卡邁勒·納哈斯/阿西莫夫出版社
最致命的傳染病
持槍賭徒霍利迪醫生的「每一次連續的出血都讓他比以前更虛弱」，瑪麗·多莉亞·拉塞爾在她的同名書《醫生》中寫道。這本書出版於 1971 年，毫不留情地描述了霍利迪日益惡化的健康狀況。 「『你會習慣的』，醫生總是說。 「你可以習慣任何事」。習慣了撕心裂肺的疼痛；習慣了痛苦。習慣了突如其來的鐵和鹽的味道；他已經習慣了肺部血液上升時掙扎著吸入空氣。
醫生患有結核病，這是一種由結核分枝桿菌引起的細菌性疾病。這種致命且傳奇的疾病贏得了許多綽號——肺癆、肺結核，以及「白色瘟疫」或「白色死亡」，因為患病者的臉色蒼白。它殺死了弗雷德里克·蕭邦、亨利·大衛·梭羅、弗朗茨·卡夫卡和埃莉諾·羅斯福。據估計，在 19 世紀的鼎盛時期，結核病導致了歐洲和美洲四分之一的死亡。
愛德華·蒙克（1885-1886）的《生病的孩子》。蒙克的妹妹15歲時死於肺結核。
結核分枝桿菌具有複雜的生命週期。毫無戒心的人吸入細菌，細菌會進入肺部。稱為巨噬細胞的免疫細胞會迅速嘗試消滅微生物。但微生物是狡猾的獵物，通常會透過分泌引發細胞死亡的因子來殺死巨噬細胞。其他白血球聚集在外來入侵者周圍，形成稱為肉芽腫的物理屏障，試圖使病原體免疫。集群中的巨噬細胞繼續對抗細菌，而其他免疫細胞則到達以加強肉芽腫壁。
在此階段，疾病仍處於潛伏期，毫無戒心的帶菌者通常不會出現任何症狀。直到結核分枝桿菌從肉芽腫爆發出來才引起活動性疾病。隨之而來的是持續黏液、血性咳嗽、疲憊、體重減輕和發燒。結核病也會導致淋巴結腫大、骨關節疼痛或癲癇發作。
如今，結核病在西方相對不常見， 在西方，結核病大多被視為一種過去的疾病。但也許令人驚訝的是，結核病仍然是地球上最致命的傳染病。在全球範圍內，它每年造成 120 萬人死亡，其中大部分死亡發生在撒哈拉以南非洲和南亞。相較之下，瘧疾在 2022 年導致約 608,000 人死亡。
地區別之結核病死亡數
結核病仍然如此猖獗，因為它很難診斷。然而，準確的診斷是了解問題嚴重程度並及早治療結核病以阻止其傳播的第一步。那麼，為什麼我們沒有在開發更便宜、更有效的病例識別方法方面取得更多進展？
原因有很多。在醫療保健條件較差的偏遠地區，許多有症狀的結核病患者因費用、交通不便或缺乏電力進行檢測而無法診斷。研究人員估計，2019 年有 290 萬名結核病患者未被發現，但由於檢測不準確，這些估計結果並不確定。即使患者確實接受了檢測，當痰液樣本中的細菌含量太低而無法檢測到時，這些檢測也常常會產生假陰性。
此外，一些診斷結核病的工具也缺乏評估抗生素抗藥性所需的特異性。 2019 年，醫生無法為 39% 的新結核病病例制定有效的抗生素治療方案，因為他們所依賴的評估無法判斷菌株對哪些抗生素敏感。
在決定誰有資格進行結核病診斷時還有另一個挑戰：由於資源有限，醫護人員往往關注有疾病跡象的人，而不是可能攜帶潛伏細菌的看似健康的人。有鑑於此，醫生仍無法掌握無症狀傳播者傳播病原體的程度。
全球結核病新發個案估計值 (低估為第5百分位及高估為第95百分位)
2014年，世界衛生組織（WHO）提出了一項名為「終結結核病策略」的倡議，旨在到2030年將病例數減少80%，死亡人數減少90%，診斷/治療費用減少100%。遺憾的是，該倡議並未得到落實。
在這篇關於結核病的兩部分系列文章的第一篇文章中，我們深入探討了這種疾病的歷史及其管理不善，同時強調了部分阻止這種在西方持續流行的關鍵突破。
一種古老的疾病
結核分枝桿菌細菌存在的時間比我們存在的時間還要長。估計三百萬年前，這種疾病的早期形式感染了東非的類人猿。作為參考，最古老的人類物種——巧人（Homo habilis）——大約在兩百萬年前出現。目前的結核分枝桿菌譜系已經困擾我們人類 1萬5千年 至 2 萬年。考古學家發現了古埃及木乃伊攜帶病原體 DNA 的證據，其中一些木乃伊已經發展為波特氏症 (Pott’s disease)－結核病擴散到骨頭引起的脊椎彎曲。這種疾病也折磨著古希臘人和古羅馬人，希波克拉底指出：「那些吐出泡沫血的人是從肺部帶來這種疾病的」。
俄克拉荷馬州的一名年輕女孩患有骨結核。攝影：多蘿西婭·蘭格，1935 年。
工業革命期間，隨著人們開始擁擠在室內，結核病病例激增，空氣中的細菌可以在室內集中和傳播。各個社會階層的人們都死於這種疾病。當藝術家和詩人開始生病時，結核病甚至成為一種令人羨慕和令人垂涎的疾病。
「我看起來多麼蒼白啊！」，詩人拜倫勳爵寫道。 「我想，我希望死於肺病……因為那時女人們都會說，『看看那個可憐的拜倫——他死的時候看起來多麼有趣啊！』」。
當有些人忙於將這種疾病浪漫化時，有些人則在努力理解它。 1882 年，德國醫生羅伯特·科赫 (Robert Koch) 對取自動物肺部的結核腫塊進行染色並在顯微鏡下檢查，發現了結核病的細菌元兇。由此，「非常細的棒狀形狀變得明顯」，他寫道。識別病原體背後的細菌將為隨後的許多診斷、治療甚至疫苗奠定基礎。但在此期間，醫生們探索了許多不知情的治療方法。
羅伯特·科赫，結核桿菌的發現者。 （左）羅伯特·科赫。
（右）科赫 1882 年繪製的結核桿菌圖畫，取自《結核病雜誌》。
在發現結核病背後的細菌後，科赫開始尋找治療方法。 1890 年，他對金化合物（例如氰化金）進行了實驗，這種化合物可以殺死純實驗室培養物中的細菌。科赫和其他人後來在動物身上測試了這些相同的化合物，但發現它們不安全。事實上，患有結核病的動物如果不接受金化合物，它們的壽命會更長。即使使用比殺死實驗室培養中的微生物所需的劑量高 1000 倍的劑量，豚鼠也無法清除其系統中的結核病。
如今，科學家會停止對具有這些特性的藥物進行臨床研究，但當時，許多研究人員都相信德國生物學家保羅·埃爾利希提出的「靈丹妙藥」理論。他認識到一些分子與細菌緊密結合，並推斷有毒化合物可以用作抗菌藥物，如果它們同樣針對細菌，同時最大限度地減少對身體的附帶損害。一些研究人員無視失敗的動物實驗，給結核病患者服用「金彈」化合物。 1912 年，德國臨床醫師給 20 位皮膚結核感染者注射了氰化金汞合金。 「我們還不能說有任何關於確定治癒的事情」，他們說。最終，他們的實驗未能治癒參與者。
研究人員接下來對劇烈運動進行了測試。也許並不奇怪，許多患者都在與體力作鬥爭。醫護人員再次改變策略，認為充足的休息和陽光會有所幫助。從 20 世紀初開始，結核病患者會前往「療養院」長期停留，在那裡他們可以專注於康復。這些療養院的描述在文獻中比比皆是。
在小說《魔山》中，托馬斯曼的伯格霍夫療養院捕捉到了這樣的氛圍，它坐落在山頂，像一家豪華酒店，周圍環繞著田野和草地。每個病人都有一個私人套間和陽台，每天至少可以吸收兩個小時的陽光。拉塞爾在《醫生》中描述這樣一個療養院時寫道：「有緩解甚至治癒的故事——有些無疑誇大了，但另一些聽起來合法。只要休息好、營養豐富的飲食和適量有益健康的葡萄酒，在那種氣候下康復似乎是可能的」。
