematodes are transmitted by mosquitoes, and different species are found all over the world.
这些寄生线虫通过蚊子传播，且世界各地品种不同。
The most common in North America is Dirofilaria immitis.
在北美最常见的是犬心丝虫
The adult parasites live and reproduce in the dog's pulmonary arteries, eventually clogging the chambers in the right side of the heart.
成年寄生虫会在狗狗的肺动脉中寄生繁殖，最终堵塞心脏右侧的心室
This leads to all kinds of nasty complications like fatigue, coughing up blood, and ultimately, heart failure.
这会导致各种严重的并发症，如疲劳、咳血，最终导致心力衰竭。
And usually, if your dog has enough worms to show symptoms, getting rid of them is really hard.
通常，如果你的狗狗体内的蠕虫达到一定数量，并出现各种症状，要摆脱它们就变得非常困难。
That's why vets encourage pet owners to use preventative medications that decrease their risk of infection and lessen the spread of disease.
因此兽医鼓励宠物主人使用预防性药物降低它们感染的风险并减少疾病传播。
And it turns out that lowering the risk of transmission is good for dogs and people,
事实证明降低传播风险对狗狗和人类都有益，
because humans can be infected with heartworm, too!
因为人类也会感染心丝虫！
Now, before you freak out too much, heartworm infections in people don't cause symptoms nearly as bad as they do in dogs.
现在在你过于惊慌之前，心丝虫感染在人体内引起的症状并不像在狗体内那么严重。
And they're super rare. Fewer than one hundred twenty cases have been reported in the United States since 1941.
而且它们很罕见。在美国，自1941年起，报道病例不足120起。
The main difference is that our immune systems aren't as easily tricked by the worms.
主要的区别是我们的免疫系统不容易被蠕虫欺骗。
You see, when a mosquito bites an animal with a heartworm infection, they suck up microfilariae, the earliest larval stage of the worm.
当蚊子叮咬感染了犬心丝虫的动物时，它们会吸走微丝虫，这是蠕虫最早的幼虫阶段。
Those mature through their next larval stages inside the mosquito,
它们在蚊子体内的下一个幼虫阶段成熟，
and then migrate to its proboscis, the stabby part it uses to suck up blood, and arguably it's least endearing feature.
然后迁移到其喙部，就是蚊子用来吸血的刺身部分，可以说是最不讨喜的特征。
When that mosquito goes for another blood meal, be it from a dog or human, the larvae bust out and get onto the skin.
当这只蚊子吃下一顿的时候，可能是狗狗或人类，幼虫就会破茧而出，爬到皮肤上。
Then they, no joke, crawl around until they find a way in, like, the tiny hole made by the mosquito.
然后它们到处爬，直到它们找到了入口，比如蚊子叮咬的小洞。
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,
从那里，它们必须通过皮肤组织蠕动进入小血管，这样它们才能在血流中穿行，
eventually making their way to the pulmonary arteries in the lungs. Inside the body, they grow and mature for about 6 months,
最终到达肺部的肺动脉。在体内，它们会生长并成熟大约6个月，
until finally, the mature worms reproduce and release their little microfilariae
直到最终成熟的蠕虫繁殖并释放它们的小微丝回到血液中，
back into the bloodstream, starting the whole process over again.
重新开始整个过程。
Now, where dogs and humans differ is that, usually, the larvae never make it into the bloodstream.
现在狗狗和人类不同之处在于，通常幼虫无法进入血液中。
This is probably because the larvae transmitted by the mosquito are in their third stage of development, or the L3 stage.
这可能是因为通过蚊子传播的幼虫正处于它们发育的第三阶段（L3阶段）。
And studies of human infections with related species of nematodes have shown that our immune system
对人类感染相关线虫的研究表明我们的免疫系统
is really good at recognizing and mounting a response against parasites when they're at this stage.
其实很善于识别和对这个阶段的寄生虫做出反应。
In fact, this is probably why human heartworm infections are considered rare to begin with.
事实上，这可能是人类犬心丝虫很罕见的原因。
It's not that we're rarely infected, it's just that we rarely stay infected long enough for anyone to notice.
并不是人类很少感染，而是感染时间短到难以引起注意。
Though sometimes, a rogue larva does find its way into a person's lungs.
虽然有时幼虫确实会进入人的肺部。
But, even when this happens, the worm never gets a chance to grow and reproduce the way it would in dogs.
但即便真的如此，蠕虫从来没有机会像在狗身上那样生长和繁殖。
The immune system always spots it and sends cells to destroy it.
