Toxoplasma gondii is a common parasite found in the guts of cats; it sheds eggs that are picked up by rats and other animals that are eaten by cats. Toxoplasma forms cysts in the bodies of the intermediate rat hosts, including in the brain. 

    Since cats don't want to eat dead, decaying prey, Toxoplasma takes the evolutionarily sound course of being a "good" parasite, leaving the rats perfectly healthy. Or are they? 

    Oxford scientists discovered that the minds of the infected rats have been subtly altered. 

    In a series of experiments, they demonstrated that healthy rats will prudently avoid areas that have been doused with cat urine. In fact, when scientists test anti-anxiety drugs on rats, they use a whiff of cat urine to induce neurochemical panic.

    However, it turns out that Toxoplasma-ridden rats show no such reaction. In fact, some of the infected rats actually seek out the cat urine-marked areas again and again. The parasite alters the mind (and thus the behavior) of the rat for its own benefit. 

    If the parasite can alter rat behavior, does it have any effect on humans? 

    Dr. E. Fuller Torrey (Associate Director for Laboratory Research at the Stanley Medical Research Institute) noticed links between Toxoplasma and schizophrenia in human beings, approximately three billion of whom are infected with T. gondii: Toxoplasma infection is associated with damage to astrocytes, glial cells which surround and support neurons. Schizophrenia is also associated with damage to astrocytes. 

    Pregnant women with high levels of antibodies to Toxoplasma are more likely to give birth to children who will develop schizophrenia. 

    Human cells raised in petri dishes, and infected with Toxoplasma, will respond to drugs like haloperidol; the growth of the parasite stops. Haloperidol is an antipsychotic, used to treat schizophrenia. 

    Dr. Torrey got together with the Oxford scientists, to see if anything could be done about those parasite-controlled rats that were driven to hang around cat urine-soaked corners (waiting for cats). According to a recent press release, haloperidol restores the rat's healthy fear of cat urine. In fact, antipsychotic drugs were as effective as pyrimethamine, a drug that specifically eliminates Toxoplasma. 

    Are parasites like Toxoplasma subtly altering human behavior? As it turns out, science fiction writers have been thinking about whether or not parasites could alter a human being's behavior, or even take control of a person. In his 1951 novel The Puppet Masters, Robert Heinlein wrote about alien parasites the size of dinner plates that took control of the minds of their hosts, flooding their brains with neurochemicals. In this excerpt, a volunteer strapped to a chair allows a parasite to be introduced; the parasite rides him, taking over his mind. Under these conditions, it is possible to interview the parasite; however, it refuses to answer until zapped with a cattle prod. 

    中文：

    美國科學家最近指出，精神分裂癥可能與一種叫做弓漿蟲的寄生蟲有很大的關系。弓漿蟲是一種人畜共有的寄生蟲，目前全世界有一半左右的人口已經感染了弓漿蟲。這種寄生蟲以貓為最終宿主，所以差不多有10％的貓帶有弓漿蟲病，而人、豬、牛、羊、狗、鳥和鼠類等都是弓漿蟲的中間宿主。 

    染上弓漿蟲 “瘋”耗子不再怕貓 

    據美國“生活科學”網站2月10日報道，美國斯坦利醫藥研究所的科學家們在利用老鼠進行一種抗焦慮藥物實驗時，用貓尿味兒來引起老鼠的神經緊張。結果卻發現感染了弓漿蟲的實驗鼠的大腦發生了微妙的變化。 

    科學家們發現，健康的老鼠會很小心地避開那些沾有貓尿的區域；而被感染了弓漿蟲的老鼠卻沒有對貓尿作出任何反應，不僅如此，這些老鼠還不止一次地跑到有貓尿的區域——“可怕”的弓漿蟲為了找到貓作為其最終宿主，竟然改變了老鼠的大腦和行為，讓老鼠自己去找死！ 

    弓漿蟲是否也能改變人類的行為？ 

    關于寄生蟲，特別是外星寄生蟲入侵人類大腦的故事，早已成為國內外科幻小說家們熱衷的題材之一。20多年前，美國電影《異形》就向我們描述了外星生物進入人體、吸食血液并控制大腦思維的故事。雖然那是科幻故事，但就寄生蟲到底能否改變人類行為這一說法，目前科學界還真掌握了不少論據： 首先，世界上約有300萬精神分裂癥患者同時感染了弓漿蟲病；感染弓漿蟲與大腦內星形膠質細胞受損有關（星形膠質細胞是一種圍繞和支持神經元的神經膠質細胞），而神經分裂癥同樣與星形膠質細胞受損有關；本身具有高水平弓漿蟲抗體的孕婦所生下的孩子長大后不易患上精神分裂癥；人類細胞感染了弓漿蟲后會與氟哌丁苯（haloperidol）之類的藥物產生反應，從而阻止弓漿蟲的生長，而氟哌丁苯是一種常用來治療精神分裂癥的強安定藥。 

    科學家們還發現了“拯救”那些被寄生蟲控制、徘徊在有貓尿地方的老鼠的辦法，那就是氟哌丁苯。這種藥可以重新培養老鼠對貓尿的恐懼。而事實上，實驗還發現這種安定藥與專門殺滅弓漿蟲的藥物——息瘧定（pyrimethamine）的作用差不多。