醫生對療養院病人的胸部進行身體檢查，尋找疾病和康復的跡象。他們感覺壓痛或異常，並透過用手指敲擊胸壁來測試肺部的叩診情況。類似鼓聲的聲音意味著肺部有空氣——這是一個好兆頭。但沉悶或扁平的聲音表示血液浸透了肺腔，或有固體物質阻塞。
胸部 X 光檢查使醫生能夠更深入地了解肺部發炎的肉芽腫。 「胸腔很明亮，但人們可以辨認出一張由深色斑點和黑色褶邊組成的網，」曼寫道。
肺部有病。兩張胸部 X 光片顯示了一名結核病患者（左）和一名健康人（右）的肺部。
圖片來源：Chauhan A. 等人。 PLOS One（2014）。
1884年，德國細菌學家加夫基 (Georg Gaffky) 制定了一個預後指數來監測療養院病患的病情進展。他認為疾病的嚴重程度取決於患者體內結核分枝桿菌的豐度，因此他的預後是基於痰液樣本中這些細菌的密度。醫生將患者分為 0 到 10 級，分數越低，細菌越少。然而，醫生發現細菌數量確實與嚴重程度沒有密切相關，到了 20 世紀 70 年代，他們不再使用加夫基的量表。
對於那些未能在休息和陽光下康復的患者，醫生嘗試透過進行人工氣胸來使患有結核病的肺部塌陷，其中他們使用皮下注射針將氮氣注入胸腔，但不注入肺部。空氣對肺部施加壓力，將其壓扁，試圖壓平因感染而凹陷的空腔壁，促使它們密封。北美和歐洲的醫生廣泛採用這種方法。在魔山，接受這種治療的患者加入了他們所謂的「半肺俱樂部」。
卡洛‧弗拉尼尼 (Carlo Forlanini) 是義大利帕維亞的醫生，負責治療人工氣胸。
圖片來源：惠康影像
氮氣氣穴會慢慢吸收到體內，使得人工氣胸所帶來的緩解只是暫時的。長期結核病患者將接受胸廓成形術，醫生將切除最多八根肋骨以壓迫胸壁並永久壓平肺部。這兩種壓制肺部的方法都不能真正治癒這種疾病。
由於歷史上此類無效的介入措施，科學家們開發了更好的方法來診斷、監測和治療結核病患者，但我們現代武器庫的缺點，仍然阻礙我們達到終結結核病策略所設定的高標準。
遠距離追蹤與溯源
工業革命期間，結核病在城市環境中大量存在，而如今，低收入國家擁擠且往往不衛生的城市也成為類似的傳播熱點。達卡是孟加拉首都，也是世界上人口最稠密的城市地區之一，也是全國疾病發生率最高的城市。然而，生活在工業化地區的人們通常更容易獲得醫療保健設施，醫生可以在那裡進行從胸部X光檢查到微生物和免疫學檢測等測試。
這意味著在一些國家，情況發生了逆轉，生活在難以獲得醫療保健的農村地區的人們感染這種疾病、被忽視並將其傳播給其他人的風險更大。例如，在中國，80%的人口居住在農村地區，居住在城市以外的人感染結核病的機率是城市居民的1.8倍。
居住在農村地區可能會因多種原因阻礙診斷。讓我們考慮一下 恩佐庫勒 (Enzokuhle) 的生活，他是一位虛構的南非男子，我們將使用他尋求結核病診斷的經歷來進行演示。 Enzokuhle 居住在 Vlaklaagte 附近的一個偏遠社區，位於人口稀少且欠發達的地區。他離最近的城鎮太遠，無法步行上下班，他在附近的金礦謀生。南非的金礦礦工在過度擁擠的條件下工作，結核病發病率是世界上最高的。
幾週前，恩佐庫勒開始出現輕微咳嗽。現在情況急轉直下，變得血腥且無情。發燒和不適阻礙了恩佐庫勒的工作能力，他擔心自己的不明疾病會傳染。恩佐庫勒迫切需要進行結核病檢測，以確定是否繼續治療，但許多障礙阻礙了他。
結核病個案的地區分佈，2022年
如果可以的話，恩佐庫勒會尋找當地的護士或醫生，但他們往往生活在已開發地區，而不是像他這樣的偏遠定居點。在南非這個結核病病例較多的國家，46%的人居住在農村地區，但只有19%的醫生居住在農村地區。醫療保健專業人員短缺，導致長時間等待。恩佐庫勒最早可以在最近的城鎮預約，還要幾個月的時間。他還必須應對交通選擇短缺、道路不足和旅行成本的問題。一項調查發現，尚比亞人平均將每月收入的 16% 用於醫療旅途。
診斷測試的費用從一美元到幾百美元不等，也會讓患者望而卻步。在馬拉威，患者接受結核病檢測需要花費約兩個半月的典型月薪，而恩佐庫勒的收入不足以支付這些費用。其他人可能沒有機會暫停工作進行醫療旅途，因為那樣他們會損失更多的錢。
遠離電網等公共設施的人有兩種接受檢測的選擇。他們可以前往醫療機構，或者提供者可以來找他們收集樣本。在南非，傳統治療師的數量比醫生多，因此醫療保健提供者與他們合作，在訪問偏遠社區時傳播測試。這可能是恩佐庫勒接受檢驗的最佳機會。
或者，恩佐庫勒可以尋找提供測試的當地非政府組織——但前提是此類組織在該地區開展工作。其中一些非政府組織已在南非興起，例如結核病聯盟和南非國家結核病協會 SANTA。其他應對結核病的國家也設立了類似的倡議主動者。在距離恩佐庫勒(Enzokuhle) 約5,000 英里的印度，一個由15,000 多名社區志工組成的非政府組織正在進行一個名為Axshya（梵文意思是「無結核病」）的計畫，旨在對偏遠社區進行有關該疾病的教育，以消除恥辱感，找出症狀，並對社區中的人們進行檢測和治療。
Axshya 進行了一種常見的結核病診斷，稱為痰液測試，類似於 20 世紀療養院中用於加夫基量表的測試。一旦患者從呼吸道中咳出足夠多的細菌黏液，志願者就會添加一種名為石炭酸品紅的紅色染色劑，這種染色劑會對所有微生物的細胞膜進行染色，但在清洗後仍會與結核分枝桿菌的表面結合。這是最便宜的測試之一對於像 Enzokuhle 這樣努力遠離家鄉的人來說，它變得更加可行。然而，紅色細菌只有在顯微鏡下才能看到，因此醫護人員必須將樣本運送到實驗室才能確定結果。因此，偏僻的定居點無法獨立進行這些測試，必須依靠非政府組織或其他醫護人員來拜訪並運送樣本，而結果可能需要數月才能收到。
結核分枝桿菌的痰測試染色。
存在其他結核病檢測，但大多數也依賴實驗室的取得。干擾素γ釋放測定檢測出對感染的免疫反應。它們以干擾素 γ 命名，干擾素是 T 細胞釋放的促發炎分子。如果這些細胞之前與感染者體內的結核菌有過接觸，它們就會在再次遇到病原體抗原之一時準備釋放這種分子。醫護人員可以從患者體內分離出 T 細胞，將其與抗原混合，並監測干擾素 γ 的釋放，以確定患者是否攜帶細菌。運行這些測試依賴於一些難以取得的機器，例如將細胞維持在體溫的培養箱、混合測試試劑的搖動平台以及檢測樣品中的干擾素伽馬的分光光度計。
由於恩佐庫勒無法輕易前往配備診斷實驗室的醫療中心，另一種選擇是縮小檢測設備的規模並將其帶給他。診斷公司 Cepheid 已經做到了這一點。 Cepheid 的測試使用聚合酶鍊式反應 (PCR)，複製了結核分枝桿菌特有的基因 rpoB，該基因編碼部分 RNA 聚合酶。
PCR 測試通常依賴實驗室設備，例如用於分裂細菌並釋放 DNA 的試劑盒、用於將 DNA 與細胞中其他成分分離的離心機，以及控制 DNA 合成反應的溫度和時間的熱循環儀 – 但是Cepheid 將所有這些打包到稱為GeneXpert® 系統的桌上型機器中，無需大型無菌實驗室和訓練有素的人員。臨床醫生只需插入痰液樣本，按下按鈕，然後等待兩到三個小時即可得到結果。
