Toxoplasma gondii is a common parasite¹ found in the guts² 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 prey³, Toxoplasma takes the evolutionarily sound course of being a "good" parasite, leaving the rats perfectly⁴ healthy. Or are they?

Oxford⁵ scientists discovered that the minds of the infected rats have been subtly altered.

In a series of experiments, they demonstrated that healthy rats will prudently⁶ avoid areas that have been doused⁷ 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 parasites⁸ 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 excerpt⁹, a volunteer strapped¹⁰ 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 prod¹¹. 

　　中文：

　　美国科学家最近指出，精神分裂症可能与一种叫做弓浆虫的寄生虫有很大的关系。弓浆虫是一种人畜共有的寄生虫，目前全世界有一半左右的人口已经感染了弓浆虫。这种寄生虫以猫为最终宿主，所以差不多有10％的猫带有弓浆虫病，而人、猪、牛、羊、狗、鸟和鼠类等都是弓浆虫的中间宿主。

　　染上弓浆虫 “疯”耗子不再怕猫

　　据美国“生活科学”网站2月10日报道，美国斯坦利医药研究所的科学家们在利用老鼠进行一种抗焦虑药物实验时，用猫尿味儿来引起老鼠的神经紧张。结果却发现感染了弓浆虫的实验鼠的大脑发生了微妙的变化。

　　科学家们发现，健康的老鼠会很小心地避开那些沾有猫尿的区域；而被感染了弓浆虫的老鼠却没有对猫尿作出任何反应，不仅如此，这些老鼠还不止一次地跑到有猫尿的区域——“可怕”的弓浆虫为了找到猫作为其最终宿主，竟然改变了老鼠的大脑和行为，让老鼠自己去找死！

　　弓浆虫是否也能改变人类的行为？

　　关于寄生虫，特别是外星寄生虫入侵人类大脑的故事，早已成为国内外科幻小说家们热衷的题材之一。20多年前，美国电影《异形》就向我们描述了外星生物进入人体、吸食血液并控制大脑思维的故事。虽然那是科幻故事，但就寄生虫到底能否改变人类行为这一说法，目前科学界还真掌握了不少论据： 首先，世界上约有300万精神分裂症患者同时感染了弓浆虫病；感染弓浆虫与大脑内星形胶质细胞受损有关（星形胶质细胞是一种围绕和支持神经元的神经胶质细胞），而神经分裂症同样与星形胶质细胞受损有关；本身具有高水平弓浆虫抗体的孕妇所生下的孩子长大后不易患上精神分裂症；人类细胞感染了弓浆虫后会与氟哌丁苯（haloperidol）之类的药物产生反应，从而阻止弓浆虫的生长，而氟哌丁苯是一种常用来治疗精神分裂症的强安定药。

　　科学家们还发现了“拯救”那些被寄生虫控制、徘徊在有猫尿地方的老鼠的办法，那就是氟哌丁苯。这种药可以重新培养老鼠对猫尿的恐惧。而事实上，实验还发现这种安定药与专门杀灭弓浆虫的药物——息疟定（pyrimethamine）的作用差不多。

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