据报道，捷克科学家近日指出，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，存在于猫的粪便中，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。自从19世纪20年代以来，医生已经确认在怀孕期间被感染的妇女会将疾病传染给胎儿，从而导致胎儿的严重大脑损坏，甚至死亡。而在成年人中，这种感染会引发类似感冒的症状，对于那些免疫系统低下的人群来说，会导致类似脑炎这种严重的并发症，这种感染大多是潜伏的，并没有明显病症。
这位63岁的捷克科学家普实克—弗莱格尔（Jaroslav Flegr）称自己就是活生生的例子，据他所述，自己已被弓浆虫感染，在过去的20年里，他的性格改变了，导致行为怪异，且常有自虐行为发生。他在接受大西洋月刊采访时表示，约在30年前，他在读了英国牛津大学动物学家理查德道金斯的一本书后，就开始禅思自己的推理。书中描述了扁形虫通过借用蚂蚁身体，感染他们的神经系统来延续生命周期。这种感染改变了蚂蚁的行为，例如，当遇到温度下降时，昆虫一般会呆在地下，但相反的是，这些蚂蚁却爬上草叶，将草叶压弯，最后它们就被牧羊群吃掉，从而扁形虫就会在动物肠道里继续生命周期。弗莱格尔说：“这是我了解到的第一例寄生虫操纵行为案例，对我影响深刻，之后我便开始注意自身行为与蚂蚁之间的相似处。”
据利兹大学首席研究员—格伦麦肯基龙（Glenn McConkey）医生研究证明，寄生虫会影响多巴胺的产量，携带信息的化学品在大脑里控制运动方向、行为认知，由此引发精神分裂症和躁郁症，寄生虫通过在细胞中形成囊肿，分泌络氨酸羟基化酶来影响大脑。研究显示，大部分精神分裂症患者都有弓形体病的临床史记录，由于导致该病情的原因包含很多因素，因此找到根治精神分裂症的疗法非常困难，科学家目前也仍不十分清楚，寄生虫为何选择人类作为寄主。而且对于老鼠来说，效果也同样很明显，科学家在研究中发现，被感染的啮齿动物相比那些未被感染的更加活跃，这就表示被感染的老鼠对于猫来说就是更具吸引力且明显的目标了。来自伦敦大学的寄生虫学家对此解释道，由于寄生虫只能在猫体内存活，因此老鼠就成为这些寄生虫操纵的目标。她还发现，正如研究人员曾经所展示的研究结果，被感染的老鼠行为表现更活跃，对于潜伏着的“敌人”也失去了一定的警惕性。(生物谷 Bioon.com)
doi:10.1371/journal.pone.0028925
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The Distribution of Toxoplasma gondii Cysts in the Brain of a Mouse with Latent Toxoplasmosis: Implications for the Behavioral Manipulation Hypothesis
Miroslava Berenreiterová, Jaroslav Flegr, Aleš A. Kuběna, Pavel Němec
Background
The highly prevalent parasite Toxoplasma gondii reportedly manipulates rodent behavior to enhance the likelihood of transmission to its definitive cat host. The proximate mechanisms underlying this adaptive manipulation remain largely unclear, though a growing body of evidence suggests that the parasite-entrained dysregulation of dopamine metabolism plays a central role. Paradoxically, the distribution of the parasite in the brain has received only scant attention.
Methodology/Principal Findings
The distributions of T. gondii cysts and histopathological lesions in the brains of CD1 mice with latent toxoplasmosis were analyzed using standard histological techniques. Mice were infected per orally with 10 tissue cysts of the avirulent HIF strain of T. gondii at six months of age and examined 18 weeks later. The cysts were distributed throughout the brain and selective tropism of the parasite toward a particular functional system was not observed. Importantly, the cysts were not preferentially associated with the dopaminergic system and absent from the hypothalamic defensive system. The striking interindividual differences in the total parasite load and cyst distribution indicate a probabilistic nature of brain infestation. Still, some brain regions were consistently more infected than others. These included the olfactory bulb, the entorhinal, somatosensory, motor and orbital, frontal association and visual cortices, and, importantly, the hippocampus and the amygdala. By contrast, a consistently low incidence of tissue cysts was recorded in the cerebellum, the pontine nuclei, the caudate putamen and virtually all compact masses of myelinated axons. Numerous perivascular and leptomeningeal infiltrations of inflammatory cells were observed, but they were not associated with intracellular cysts.
Conclusion/Significance
The observed pattern of T. gondii distribution stems from uneven brain colonization during acute infection and explains numerous behavioral abnormalities observed in the chronically infected rodents. Thus, the parasite can effectively change behavioral phenotype of infected hosts despite the absence of well targeted tropism.