儘管 GeneXpert® 系統與其他方法相比具有低保真度，但它和其他緊湊型 PCR 測試一樣仍然需要電源。這意味著大約 有7 億人無法使用電力。大多數沒有電力的人生活在結核病猖獗的撒哈拉以南非洲地區。幸運的是，恩佐庫勒生活在一個有電力的定居點，但他知道附近地區沒有能源的人們不會從這種方法中受益。預算也仍然是一個障礙。 2023 年，每個 GeneXpert 系統的診所成本約為 19,000 美元。由於沒有健康保險且收入有限，恩佐庫勒決定不再花費 260 南非蘭特（約 15 美元）進行個人 PCR 檢測。
然而，有一種結核病檢測不需要電力、昂貴的設備或進入實驗室，也不會給患者或提供者帶來巨額費用——結核菌素皮膚試驗。這個測試可以在恩佐庫勒的家中進行。它包括將細菌的一種蛋白質衍生物（稱為結核菌素）注射到皮膚下，看看 T 細胞是否會對其做出反應。如果恩佐庫勒的系統中有結核分枝桿菌，他的 T 細胞將識別抗原並在注射部位產生強烈的免疫反應，留下一個牛眼狀的紅腫。
在校學生接受結核菌素皮膚測試，以確定他們是否接觸過結核病。皮膚注射了結核病抗原。若皮膚變紅，則為陽性反應；免疫反應升高是由於較早接觸該疾病所致。圖片來源：阿拉米
發現結核分枝桿菌的德國醫生羅伯特·科赫(Robert Koch) 於1890 年意外地開發出了這種幾乎無處不在的診斷方法。他的目標是透過分離死亡細菌的萃取物並將其施用於患者皮下來開發一種治療結核病的方法。這種療法並沒有達到預期的效果；他的受試者很快就出現寒顫、發燒和嘔吐。這可能是因為他們的免疫系統本來就在與細菌作鬥爭，突然面對過量的抗原，導致它反應過度。然而，經過仔細改進，這項技術演變成一種更溫和的結核病皮膚測試。
撇開邏輯因素不談，結核病的各種診斷通常容易出錯，準確率在 67% 到 100% 之間。測試品質通常透過兩個因素來衡量：敏感性和特異性。敏感性是指被正確識別為陽性的病原體攜帶者比例。痰液檢測對兒童來說不是很敏感，因為他們經常難以咳出足夠的黏液供醫護人員檢測細菌，從而導致假陰性。這種診斷只能發現 7% 的 15 歲以下兒童病例和 1% 的 4 歲及以下兒童病例。即使在成人中，痰液中可能只含有少數微生物，但每毫升痰液中至少需要 1,000 個細菌才能檢測到病原體。因此，該測試只能檢測到大約 75% 的病例。
Enzokuhle 無法承受的 PCR 檢測是最敏感的工具，經常在痰液檢測失敗的情況下發現感染。即使樣本中含有少量結核分枝桿菌，PCR 也會將其 DNA 倍增多次，放大訊號，使它們不可能被遺漏。
檢測細菌免疫反應的測試，即結核菌素皮膚試驗和干擾素γ釋放測定，也可能有敏感性問題。對於免疫力低下的人來說，這些診斷效果尤其差，例如愛滋病毒治療不充分的人或愛滋病患者，他們缺乏足夠的 T 細胞來對細菌產生免疫反應。愛滋病毒/愛滋病和結核病在地理上常常重疊，例如在南非，這使得這些檢測在那裡不太有用。然而，恩佐庫勒同時接受了愛滋病毒檢測（這是南非的常見做法），結果呈現陰性。
在恩佐庫勒的案例中，他的結核菌素皮膚測試結果（紅色腫塊）比痰測試的結果要早，痰測試必須送到實驗室。隨著皮膚試驗證實他懷疑自己感染了結核病，恩佐庫勒準備開始治療。也就是說……直到痰液結果呈現陰性。
測試之間的差異可能與另一個品質控制因素有關：特異性——沒有攜帶病原體的人被正確識別為陰性的比例，例如被其他微生物感染的人。結核菌素皮膚試驗和痰液試驗在這方面均有不足。接觸與結核分枝桿菌密切相關的無害細菌物種可能會產生假陽性結果。由於它們在結構上與結核分枝桿菌非常相似，因此它們的膜在痰液測試中保留了紅色染色，並且它們可以啟動 T 細胞對抗其蛋白質衍生物，從而扭曲皮膚測試的結果。
同樣的挑戰也出現在另一個更不尋常的測試中：在一些撒哈拉以南非洲國家，例如莫三比克，技術人員訓練非洲巨袋鼠從一批痰液樣本中嗅出結核分枝桿菌，當它們發現時，會抓破籠子的地板。老鼠的診斷速度比照顧它們的實驗室技術人員更快，但是，就像標準痰測試一樣，它們很難區分病原體和分枝桿菌的「野生」菌株。
幾乎所有南非人都建議接種卡介苗 (BCG) 疫苗，接種者也會將 T 細胞暴露於結核菌素中，導致皮膚檢驗出現偽陽性。這是因為卡介苗疫苗由一種稱為牛分枝桿菌的活細菌組成，這種細菌在體外培養了足夠的時間，使其不太適應感染。因此，儘管它提供了一些保護，但該疫苗與結核分枝桿菌的親緣關係太近，這意味著它可能會混淆最廣泛使用的結核病診斷結果。
一歲兒童未接種對抗結核病之卡介苗的人數，2021年
于擾素釋放測定是上述的結核病鑑定方法，具有較好的特異性。這是因為該檢測使用了相關細菌或卡介苗疫苗中缺少的結核分枝桿菌抗原，這意味著只有先前遇到病原體的 T 細胞才能產生可檢測的免疫反應。
即便如此，GeneXpert® Systems PCR 測試總體上仍具有最佳特異性，因為它使用三個引子（即引起基因加倍反應的短鏈 DNA），作為三因素驗證的一種形式，確保系統複製正確的基因，而不是相關細菌物種中存在的基因。較新的 GeneXpert® 系統甚至具有額外的特異性，因為它們可以檢測賦予抗生素抗藥性的基因，從而使醫生能夠就治療方案做出更明智的決策。
由於不確定如何進行陽性皮膚測試和陰性痰測試後，Enzokuhle 做出了艱難的決定，即支付 GeneXpert® 系統測試費用來解決問題。結果呈陽性，醫生向他保證結果是準確的。他的痰樣本中的細菌可能太少，無法檢測出陽性。對 Enzokuhle 來說幸運的是，PCR 測試沒有發現任何抗生素抗性基因，醫生給他開了常規抗生素療程，應該會在六個月內清除感染。
混亂的流行病學
在人口層面，誇大的統計數據、未報告的死亡人數以及傳播盲點使我們無法充分了解結核病的負擔。
皮膚試驗對恩佐庫勒的病例是正確的，但對許多其他患者來說卻失敗了，這扭曲了我們對結核病傳播的全球理解。對於先前接種過疫苗或被某些細菌感染的患者來說，皮膚試驗常常會失敗，產生假陽性的機率高達 40%。劍橋大學的一些科學家認為，對皮膚測試的錯誤解釋導致了對這種疾病最普遍的誤解之一 ——大約20 億人（或全球四分之一的人）在許多人身上都隱藏著潛伏狀態的病原體。這種虛構的觀點讓一些人相信，這種不起眼的細菌隨時可能爆發為全面的疾病。
事實上，儘管尚未獲得最新的估計，但沒有疾病跡象的結核分枝桿菌攜帶者人數可能會更少。模型顯示，十分之九的人會在十年內藉助免疫系統自然清除病原體，大多數人會在兩年內清除感染。這打破了潛伏感染是終生的神話。更重要的是，98% 的結核病病例出現在感染後兩年內，這顯示結核分枝桿菌通常不會等到幾年後才會引起疾病。
報告的結核病死亡人數——每年 120 萬人——也不準確，給人一種錯誤的印象，認為這種疾病的致死率實際上比實際情況要低。 HIV 感染治療不充分的人的 T 細胞會下降，T 細胞會對抗受感染的細胞，並形成肉芽腫，從而阻止結核分枝桿菌的傳播。因此，愛滋病患者更有可能罹患上活動性結核病，許多人死於細菌感染，但他們的死亡證明只將愛滋病列為死因，以避免重複計算。 2019 年，三分之一死於愛滋病的人也患有結核病，導致與結核病相關的死亡人數之落差達到 208,000 人。再加上無症狀攜帶者的數量可能被誇大的事實，這種微生物造成的致命性威脅變得更加令人震驚。
