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细菌耐药恐成为人类致命威胁

Clifton Leaf 2017年11月14日

耐药性是对全球健康和安全最严重的威胁之一,这个威胁很大程度上是人类造成的。

美国疾病中心(CDC)预计,未来12个月里,细菌或真菌感染对抗生素免疫的情况,将导致至少200万起病例和2.3万人死亡。不幸的是,这一问题毫无好转之势,还在不断恶化:如今在全球范围内,每年有70万人因为这样的耐药微生物而死亡;而英国政府委托的一项权威研究显示,到2050年,死亡数字会飙升到1,000万,甚至超过每年因癌症死亡的人数。

世界卫生组织(World Health Organization)警告称,耐药性——即通俗小报所说“超级细菌”的兴起——是“对全球健康和安全最严重的威胁之一”。而且不要搞错:这个威胁很大程度上是人类造成的。

在把问题归咎于人类之前,我们首先谈谈自然在其中扮演的角色。简而言之,问题在于大部分细菌极快的分裂速度,会不可避免地导致它们极快的演化速度。举个例子,一个大肠杆菌(E. coli)在合适的情况下,仅需7个小时就能分裂成拥有2,097,152个细菌的巨大菌落,而细菌每次分裂都有可能出现突变和适应性变化,尤其是当它们处于强大的选择压力之下时。

这里就要谈到我们自己了。在帮助细菌迅速演化上,我们人类起到了至少两大作用。首先,我们长期对病人过度和不恰当地开具抗生素处方。这些无效的治疗往往会让存活下来的细菌进化出耐药性,并将这种耐药性传递给后代。正如俗话所说:“杀不死你的,会让你更强大。”(看看这条可怕的时间轴,我们就能知道抗病菌株的诞生有多快。)

其次,另一种潜在的行为也推动了细菌的演化。我们惯于给牲畜喂食低剂量的抗生素——自20世纪40年代进入抗生素时代以来,农业领域已经这样做了近80年。

我们如何做出了这样的决定?这是一个曲折的故事。在9月出版的非凡新著《大鸡:抗生素创造现代农业、改变全球饮食的惊人往事》(Big Chicken: The Incredible Story of How Antibiotics Created Modern Agriculture and Changed the Way the World Eats)中,作者玛丽安·麦肯纳对此娓娓道来。

麦肯纳写道:“此刻,遍布这个星球大部分地区的大部分肉用家畜,在生命的大部分时间里都服用了抗生素,其消耗量多达每年63,151吨,这是养殖的辅助手段。”农民们发现,抗生素有助于“让饲料有效地转化为可口的肌肉”,于是他们这么用了。抗生素会被放在饲料或是水里,它们可以帮助牲畜抵御疾病,农民也能因此在畜棚里养上更多牲口,并把老式农业转换为现代的产业化模式。

麦肯纳表示,动物肠道内有了耐药细菌后,接下来的情况大抵如下:被拉进屠宰场后,消化道内的突变细菌有时就会“溅到肉上”。随后,这些肉上的细菌可能会被直接食用,或是被带到家里或餐厅的厨房,从而污染柜台、砧板或其他食物。最终,它们可能感染人类。

麦肯纳说:“这只是途径之一。另一种途径下,那些肠胃中的物质,那些耐药细菌,会通过粪便离开动物。”粪便会干燥,沾满细菌的尘土会随风飘散、渗入地下水,或是变成肥料撒到其他土地里。她表示:“总之,这些耐药细菌会通过多种方式进入环境,随后迁移到人类身上。更麻烦的是,它们携带的基因——那些控制耐药性的基因——可能还会分裂出来,并被其他细菌吸收。途径太多了,简直随心所欲。”

尽管在美国和欧洲,对牲畜使用抗生素来“促进生长”都不再合法,但农民仍然可以在牲畜群中用抗生素来防治疾病。麦肯纳表示,这里有个很广泛和“模糊的中间地带”,因为许多情况下抗生素产生的效果是一样的:“人们依旧在使用少量或小剂量的抗生素”,从而营造了耐药菌种的温床。她保守地表示:“如果我们对人类这样做,我们会认为这不妥当。”

本周二,世界卫生组织发表报告,呼吁终止抗生素在食品动物上的常规使用。医学期刊《柳叶刀》(The Lancet Planetary Health)也新发一篇分析,阐述了这种做法的危险性。(这篇文章也值得一读。)

对此,美国农业部(Department of Agriculture)发布了新闻稿作为回应,声称:“世界卫生组织的指导方针与美国政策不符,也没有可靠的科学依据。”你可以阅读完整声明,做出自己的判断。农业部提出批评的原因,部分在于世界卫生组织的建议所采用的一些论据,连世卫组织本身都认为“证据质量较低”。

我致电了农业部,希望能得到更加彻底的说明。不过目前为止,农业部还没有人正式对我公布任何信息。(财富中文网)

译者:严匡正 

During the next 12 months, the CDC estimates that at least 2 million illnesses and 23,000 deaths will be caused by bacterial or fungal infections that no longer respond to antibiotics. And this problem, unfortunately, is getting worse, not better: Across the globe, 700,000 now die each year from such drug-resistant microbes; by 2050, according to a formidable blue-ribbon study commissioned by the UK government, that figure could well soar to 10 million, surpassing even worldwide deaths from cancer.

