A parasite that infects the human brain, subtly changing its personality and social behavior, and capable of passing from mother to infect an infant in utero? That is the essence of a body horror, but this little rascal isn’t fiction. And it gets better: this parasite is considered to be one of the most successful parasites in the world due to its widespread, global distribution as well as its capacity to infect nearly every type of body tissue in all warm-blooded vertebrates (a). Schedule a phone conference with Spielberg and Cruise ASAP, guys, we’ve got the next sci-fi-action blockbuster on our hands (brains?). We’re looking at the ubiquitous protozoa Toxoplasma gondii and research on its capacity to modulate human personality and behavior.
T. gondii is an obligate intracellular parasite that has a vast host distribution, capable of infecting all species of mammals. Domestic cats and other felines are, however, the definitive host for the parasite’s reproductive stage. As such, all other animals serve as intermediate hosts of this polyxenous parasite. Humans are usually infected through consumption of infected raw or undercooked meats that happen to be studded with tissue cysts, typically lamb or pork (a). Turns out that T. gondii infection, known as “toxoplasmosis”, is the most common food-borne parasitic infection that requires hospitalization, and the third most common food-borne illness overall (b). Indeed, a study in 2002 conducted in the United Kingdom examined commercial meat sold in grocery stores and found that 38% of samples were infected with T. gondii cysts (h).
Examples of Toxoplasma gondii transmission. Image: Nature Education. Click for source.
Humans can also acquire infection through contaminated water and contact with cat feces that contain oocysts. Depending upon one’s diet and exposure to cats, researchers estimate that up to 80% of the population may be infected with T. gondii (k). Unwelcome news for lovers of carpaccio and cats.
Acute toxoplasmosis includes symptoms very similar to a cold - fever, headache, sore throat and coughing - along with a few psychopathological features including depression, anxiety, apathy and paraesthesia (“pins and needles” sensation) (c). The parasites (in the form of tachyzoites, one stage of its life cycle) infect macrophages and are distributed throughout the body. Over time, parasite-containing cysts form as a response to the body’s acquired immunity and are commonly found in the brain, lymph nodes, lungs and liver (a). In 60% of cases, infection is asymptomatic and many people are unaware that they’re even infected (g). The parasite does its worst damage among immunocompromised patients, either as a primary infection or as a recrudescent infection, and is a dangerous complication for those who are HIV-positive. Congenital transmission in which a woman becomes infected during pregnancy is also of public health concern. A newborn exposed in utero may be born with profound neurological and ocular sequelae, such as microcephaly, deafness, retina damage and mental retardation (g). Spontaneous abortion or stillbirth may also be another heart-wrenching outcome of primary toxoplasmosis.
T. gondii oocysts in a fecal floatation. Image: CDC. Click for source
A great literature review by Pappas et al. examined worldwide seroprevalence levels to craft a global epidemiological picture of the disease. Click here to check out the map. Prevalence of the disease was found to be highest in Europe and South America and may be attributed to a greater consumption of undercooked meats (b). Most infectious diseases are geographically limited to specific locales due to their climatic requirements, access to intermediate hosts and other factors. The most interesting aspect of toxoplasmosis is that these little buggers can be found everywhere in pretty much every population group of humans studied! Toxoplasma is a remarkably adaptive and successful parasite, and not only because of its ubiquitousness - it also keeps its hosts alive (for the most part) instead of scrambling our brains by provoking an inflammatory immune response.
An intriguing hypothesis is that infection can bring about personality and behavior changes (k). T. gondii is neurotrophic and exhibits a particular preference for the glial cells that provide support to the brain’s neurons. That toxoplasmosis may modulate human behavior is fascinating in itself as it’s already been proven with rodents; rats infected with the parasite show behaviors associated with decreased anxiety and neophobia, as well as increased levels of aggressiveness (d). Such marked changes have been interpreted as the parasite increasing the rodent’s risk of cat predation, adapting to its intermediate rodent host by enhancing the likelihood of transmission to its definite feline host (d). The mechanism is unclear but many speculate that the parasite’s location in such a privileged spot in the brain may affect neuro-immunomodulation and neurotransmission (f). Sneaky stuff.
So consider this: Toxoplasma may also affect its human host’s behavior so as to enhance the parasite’s transition to its definite host in the cat. In other words, the parasite may be manipulating how we think and act to maximize the possibility ... of our brain being eaten by our feline friends.
Of course no such thing happens. But indeed, there are a few studies that have found correlations between toxoplasmosis and changes in personality profiles. One such study conducted in 2000 by Flegr et al. used Cattel’s 16 Factor Questionnaire, a self-reported personality profile survey, to look at differences in personality traits amongst acute and latent toxoplasmosis patients. Both stages of the disease were correlated with increases in warmheartedness, outgoingness, easygoingness and high will-power in women, while the same factors were associated with decreases in men (d). Infected women are more likely to respect social rules, be amicable, attentive and loyal to others than women who are uninfected; infected men are more introspective, suspicious and have low self-esteem (g). Both women and men score higher for levels of anxiety than those uninfected (d)(g). Women seem to luck out with toxoplasmosis, whereas men turn into something like a jealous ex-boyfriend stalking you on Facebook.