全球HIV/AIDS個案中因結核病和其他與HIV/AIDS相關之疾病死亡之人數
終結結核病策略的目標之一是到 2030 年要將新病例減少 80%，但距離這一目標還差 9%。無症狀傳播者可能是造成這種短缺的部分原因。醫療保健專業人員經常只為身體不適的人保留診斷測試和治療。這在疾病的流行病學中造成了一個盲點，因為研究人員對沒有表現出疾病跡象的人所造成的影響上缺乏規模感。
當世界衛生組織比較九個亞洲國家無症狀和有症狀人群的結核病患者盛行率數據時，他們發現，34-68%的痰液檢測呈陽性的人沒有表現出結核病症狀。約翰霍普金斯大學的研究人員認為，根據微生物檢測呈陽性但沒有表現出任何症狀，未被發現的亞臨床結核病的個體之比例可能佔盛行率的 10%（多達 1000 萬人）。攜帶者通常含有大量細菌，每毫升痰中至少含有 10,000 個微生物，其濃度足以透過空氣傳播，甚至僅透過呼吸即可傳播。有時咳嗽等症狀很容易被誤認為是普通感冒，患者通常會等待這些症狀消失才去看醫生並接受檢查。此時間滯後之落差進一步加劇了傳播。
由於有這麼多的人患有這種疾病的活躍形式，醫護人員已經不再優先考慮潛伏感染的篩檢。然而，追蹤潛隱疾病可能對於消除結核病至關重要。最終，迫切需要開發一種新的診斷方法，其靈敏度足以發現潛在感染，但價格足夠便宜，易於使用。
明天的結核病檢測
醫生經常透過在顯微鏡下觀察痰液塗片來辨識結核分枝桿菌。但越來越多的科學家開始嘗試天文學而不是微生物學的原理，以更深入地了解患者的感染情況。
當天文學家使用望遠鏡觀察恆星時，他們不僅捕捉恆星質量的圖像：他們還收集恆星吸收和發射的波長光譜，從而深入了解其元素組成。新的結核病診斷方法具有類似的作用——它們使科學家能夠捕獲結核分枝桿菌的光譜，揭示有關感染的詳細信息，從而指導治療過程。
泰國孔敬大學的工程師正在開發一種基於拉曼散射的技術，拉曼散射是一種用一種波長的光激發的化合物在電磁波譜的不同部分發射光子的現象。發射光的波長取決於化合物的性質，不同化合物的溶液會產生類似山丘和山谷的複雜光譜。
由石英比色皿中的丙酮分子產生的光譜。圖片來源：迪內什·丹卡
研究團隊假設，結核病患者的血清攜帶結核分枝桿菌產生的生物分子，會產生獨特的光譜。為了在一項小型概念驗證研究中檢驗這個理論，他們招募了四組人：26 人患有活動性結核病感染；。 20 人患有潛伏性結核病，這意味著他們沒有表現出結核病症狀，但透過其他方法檢測呈陽性； 34 人儘管接觸過結核病患者，但沒有感染跡象；以及38名健康志願者。
從這四組人的血液中分離出血清並乾燥樣本後，他們用綠色雷射激發樣本並收集拉曼光譜。從遠處看，所有四組的光譜看起來都是相同的，正如人們所期望的，血清主要由相同的物質組成，即血液蛋白質和脂質。但經過仔細檢查，每組的山丘和山谷的高度不同，凸顯了由細菌或應對感染的免疫因素引起的個體差異。
研究人員認為，所產生的特徵景觀可以在未來進行更精確和詳細的診斷，區分活動性、潛伏性和清除性結核感染。這可以糾正該疾病的流行病學記錄，並有助於在疾病有機會傳播之前發現並治療它。
該測試仍然只是一個概念驗證，當工程師將測試打包為診斷時，他們將不得不應對拉曼光譜依賴複雜設備的限制。它們可能會面臨與緊湊型 PCR 檢測類似的障礙：在沒有電力的地方無法使用，而且對於像 Enzokuhle 這樣的患者來說，價格昂貴得令人望而卻步。
史丹佛大學的工程師可能有不同的答案。他們製造了一款名為 Octopi 的便攜式、經濟實惠的顯微鏡，具有一系列可用於診斷的成像模式，包括拉曼光譜。 Octopi 使用拉曼散射來區分對四種常用抗生素中的任何一種產生抗藥性的結核分枝桿菌，這可以快速告知醫生最佳的治療方案。
從用手指敲擊病人的胸部來聆聽結核病的跡象，科學家們已經取得了很大的進展。雖然一些過時的診斷方法，包括痰液測試和皮膚測試，仍然被廣泛使用，但研究人員發明了具有更好敏感性和特異性的新技術。現在，發明者正在應對下一個診斷挑戰。使用拉曼光譜，他們可以從一次測試中收集更多信息，包括病原體生命週期的階段或其抗藥性。
隨著這些新型診斷測試對結核病流行病學的了解不斷加深，我們離控制疾病又更近了一步。然而更好的診斷還不夠。在第二部分中，我們深入研究目前疫苗和抗生素治療的限制。我們也討論了替代方案的潛力。與正在開發的下一代診斷技術一樣，這些技術為結核病有一天可能逐漸成為藝術、小說和浪漫詩歌的遺跡帶來了希望。
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感謝 Saloni Dattani、Xander Balwit、Devon Balwit、Niko McCarty 和 Merrick Pierson Smela 編輯本文的草稿。
卡邁勒‧納哈斯 (Kamal Nahas) 是一位來自英國牛津的研究員出身的記者，報導生物學、健康和科技領域的故事。
引用：納哈斯，卡邁勒。「被遺忘的流行病」。阿西莫夫出版社（2024）。 DOI：https://doi.org/10.62211/62ur-55df
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註腳
1. 儘管結核病在西方不太常見，但在被監禁的人中結核病的盛行率較高，因為擁擠的監獄條件可能有助於空氣中病原體的傳播。
2. 如果診斷機構缺乏足夠的生物安全預防措施，痰檢中的黏液樣本可能會將結核分枝桿菌傳播給醫護人員。然而，哈佛大學和麻省理工學院的科學家正在尋找尿液作為更安全的檢測替代方案。尿液很少攜帶結核分枝桿菌，但經常含有一些經由腎臟過濾的生物標記。
The Forgotten Pandemic
Kamal Nahas / Asimov Press
The Most Deadly Infectious Disease
“Each successive episode of bleeding left him weaker than before,” wrote Mary Doria Russell of Doc Holliday, the gunslinging gambler, in her eponymous book, Doc. Published in 1971, the book contains unsparing descriptions of Holliday’s deteriorating medical condition. “‘You get used to it,’ Doc always said. ‘You can get used to anything.’ Used to the gnawing pain; used to the sudden taste of iron and salt; used to the struggle to pull air in as blood from his lungs rose.”