Antimicrobial resistance—or the rise of “Superbugs,” as the tabloids call it—is “one of the most serious threats to global health and security,” the World Health Organization warns. And make no mistake: the threat is also, largely, human-made.

Before I get to our culpability on this front, let’s start with nature’s. The problem, in a nutshell, is the superfast division speed of most bacteria, which leads inevitably to a revved up process of evolution. Under the right circumstances, a single E. colibacterium, for instance, can divide into a 2,097,152–strong colony in a mere seven hours—and with each division comes the potential for mutation and adaptation, particularly if these organisms are exposed to strong selective pressures.

That’s where we come in. We mortals help push that fast evolutionary process into warp speed in at least two ways. First, we do it through our long practice of overprescribing and inappropriately prescribing antibiotics to patients. These ineffective treatments often leave in their wake surviving microbes that develop resistance to the drugs used and then pass along those adaptations to subsequent generations. As the saying goes: “Whatever doesn’t kill you, makes you stronger.” (To see how feverishly fast resistant strains can emerge, see this scary timeline.)

Secondly, we egg on evolution through another insidious process: routinely giving sub-therapeutic antibiotics to livestock—something that the agriculture industry has been doing for nearly eight decades, or since the age of antibiotics began in the 1940s.

How we came to do this is a twisting tale that science writer Maryn McKenna elegantly unspools in her extraordinary new book, Big Chicken: The Incredible Story of How Antibiotics Created Modern Agriculture and Changed the Way the World Eats, which was published in September.

“At this moment, most meat animals, across most of the planet, are raised with the assistance of doses of antibiotics on most days of their lives: 63,151 tons of antibiotics per year,” McKenna writes. Farmers began to use the drugs when they discovered that it helped “convert feed to tasty muscle more efficiently.” The drugs, which could be administered in both feed and water, helped shield the livestock from disease, which also allowed farmers to pack more animals into barns and transformed old-fashioned agriculture into its modern industrialized form.

Once resistant bacteria are in the gut of an animal, then one of several things happens, McKenna says: When the animal is taken to the slaughterhouse, the mutated microbes in their digestive tracts can sometimes “get splashed on the meat.” And then, those resistant bacteria on the meat might either be consumed directly or be carried into a home or restaurant kitchen, where they might also contaminate a counter, cutting board, or other food. Eventually, they can infect people.

“That’s one pathway,” she says. “Another is when those gut contents, those resistant bacteria, exit the animal through manure.” That waste can dry up, leaving its bacteria-strewn dust to be blown away by the wind, or it can seep into groundwater, or be sprayed as fertilizer onto other fields. “So in a variety of ways,” she says, “these resistant bacteria make their way into the environment and they can then migrate to people in that manner. Or more troubling, the genes that they contain—the genes that control those processes of becoming resistant—can break free of the bacteria and be taken up by other bacteria. It’s a choose-your-own-adventure set of pathways.”

While it’s no longer legal in either the United States or in Europe to use antibiotics for “growth promotion” of livestock, farmers can still rely on them to prevent or control disease in a flock or herd. And herein lies a very broad and “mushy middle,” says McKenna, with the effect, in many cases, being the same: “It’s still using smaller-than-treatment doses, or sub-therapeutic doses of antibiotics,” which creates a literal breeding ground for resistant microbial strains. “If we did that in humans, we would call it inappropriate,” she says with understatement.

On Tuesday, the WHO issued a report calling for the end to the routine use of antibiotics in food-producing animals, which was accompanied by a fresh analysis of the dangers of this practice in The Lancet Planetary Health. (Also worth reading.)

The U.S. Department of Agriculture responded with a press release of its own, stating: “The WHO guidelines are not in alignment with U.S. policy and are not supported by sound science.” You can read the agency’s full statement here and decide for yourself. Part of their criticism is that some of the World Health Organization’s recommendations are supported by what the WHO itself terms “low-quality evidence.”

I called the USDA looking for a more thorough explanation than what’s provided here. But no one at the department was able to speak to me on the record.

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