In 1896 Scientific American ran an editorial titled "Is insanity due to a microbe?", and thus started a lively discussion on infectious causes of schizophrenia, epilepsy and other diseases of the mind. Apparently, I have benn living under a stone for all of my life, because word of this idea - that microbial parasites could alter human brain function and cause real changes in human behavior - did not reach me until very recently, when WYNC Radio Lab aired an entire episode on parasites.
Of course I had heard - in undergraduate zoology classes - about parasites that infect and release chemicals into ants brains and make them perform all kinds of suicidal stunts, so that the parasite may land comfortably in the stomach of a bird, sheep or whatever else is its preferred definite host, but at the time I was more surprised to learn that ants actually had a brain, than that I seriously contemplated the possibility that tiny parasites could cause similar alterations in human behavior.

Anyway, after listening to the radio lab episode, and subsequently an interview with legendary professor Robert Sapolsky, my interest in the link between parasites and human brain diseases was sufficiently awoken to make me look up some scientific papers on the topic. What I learned - although it is only peripherally a social science matter -, I believe, is interesting enough to warrant a blog post here:
One of the apparently best studied parasites in regards to changes in human behavior (possibly besides certain fungi), are toxoplasmosis gondii; or short Toxo.

Toxo's primary claim to fame is that it makes rodents become sexually attracted to cats; (possibly even very big cats, as might be an interpretation for the above picture by Casey Gutteridge/
Toxo's life cycle includes a number potential intermediate warm-blooded hosts, but its main goal is to end up in the stomach of a feline; which is where it reproduces.
Inside of intermediate hosts, Toxo can cause severe physiological damage, by attacking muscle tissue as well as the brain. As a recent report in the Journal for Parasitology adds,

"[Toxo] is opportunistic infection and a risk for newborns-causing fetal damage in humans-and for people showing any degree of immunodeficiency, such as carriers HIV or other immunocompromising conditions"

Toxo comes in three infective forms

"tachyzoites, found in bodily fluids during acute infection; bradyzoites, found inside tissue cysts during chronic infection; and oocysts, a resistant, immature form shed in the feces of felids."

Humans usually become infected by picking up oocysts from poorly washed food, or from eating undercooked meat, but Toxo tachyzoites can also be transmitted from mother to child during pregnancy; which is why doctors usually tell pregnant women to avoid cats.
60% of Toxo infections are harmless and do not cause any averse symptoms for the host, hence the incidence estimates for the infection might be rather crude, but for what it is worth

"15 to 85% of the adult human population is chronically infected by T.gondii, depending on the geographical area, feeding habits,and contact with cats. In the USA, 1,500,000 infections are estimated to occur annually, with 15% of them being asymptomatic."

Reading about what Toxo does, once inside the human body reads a little like the script for a war movie, so I figure I'm best of quoting researchers Costa sa Silva and Lagoni, who write

"Once inside the host, T. gondii has the ability to cross nonpermissive biological barriers. Tachyzoites cross the placental or intestinal epithelium using paracellular transmigration and enter circulating cells such as macrophages and dendritic cells. They cross the blood-brain barrier and gain access to important sites in the brain. During activation, rhoptries release a family of proteins essential for the invasion and protection of parasitophorous vacuoles."

[Note: Rhoptries are essentially specialized secretory organelles] and parasitophorous vacuoles is a fancy way of describing the protective layer around the parasite].

Unlike BSE causing prions, Toxo goes to work immediately after infecting the brain.
Known effects seem to include almost any behavior you could think of, but one thing that struck me is that Toxo seems to have it out for the men, more than for women. Look at this list of effects for men:

reduced brain function, increased jealousy, introspectiveness, boredom; reduced psychomotor activity and reaction times, increased emotional instability, suspicion, and short temperdness; lowered self-esteem, and disregard for social rules.

Essentially, Toxo turns men into DC Comic's Joker...
Additionally, men also become more prone to guilt and start showing greater group dependency after infection with Toxo.

Women on the other hand showgreater self-esteem, and exhibit more intelligence, awareness, cordiality, amicability, attentiveness to others, loyalty and self-sufficiency. Women reportedly also become more sentimental, socially precise and affective. (Toxo does appear, however, to be linked with schizophrenia in women more often than in men).

As a social scientist, I do not posess any particular insight into the subject matter, yet despite my confidence in the the peer review process for the journals in which these findings were reported, I do maintain some reservations towards the above cited above results. Given that measuring most of the above stated effects is challenging in and of itself, my guess is that the relationship between Toxo and these effects is a little more subtle than stated...but that's just my personal view, and
whatever the case, one very well documented side effect of Toxo infection, for both men and women, concerns loss of psychomotor skills, lack of concentration and increased risk taking. Indeed the effect are so dramatic that

"T. gondii were significantly associated with traffic accidents", and in the Czech Republic" and
"the risk for car accidents [is] 2.65 times greater in infected individuals, no matter if they were drivers or passengers."

Toxo is now also being implicated with schizophrenia, due to the increased number of antibodies against Toxo that are regularly found in schizophrenics. Additionally, there appears to be a general understanding of the chemical processes in which Toxo influences schizophrenic disorders, but here I am totally leaving my realm of competency, and shall therefore leave it up to the inspired chemistry student to blog about this...Suffice it to say, though, that Schizophrenic subjects infected with Toxo show a five-fold increase in mortality rates, over those without Toxo, and that research into parasites has recently (again) become a hot topic for many areas of scientific inquiry; including Evolutionary Anthropology, the Behavioral Sciences, Neurology and Epidemiology.

In case you're interested in some more information about parasites, there are a host (and yes, I thoroughly enjoyed the dual meaning of the word in this context) of excellent sources on the web. I also link to the Radio Lab episode and the Robert Sapolsky video, mentioned above in a recent post at Evolved Primate's sister blog "Ingenious Monkey".

Main Reference: da Silva RC, & Langoni H (2009). Toxoplasma gondii: host-parasite interaction and behavior manipulation. Parasitology research, 105 (4), 893-8

[Note: Picture Credits go to Casey Gutteridge]

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