免疫系统总是会发现它并派出细胞摧毁它。
This destruction process forms nodules in the lung tissue, which is usually how we end up figuring out that someone had heartworm infection.
这种破坏过程会在肺组织中形成结节，这通常是我们最终确定某人患有犬心丝虫感染的原因。
These nodules, called coin lesions, are rarely harmful.
这些结节被称为硬币样，通常不会造成伤害。
They're mostly just annoying, because other, deadlier conditions also cause them, so doctors have to take them seriously.
它们大多只是令人讨厌，因为其他更致命的情况也会导致它们，所以医生必须认真对待它们。
As for why our immune systems always end up finding and killing those worms:
至于为什么我们的免疫系统总能发现并杀死这些蠕虫：
well, it probably has as much to do with bacteria as it does with the parasite itself.
这可能与细菌和寄生虫本身有同样多的关系。
Filarial nematodes like Dirofilaria have an intimate partnership with bacteria called Wolbachia.
像恶丝虫属这样的丝虫线虫有一个和细菌非常亲密的伙伴—沃尔巴克氏菌。
The bacteria live inside the worm's cells, and it's a mutually beneficial relationship.
这种细菌生活在蠕虫的细胞内，它们是一种互利的关系。
The worms provide the bacteria with amino acids for growth, while the Wolbachia are essential for the development of the worm's larvae.
蠕虫为细菌提供生长所需的氨基酸，而沃尔巴克氏菌是蠕虫幼虫发育的必需品。
And the bacteria also play an important role in the parasite's ability to infect a host animal.
并且这种细菌也在寄生虫感染宿主动物的能力方面起着重要作用。
Proteins produced by Wolbachia cause the host's immune system to start fighting a bacterial infection,
沃尔巴克氏菌生产出的蛋白质导致宿主的免疫系统开始抗击细菌感染，
a type of immune response called a Th1 response, even though there isn't a bacterial infection going on!
这是一种免疫反应，被称为Th1反应，虽然其实根本没有细菌感染！
Now, here's why this is important: Th1 responses are counterbalanced by immune responses called Th2 responses.
现在这一点重要的原因在于：Th1反应被称为Th2反应的免疫反应所抵消。
They basically do opposite things: Th1 responses promote inflammation, while Th2 responses dampen it.
它们做着相反的事情：Th1反应促进炎症，而Th2反应抑制炎症。
So, the presence of Wolbachia can shift the immune system toward more of a Th1 response.
沃尔巴克氏菌的存在可以使免疫系统产生更多的Th1反应。
But the Th2 responses are what our immune systems use to attack worms!
但是Th2反应是我们的免疫系统用来攻击蠕虫的！
So by inducing the Th1 response, the bacteria seem to essentially distract the immune system, allowing the worms to sneak around and proliferate.
通过引发Th1反应，细菌似乎从本质上分散了免疫系统的注意力，让蠕虫四处潜伏和繁殖。
Some scientists even think a strong Th1 response helps the larval worms grow and mature.
一些科学家甚至认为强烈的Th1反应帮助幼虫蠕虫生长和成熟。
Also, dogs really get the short end of the stick here,
狗在这方面确实吃亏了，
because studies have shown the Th1 response is increased when the host has more microfilariae in their system.
因为研究表明，当宿主体内有更多的微丝蚴时，Th1反应会增加。
So once a few worms have set up shop and started breeding, the dog's immune system gets even worse at fighting them off.
所以一旦一些蠕虫开始繁殖，狗狗的免疫系统在击退它们方面就会变得更差。
So humans can totally be infected with heartworm, but the reason the disease hits dogs way harder has to do
所以人类完全可以感染上心丝虫病，但是这种病更容易找上狗狗与
with how our different immune systems react to the worms and their bacterial allies.
人类免疫系统对这种蠕虫及其细菌盟友所产生的反应有关。
And that's why it's really important to talk to your vet about heartworm meds for your pets.
所以和你的兽医谈谈给宠物吃心丝虫药真的非常重要。
Your furry friends will breath a little easier; and so, presumably, will you.
你那些毛茸茸的朋友们会松一口气；想必你也是。
Heartworms are pretty nasty, so it's great that our bodies have figured out how to kill them.
心丝虫非常讨厌，所以我们的身体找到了杀死它们的方法，这很好。
But you might not want your body to get rid of every worm that wiggles its way in.
但你可能不想让你的身体摆脱每一条蠕动的虫子。
In fact, having worms may be a good thing for your immune system!
事实上，体内有蠕虫对你的免疫系统或许是件好事！
And we have a whole episode that explains why, which you can watch next.
我们有一期节目就是讲述其中原因的，大家可以去看看。