Doc suffered from tuberculosis, or TB, a bacterial disease caused by Mycobacterium tuberculosis. Both deadly and storied, the disease has earned many monikers — consumption, phthisis, and “the white plague” or “white death,” owing to the paleness of those afflicted. It killed Frédéric Chopin, Henry David Thoreau, Franz Kafka, and Eleanor Roosevelt. At its peak in the 19th century, TB is estimated to have caused one in four deaths in Europe and America.
The Sick Child, by Edvard Munch (1885-1886). Munch’s sister died from tuberculosis at the age of 15.
M. tuberculosis has a complex lifecycle. Unsuspecting people inhale the bacteria, which travel down into
the lungs. Immune cells, called macrophages, quickly attempt to destroy the microbes. But the microbes are wily prey, often killing the macrophages instead by secreting factors that trigger cell death. Other white blood cells cluster around the foreign invaders, forming a physical barrier, called a granuloma, that immures the pathogen. Macrophages in the cluster continue fighting the bacteria, and other immune cells arrive to reinforce the granuloma wall.
At this stage, the disease is still latent, and unsuspecting carriers typically experience no symptoms. It is not until M. tuberculosis erupts from the granuloma that it causes active disease. Persistent mucus, a bloody cough, exhaustion, emaciation, and fevers follow. Tuberculosis can also cause swollen lymph nodes, bone and joint pain, or seizures.
Nowadays, TB is relatively uncommon in the West,1 where it is mostly shrugged off as a disease of the past. But perhaps surprisingly, TB remains the deadliest infectious disease on the planet. Globally, it kills 1.2 million people a year, with the majority of fatalities occurring in Sub-Saharan Africa and South Asia. Malaria, in contrast, killed about 608,000 people in 2022.
TB remains so rampant because it is difficult to diagnose. Yet accurate diagnoses are the first step toward understanding the scale of the problem and treating TB early to halt its spread. So why haven’t we made more progress toward developing cheaper and more efficient ways to identify cases?
There are many reasons. In hard-to-reach places with poor access to healthcare, many people with symptomatic TB cannot receive a diagnosis due to cost, inadequate transport, or a lack of electricity to run tests. Researchers estimate that 2.9 million people with TB went undetected in 2019, but these estimates are uncertain due to inaccuracies in testing. And even when patients do get tested, those tests often produce false negatives when the bacterial load is too low to detect in sputum samples.
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Additionally, some of the tools for diagnosing TB also lack the specificity required to assess antibiotic resistance. In 2019, physicians were unable to tailor effective antibiotic regimens for 39 percent of new TB cases because the assessments they relied on could not gauge which antibiotics the strains would be susceptible to.
Yet another challenge arises in deciding who qualifies for a TB diagnosis: With limited resources, healthcare workers tend to focus on people showing signs of the disease rather than seemingly healthy people who may carry the latent bacteria. Given this, physicians still don’t grasp the degree to which asymptomatic spreaders transmit the pathogen.
In 2014, the World Health Organization (WHO) proposed an initiative called the End TB Strategy, aimed at reducing cases by 80 percent, deaths by 90 percent, and diagnostic/treatment costs by 100 percent by 2030. Regrettably, the initiative is not on track to meet any of those targets.
In this essay, the first in a two-part series on TB, we dive into the history of this disease and its mismanagement while highlighting key breakthroughs that have partially thwarted this persistent pandemic in the West.
An Ancient Malady
The M. tuberculosis bacterium has been around longer than we have. Early forms of the disease infected great apes in East Africa an estimated three million years ago. For reference, the oldest human species, Homo habilis, came into the picture roughly two million years ago. The current lineage of M. tuberculosis has plagued our species for 15 to 20 thousand years. Archaeologists have found evidence of ancient Egyptian mummies carrying the pathogen’s DNA, including some that had developed Pott’s disease — a bending of the spine brought on by TB spreading to bones. The disease also tormented the ancient Greeks and Romans, with Hippocrates noting: “Those who spit up frothy blood bring this up from the lung.”
A young girl in Oklahoma suffering from bone tuberculosis. Photograph by Dorothea Lange, 1935.
Tuberculosis cases surged during the Industrial Revolution as people began crowding indoors, where airborne bacteria could concentrate and spread with ease. People across social classes succumbed to the disease. When artists and poets began to fall ill, tuberculosis even became a glamorized and coveted infirmity.
“How pale I look!” wrote the poet, Lord Byron. “I should like, I think, to die of consumption … because then the women would all say, ‘see that poor Byron — how interesting he looks in dying!’”
While some were busy romanticizing the disease, others were working hard to understand it. In 1882, German physician Robert Koch discovered the bacterial culprit behind TB by staining a tubercular mass taken from an animal lung and examining it under a microscope. From this, “very fine rod-like forms became apparent,” he wrote. Identifying the bacteria behind the pathogen would later set the stage for many upcoming diagnostics, treatments, and even a vaccine. But doctors explored numerous uninformed remedies in the interim.
Robert Koch, discoverer of the tuberculosis bacillus. (Left) Robert Koch. (Right) Koch’s 1882 drawings of the tuberculosis bacilli, from Die Ätiologie der Tuberkulose.
Upon discovering the bacterium behind TB, Koch began searching for cures. In 1890, he experimented with gold compounds, such as gold cyanide, which killed the bacteria in pure lab cultures. Koch and others later tested these same compounds in animals, but found them to be unsafe. In fact, animals with TB lived longer if they didn’t receive the gold compounds; even with a dose 1,000-times greater than what was needed to kill the microbes in lab cultures, guinea pigs couldn’t clear TB from their systems.