Light micrograph of a cyst containing T. gondii parasites (stained red) in brain tissue. Thousands of resting parasites can be seen enveloped by the thin parasite cyst wall. Image: Science Photo Library. Click for source.
I read these results and am hesitate to fully endorse them. Don’t get me wrong, it’s a neat idea. Brain zombies? I’m totally there! But maybe this is the molecular biologist in me, reluctant to believe self-reporting psychological surveys. These ideas of behavior and personality modulation are also pretty controversial. And, of course, there’s the issue of correlation versus causation. Does toxoplasmosis cause a change in one’s personality or does a type of personality (say, a Brazilian cat-lover who is fond of undercooked pork) predispose one to infection with toxoplasmosis?
The researchers claim that the fact that identical personality changes can be seen in both the acute and latent cases serve as an indication of the persistence of the parasite’s effect. In another study by Flegr that looked at infected women over the course of fourteen years, he claimed that the high anti-Toxoplasma antibody titre seen in these patients reflects the long-term duration of the infection and can thusly be attributed to long-term personality shifts (d). However, IgG antibody levels typically level off in chronic infections due to a reduction in antigenic stimulation as the parasite settles down into its new human home (i). Does the parasite permanently change a person’s personality upon acute infection or does it continually modulate it over time? There’s a lot of questions raised in the paper but, as a thought experiment, it’s an interesting read.
In a similar vein is an article written by Kevin Lafferty that uses Flegr’s results. In “Can the common brain parasite, Toxoplasma gondii, influence human culture?” Lafferty speculates that the personality profiles resulting from toxoplasmosis, with increased feminine qualities in women and more masculine ones in men, may explain “greater differentiation of sex roles” in countries with rigid gender norms (e). The relationship between toxoplasmosis and the common personality profiles of a population may explain some aspects of the “personality” of a culture, in particular gender roles and expectations (e). You can find it here. More thought experiment stuff, really.
So. Brains. Toxo. What to make of it? There’s a wealth of research and reviews out in Pubmed-land that look at behavioral and mental disorders and the prevalence of toxoplasmosis infection. Schizophrenia and epilepsy are the two main neurological disorders that seem to continually be coincident with toxoplasmosis (g). Of course, everything will boil down to the boxing match between correlation vs. causation, as well as the incontrovertible fact that infection, mental disease, personality and the reason why people like cats (no, seriously, why?) result from a multitudinous array of factors - genetics, history, culture and culinary taste - that probably have nothing to do with this parasite. But maybe that steak should be more on the medium rare side, hmm? It’s food for (neuromodulated) thought!
ResourcesT. gondii is one of those crazy protozoan parasites with complicated life cycles. I greatly simplified the details and you should go to the CDC site
to get more info about the biology and such. If you're curious about that Cattel 16 Factor Questionaire, you can take a look at it here
. References (a) Despommier, D, Gwadz RW, Hotez PJ and Knirsch CA. Parasitic Diseases. 5th ed. New York: Apple Trees Production, LLC. 2006 (b) Pappas G, Roussos N & Falagas ME. (2009) Toxoplasmosis snapshots: Global status of Toxoplasma gondii seroprevalence and implications for pregnancy and congenital toxoplasmosis. Int J for Parasitology.39:1385–1394 (c) Flegr J, Zitková S, Kodym P, Frynta D. (1996) Induction of changes in human behaviour by the parasitic protozoan Toxoplasma gondii. Parasitology. 113 (Pt 1):49-54 (d) Flegr J, Kodym P, Tolarová V. (2000) Correlation of duration of latent Toxoplasma gondii infection with personality changes in women. Biol Psychol.53(1):57-68 (e) Lafferty KD (2006) Can the common brain parasite, Toxoplasma gondii, influence human culture? Proc Biol Sci. 273(1602): 2749-55 (f) Fekadu A, Shibre T, Cleare AJ. (2010) Toxoplasmosis as a cause for behaviour disorders--overview of evidence and mechanisms. Folia Parasitol (Praha). 57(2): 105-13 (g) da Silva RC, Langoni H. (2009) Toxoplasma gondii: host-parasite interaction and behavior manipulation. Parasitol Res.105(4):893-8 Epub (h) Aspinall TV, Marlee D, Hyde JE, Sims PFG. (2002) Prevalence of Toxoplasma gondii in commercial meat products as monitored by polymerase chain reaction—food for thought? Int J Epidemiol. 32:1193–1199 (i)Remington JS and R McLeod. Toxoplasmosis. Infectious Diseases in Medicine and Surgery (3rd Edition). J Bartlett, S. Gorbach, N Blacklow (Eds). Lippincott Williams & Wilkins: Philadelphia, 2003 (j) Daryani A, Sharif M, Hosseini SH, Karimi A and Gholami S. (2010) Serological survey of Toxoplasma gondii in schizophrenia patients referred to Psychiatric Hospital, Sari City, Iran. Tropical Biomedicine. 27(3): 476–482
Flegr J, Kodym P, & Tolarová V (2000). Correlation of duration of latent Toxoplasma gondii infection with personality changes in women. Biological psychology, 53 (1), 57-68 PMID: 10876065