Today, scientists would halt clinical research on drugs with these properties, but back then, many researchers put stock in the “magic bullet” theory proposed by Paul Ehrlich, a German biologist. He recognized that some molecules bind tightly to bacteria and reasoned that toxic compounds could be used as antimicrobial drugs if they similarly targeted the bacteria while minimizing collateral damage to the body. Ignoring the failed animal experiments, some researchers administered “gold bullet” compounds to TB patients. In 1912, German clinicians gave an amalgam of gold cyanide to 20 patients with a skin TB infection. “We cannot say anything yet about definite healing,” they said. In the end, their experiment failed to cure the participants.
Researchers next put intense exercise to the test. Perhaps unsurprisingly, many patients struggled with stamina. Changing tack yet again, healthcare workers reasoned that plenty of rest and sunshine would help. Beginning in the early 20th century, people with TB would travel to “sanatoriums” for extended stays where they could focus on recovery. Depictions of these sanatoriums abound in literature.
In the novel, The Magic Mountain, Thomas Mann’s Berghof sanatorium captures the ambiance, sitting at the top of a hill like a luxury hotel, surrounded by fields and meadows. Each patient had a private suite and balcony to absorb sunlight for at least two hours each day. Describing such a sanatorium in Doc, Russell writes: “There were stories of remission and even cures — some undoubtedly exaggerated, but others that sounded legitimate. With rest, a nutritious diet, and moderate amounts of healthful wine, convalescence in that climate seemed possible.”
Doctors physically examined the chests of their sanatorium patients, looking for signs of disease and recovery. They felt for tenderness or abnormalities and tested the percussion of the lungs by striking the chest wall with their fingers. A drum-like sound meant that there was air in the lungs — a good sign. But a dull or flat sound suggested blood drenched the lung cavity or that there was an obstruction by a solid mass.
Chest X-rays gave doctors additional insight into the inflamed granulomas within the lungs. “The chest cavity was bright, but one could make out a web of darker spots and blackish ruffles,” Mann wrote.
Affliction of the lungs. Two chest x-rays showing the lungs of a TB patient (left) and a healthy individual (right). Credit: Chauhan A. et al. PLOS One (2014).
In 1884, German bacteriologist Georg Gaffky developed a prognostic index to monitor the progress of sanatorium patients. Reasoning that disease severity would depend on the abundance of M. tuberculosis bacteria in the patient, he based his prognostic on the density of these bacteria in sputum samples. Physicians placed patients on a scale of 0 to 10, ascribing a low number to those with fewer bacteria. However, doctors discovered that bacterial numbers did not closely correlate with severity, and by the 1970s, they moved away from Gaffky’s scale.
For those patients who failed to recover with rest and sunshine, physicians tried collapsing a TB-ridden lung by performing an artificial pneumothorax, in which they infused nitrogen gas into the chest cavity — but not into the lungs — using a hypodermic needle. The air applied pressure to the lung, squashing it down in an attempt to flatten the walls of cavities hollowed out by the infection, encouraging them to seal. Physicians in North America and Europe widely adopted this method. In The Magic Mountain, patients receiving this treatment joined what they dubbed the “Half-Lung Club.”
Carlo Forlanini, a medical doctor in Pavia, Italy, administers an artificial pneumothorax.
Credit: Wellcome Images
The nitrogen air pockets would slowly absorb into the body, making the relief granted by an artificial pneumothorax only temporary. Long-term TB sufferers would receive a thoracoplasty instead, whereby doctors would remove up to eight ribs to compress the chest wall and permanently flatten a lung. Neither of these lung-squashing methods actually cured the disease.
Since such ineffective historic interventions, scientists have developed better methods to diagnose, monitor, and treat TB patients, but the shortcomings of our modern-day arsenal still hold us back from reaching the high bar set by the End TB Strategy.
Track and Trace, Far and Wide
TB abounded in urban environments during the Industrial Revolution, and today, crowded and often unsanitary cities in lower-income countries act as similar hot spots for transmission. Dhaka, the capital city of Bangladesh and one of the world’s most densely populated urban areas, has the highest disease incidence in the country. However, people living in industrialized areas generally have better access to healthcare facilities, where physicians can run tests from chest radiography to microbial and immunological assays.
This means that in some countries, the situation flips, and people living in rural areas with poor access to healthcare are at greater risk of catching the disease, slipping under the radar, and spreading it to others. In China, for example, 80 percent of the population resides in rural areas, and people who live outside cities catch TB 1.8 times more often than urban dwellers.
Living in rural areas can impede diagnoses for multiple reasons. Let’s consider the life of Enzokuhle, a fictitious South African man whose experience seeking a TB diagnosis we’ll use for the purpose of demonstration. Enzokuhle resides in a remote community near Vlaklaagte in an underpopulated and underdeveloped region. Too far from the nearest town to commute on foot, he makes a living at a nearby gold mine. Gold miners in South Africa work in overcrowded conditions and experience the highest rates of TB in the world.
Enzokuhle began to experience a mild cough a couple of weeks back. It now takes a downward turn, becoming bloody and unrelenting. Fever and malaise hinder Enzokuhle’s ability to work, and he worries that his unidentified illness is contagious. Enzokuhle desperately needs a TB test to know whether or not to pursue treatment, but many obstacles stand in his way.
Enzokuhle would seek out a local nurse or doctor if he could, but they tend to live in developed areas rather than remote settlements like his. In South Africa, a country with a large number of TB cases, 46 percent of people live in rural areas, but only 19 percent of physicians do. Healthcare professionals are in short supply, causing long waits. The earliest that Enzokuhle can book an appointment in the nearest town is months away. He also has to navigate around a shortage of transportation options, inadequate roads, and the cost of travel. One survey found that people in Zambia spend an average of 16 percent of their monthly income on medical travel.
The costs of diagnostic tests, which range from a single dollar to several hundred, can deter patients, too. In Malawi, it costs about two-and-a-half months of typical monthly wages for patients to get tested for TB, and Enzokuhle doesn’t make enough to cover such expenses. Others may not have the opportunity to pause work for medical travel, because then they’d lose even more money.
People living off the grid have two options for getting tested. They can either travel to a healthcare facility, or providers can come to them to collect samples. In South Africa, traditional healers are more numerous than medical doctors, so healthcare providers work with them to disseminate tests upon visiting isolated communities. This may be Enzokuhle’s best chance of being tested.
Alternatively, Enzokuhle could seek out a local NGO that provides tests — but only if such groups work in the region. A few of these NGOs have emerged in South Africa, such as TB Alliance and SANTA, the South African National Tuberculosis Association. Other countries grappling with TB have set up similar initiatives. Around 5,000 miles away from Enzokuhle, in India, one such NGO, a group of over 15,000 community volunteers, runs a project called Axshya (meaning TB-free in Sanskrit) that seeks to educate remote communities about the disease to combat stigma, pinpoint symptoms, and test and treat people in their communities.
Axshya runs a common TB diagnostic called a sputum test — similar to the one used for the Gaffky scale in 20th-century sanatoriums. Once patients cough up enough bacteria-ridden mucus from their airways, the volunteers add a red stain called carbol fuchsin that dyes the membranes of all microbes but remains bound to M. tuberculosis’ surface following a wash.2 This is one of the cheapest tests available, making it more viable for people like Enzokuhle who struggle to journey far from their homes. However, the red-painted bacteria are only visible under a microscope, so healthcare workers must transport the samples to a laboratory to determine the results. As a consequence, secluded settlements cannot run these tests independently and must rely on NGOs or other healthcare workers to visit and ship off their samples, the results of which can take months to receive.
A sputum test stain for M. tuberculosis.
Other TB tests exist, but most also rely on laboratory access. Interferon gamma release assays pick up an immune response to the infection. They are named after interferon gamma, a pro-inflammatory molecule released by T cells. If the cells previously crossed paths with TB bacteria in an infected person, they become primed to release this molecule upon reencountering one of the pathogen’s antigens. Healthcare workers can isolate T cells from a patient, mix it with an antigen, and monitor interferon gamma release to determine if the patient has the bacterium. Running these tests relies upon several harder-to-access machines, such as incubators to maintain the cells at body temperature, shaking platforms to mix test reagents, and spectrophotometers to detect interferon gamma in the samples.
Since Enzokuhle cannot travel easily to a medical center equipped with a diagnostic laboratory, another option is to shrink down the testing equipment and bring it to him. The diagnostic company Cepheid has done just that. Using a polymerase chain reaction (PCR), Cepheid’s test makes numerous copies of a gene specific to M. tuberculosis known as rpoB, which codes for part of its RNA polymerase.
PCR testing normally relies on laboratory equipment, such as kits to split open the bacteria and release their DNA, a centrifuge to separate DNA from other components in the cell, and a thermal cycler that controls the temperature and timing of the DNA synthesis reaction — but Cepheid has packaged all of these into a table-top machine called a GeneXpert® System, negating the need for a large, sterile laboratory and trained personnel. Clinicians just insert a sputum sample, press a button, and wait two to three hours for the results.
As low-fi as the GeneXpert® System is compared to other methods, it and other compact PCR tests still require a power supply. This means they are unavailable to approximately 700 million people with no access to electricity. The majority of people without power live in Sub-Saharan Africa, where TB is rampant. Fortunately, Enzokuhle lives in a settlement with power, but he knows people in a nearby region with no energy source that would not benefit from this method. Budgeting remains a barrier, too. In 2023, each GeneXpert System cost clinics about 19,000 U.S. dollars. With no health insurance and limited income, Enzokuhle decides not to shell out 260 South African Rand (roughly 15 U.S. dollars) for an individual PCR test.
However, there is one TB test that does not require electricity, expensive equipment, or access to a laboratory and does not leave patients or providers with a huge bill — the tuberculin skin test. This test can take place in Enzokuhle’s home. It involves injecting a protein derivative from the bacteria, called tuberculin, under the skin to see if T cells react to it. Should Enzokuhle have M. tuberculosis in his system, his T cells will recognize the antigen and mount an intense immune response at the injection site, leaving behind a red and swollen bull’s-eye.
School students receive tuberculin skin tests to determine if they’ve been exposed to tuberculosis. The skin is injected with a TB antigen. If the skin reddens, that indicates a positive reaction; the elevated immune response is caused by an earlier exposure to the disease. Credit: Alamy
Robert Koch, the German physician who discovered M. tuberculosis, accidentally developed this nearly-ubiquitous diagnostic back in 1890. He aimed to develop a treatment for tuberculosis by isolating extracts from dead bacteria and administering them to patients under the skin. This therapy didn’t have the desired outcome; his subjects quickly developed chills, fever, and vomiting. This was probably because their immune systems, which had already been fighting the bacteria, suddenly faced an excess of its antigens, causing it to overreact. With careful refinement, however, this technique evolved into a milder TB skin test.
Logistic considerations aside, the various diagnostics for TB are generally error-prone, with a range in accuracy from 67 to 100 percent. Test quality is usually measured by two factors: sensitivity and specificity. Sensitivity refers to the share of people with the pathogen that are correctly identified as positive. The sputum test isn’t very sensitive in children because they often struggle to cough up enough mucus for healthcare workers to detect the bacterium, resulting in false-negatives. This diagnostic only catches 7 percent of cases in children under 15 and 1 percent of cases in children aged four and under. Even in adults, sputum may only contain a few of the microbes, but at least 1,000 bacteria per milliliter of sputum are needed to detect the pathogen. As a result, the test only catches about 75 percent of cases.
The PCR test, which Enzokuhle wasn’t able to afford, is the most sensitive tool and often picks up infections where the sputum tests fail. Even if the sample contains a few M. tuberculosis bacteria, the PCR will double up their DNA many times over, thus amplifying the signal and making them impossible to miss.
Tests that detect an immune response to the bacterium, namely the tuberculin skin test and interferon gamma release assays, can also have problems with sensitivity. These diagnostics are especially poor for people with compromised immunity, such as individuals with inadequately treated HIV, or AIDS patients, who lack enough T cells to mount an immune response against the bacteria. HIV/AIDS and TB often overlap geographically, such as in South Africa, rendering these tests less useful there. However, Enzokuhle undergoes an HIV test at the same time (as is common practice in South Africa), and it comes back negative.
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In Enzokuhle’s case, the results of his tuberculin skin test — the red swollen lump — comes in sooner than his sputum test that has to be shipped to a laboratory. With the skin test confirming his suspicion that he has contracted TB, Enzokuhle is all set to begin treatment. That is … until the sputum results come back negative.
The discrepancy between the tests might lie with the other quality-control factor: specificity — the share of people who don’t have the pathogen that are correctly identified as being negative, such as people who are infected by other microbes. Both the tuberculin skin test and the sputum test fall short in this regard. Exposure to harmless bacterial species closely related to M. tuberculosis can yield a false-positive result. Because of how similar they are in structure to M. tuberculosis, their membranes retain the red stain in the sputum test, and they can prime T cells against their protein derivatives, thereby skewing the results of the skin test.
This same challenge appears in another, more unusual test: In some Sub-Saharan African countries, such as Mozambique, technicians train the African Giant Pouched Rat to sniff out M. tuberculosis from a batch of sputum samples, scratching the floor of their cage when they come across a sample carrying the microbes. The rats are speedier diagnosticians than the laboratory technicians who care for them, but, just like with the standard sputum tests, they struggle to distinguish the pathogen from “wild” strains of mycobacteria.
People who received the Bacillus Calmette–Guérin (BCG) vaccine, which is recommended to nearly everyone in South Africa, also expose their T cells to tuberculin, leading to false positive skin tests. This is because the BCG vaccine consists of a live bacterium, called Mycobacterium bovis, which has been cultured outside the body enough times to render it poorly adapted for infection. So although it provides some protection, the vaccine is too close a relative of M. tuberculosis, meaning it can muddle the results of most widely-accessible TB diagnostics.
The interferon release assay, an aforementioned method for identifying TB, has better specificity. That’s because the assay uses M. tuberculosis antigens that are missing from related bacteria or the BCG vaccine, meaning only T cells that previously encountered the pathogen can mount a detectable immune response.
Even so, the GeneXpert® Systems PCR test boasts the best specificity overall because it makes use of three primers (i.e. short strands of DNA that cause the gene-doubling reactions), serving as a form of three-factor authentication, ensuring the system copies the correct gene and not one present in a related bacterial species. Newer GeneXpert® Systems even come with an extra layer of specificity in that they can detect genes that confer resistance to antibiotics, allowing physicians to make better-informed decisions about treatment regimens.
Unsure how to proceed with a positive skin test and a negative sputum test, Enzokuhle makes the difficult decision to pay for a GeneXpert® System test to resolve the matter. It comes back positive, and the doctor assures him the result is accurate. He may have had too few bacteria in his sputum sample to test positive. Fortunately for Enzokuhle, the PCR test does not spot any antibiotic resistance genes, and the doctor prescribes him a routine course of antibiotics, which should clear the infection in six months.
A Muddied Epidemiology
At the population level, exaggerated statistics, unreported fatalities, and a blind spot for transmission prevent us from fully grasping TB’s burden.
The skin test is correct in Enzokuhle’s case, but it fails for many other patients, skewing our global understanding of TB’s spread. The skin test often fails for patients who have previously been vaccinated or infected by certain bacterial species, producing false positives as often as 40 percent of the time. Some scientists at the University of Cambridge argue that incorrect interpretations of the skin test contributes to one of the most widespread misconceptions about the disease — that roughly two billion people (or one-quarter of the globe) harbor the pathogen in a latent state for many years or even for the rest of their lives. This apocryphal view leads some to believe that the unassuming bacteria could at any point erupt into full-blown disease.
In reality, the number of people carrying M. tuberculosis with no signs of disease is probably lower, although updated estimates are not yet available. Modeling suggests that nine out of ten people will naturally clear the pathogen with the help of their immune system within a decade, and most will clear the infection in under two years. This busts the myth that latent infections are lifelong. What’s more, 98 percent of TB cases appear within two years of infection, revealing that M. tuberculosis does not often wait to cause disease several years down the line.
The reported number of deaths from TB — 1.2 million people each year — is also inaccurate, creating a false impression that the disease is actually less lethal than it really is. People with inadequately treated HIV infections experience a drop in their T cells, which fight infected cells and contribute to the granuloma that prevents M. tuberculosis from spreading. AIDS patients are more likely to develop active TB as a result, and many succumb to the bacterial infection, but their death certificates only report AIDS as the cause to avoid double-counting. In 2019, one in three people who died of AIDS also had TB, creating a gap of 208,000 TB-related deaths. Combined with the knowledge that the number of asymptomatic carriers is likely exaggerated, the threat of lethality posed by this microbe becomes more alarming.
One of the goals of the End TB Strategy is to reduce new cases by 80 percent before 2030, but they are falling short of this target by 9 percent. Asymptomatic spreaders could partly contribute to this shortfall. Healthcare professionals often reserve diagnostic tests and treatments for the unwell. This creates a blind spot in the disease’s epidemiology as researchers lack a sense of scale for the impact posed by people who show no outward signs of sickness.
When the WHO compared TB prevalence data among asymptomatic and symptomatic people in nine Asian countries, they discovered that 34–68 percent of people with positive sputum tests showed no TB symptoms. Researchers at John Hopkins University argued that undetected, subclinical TB could account for up to 10 percent of prevalence — as many as 10 million people — based on the proportion of individuals who test positive in a microbiological assay but show no symptoms. Carriers often host copious volumes of the bacteria, at least 10,000 microbes for each milliliter of sputum, which is high enough to transmit them through the air, even by breathing alone. Sometimes symptoms like coughing are easily mistaken for a common cold, and patients often wait for such symptoms to pass before seeing a physician and getting tested. This lag time further exacerbates transmission.
With so many people suffering from the active form of the disease, healthcare workers have deprioritized screening for latent infections. However, tracking the dormant disease might prove essential for quashing TB cases. Ultimately, there is a massive need to develop a new slate of diagnostics sensitive enough to pick up latent infections but inexpensive enough to be readily available.
TB Tests of Tomorrow
Physicians often identify M. tuberculosis by viewing sputum smears under a microscope. But increasingly, scientists have begun experimenting with a principle from astronomy, rather than microbiology, to gain deeper insight into patient infections.
When astronomers use telescopes to view the stars, they don’t only capture an image of the stellar masses: They also collect the spectra of wavelengths that the stars absorb and emit, providing insight into their elemental makeup. The new TB diagnostics do something similar — they allow scientists to capture spectra from M. tuberculosis, revealing details about infections that could guide the course of treatment.
Engineers at Khon Kaen University in Thailand are developing one such technique based on Raman scattering, a phenomenon in which compounds excited with one wavelength of light emit photons in a different part of the electromagnetic spectrum. The wavelength of emitted light depends on the properties of the compound, and a solution of different compounds produces a complex spectrum resembling hills and valleys.
A spectrum produced by acetone molecules held in a quartz cuvette. Credit: Dinesh Dhankhar
The team hypothesized that sera from TB patients, carrying biomolecules produced by M. tuberculosis, would produce a unique spectrum. To test this theory in a small proof-of-concept study, they recruited four groups of people: 26 who had active TB infection; 20 with latent TB, meaning they showed no TB symptoms but tested positive by other methods; 34 people with no signs of infection despite exposure to TB patients; and 38 healthy volunteers.
After isolating sera from the blood of these four groups and drying out the samples, they excited them with a green laser and collected their Raman spectra. From a distance, the spectra of all four groups looked identical, as one would expect for sera mostly composed of the same material, namely blood proteins and lipids. But upon closer examination, the hills and valleys for each group differ in height, highlighting the individual differences caused by either the bacteria or the immune factors tackling the infection.
The researchers argue that the signature landscapes produced could allow for more precise and detailed diagnoses in the future, discriminating between active, latent, and cleared TB infections. This could correct the record on the disease’s epidemiology and help catch and treat the condition before it has a chance to spread.
This test is still only a proof-of-concept, and the engineers will have to contend with the constraint that Raman spectroscopy relies on complex equipment when they come to package the test as diagnostic. They could face similar obstacles as the compact PCR tests: unusable in places with no electricity and prohibitively expensive for patients like Enzokuhle.
Engineers at Stanford University may have a different answer. They built a portable, affordable microscope called Octopi with a range of imaging modalities useful for diagnostics, including Raman spectroscopy. Octopi uses Raman scattering to discriminate between M. tuberculosis bacteria that confer resistance to any one of four commonly prescribed antibiotics, which could quickly inform physicians about the best treatment options.
Scientists have come a long way from tapping their fingers against the chest of a patient to listen for signs of TB. And while some of the antiquated diagnostics, including the sputum test and the skin test, remain widely used, researchers have invented new techniques with better sensitivity and specificity. Now, inventors are tackling the next diagnostic challenge. Using Raman spectroscopy, they can collect more information from a single test, including the stage of the pathogen’s lifecycle or its drug resistance.
With the increased understanding of TB’s epidemiology that these novel diagnostic tests offer, we inch closer to controlling the illness. Yet better diagnostics will not suffice. In part two, we delve into the limitations posed by current vaccines and antibiotic treatments. We also discuss the potential for alternatives. As with the next-gen diagnostics under development, these provide hope that one day TB may fade into a relic of art, novels, and romantic poetry.
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Thanks to Saloni Dattani, Xander Balwit, Devon Balwit, Niko McCarty, and Merrick Pierson Smela for editing drafts of this essay.
Kamal Nahas is a researcher-turned-journalist based in Oxford, UK, who covers stories in biology, health, and technology.
Cite: Nahas, Kamal. “The Forgotten Pandemic.” Asimov Press (2024). DOI: https://doi.org/10.62211/62ur-55df
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Footnotes
- Although TB tends to be less common in the West, it has a higher prevalence among incarcerated individuals as crowded prison conditions might aid the spread of the airborne pathogen.
- The mucus samples from sputum tests can spread M. tuberculosis to healthcare workers if the diagnostic facility lacks adequate biosafety precautions. However, scientists at Harvard University and Massachusetts Institute of Technology are looking to urine as a safer testing alternative. Urine rarely carries M. tuberculosis bacteria but often holds some of its biomarkers that filter through the kidneys.