Fatal Attraction

(consciousness redux) Fatal Attraction Some protozoa infect the brain of their host, shaping its behavior in ways most suited to the pathogen, even if it leads to the suicide of the host By christof Koch

The ancient debate surrounding pose, replication—like a computer virus ity, these invaders make the host’s brain the existence of free will appears unre- that reprograms an infected machine. do things counterproductive to its own solvable, a metaphysical question that Such is the case with T. gondii. It sex- survival. generates much heat yet little light. Com- ually reproduces only in the intestines of Toxoplasmosis has been most thor- mon sense and volumes of psychological cats yet can maintain itself indefinitely in oughly studied in rats and mice. Both and neuroscientific research reveal, how- any warm-blooded animal. Infected cats species have a deep-seated, innate fear of ever, that we are less free than we think cats for obvious reasons. Spray a bit of we are. Our genes, our upbringing and cat urine into a corner, and the rodent our environment influence our behav- will avoid this location, well, like the iors in ways that often escape con- plague. In contrast, an infected an- scious control. Understanding imal loses its innate fear of cats. this influence, the advertise- By some measures, it even ap- ment industry spent appears to be mildly attracted to proximately half a trillion the smell of felines. This is an dollars worldwide in unfortunate turn of events for 2010 to shape the buy- the rodent, because it is now ing decisions of con- more likely to be successfully sumers. And extreme hunted by a cat. On the other dictatorships, such hand, this is a great deal for T. as that in North gondii. When the cat devours Korea, remain in the sick critter and its contami- power through nated brain, T. gondii moves the effective use into its final host, where it repro- of insidious and duces, completing its life cycle. all-pervasive forms Not quite what the romantics of propaganda. Yet have in mind when they write nothing approaches about “the circle of life”! the perfidy of the one- The behavioral manipula- celled organism Toxo- tion induced by T. gondii is plasma gondii, one of quite specific. The infected the most widespread rodent doesn’t look sick; of all parasitic proto- its weight is normal; it zoa. It takes over the We humans think moves about nor ) we have free will, but brain of its host and we are less free than we think: mally, possibly a bit makes it do things, parasites can influence the behavior of ani- more frantically even actions that will mals—including our own species. than other mice; cause it to die, in the it grooms itself; and head and earwig ( H service of this nasty hitchhiker. It sounds shed millions of their oocysts in their fe- it interacts routinely with its conspecifics. U like a cheesy Hollywood horror flick, ex- ces. Taken up by all kinds of animals, Think how different this case is from NR U cept that it is for real. including dogs, rodents and humans, what happens in rabies, another nasty in- ACK

We know that illness in general can they infect muscle and the brain to es- fection. The animal loses its instinctual ); J slow us down, incapacitate us and, in the cape attacks by the host’s immune sys- shyness, aggressively attacking others Koch worst case, kill us. Yet this organism is tem. Hidden away, they remain dormant (the proverbial mad dog), thereby spread- much more specific. Natural selection as cysts, surrounding themselves with a ing the rabies virus through its bite. But has given rise to pathogens that infiltrate tough cell wall. Yet this quiet stage of in- because T. gondii can reproduce only in the nervous system and change that sys- fection, called toxoplasmosis, is decep- felines, it wants its host to be eaten by

tem’s wiring to achieve its ultimate pur- tive. Violating all rules of good hospital- cats, not by just any carnivore. And be- ( koch C hristof

16 scientific american mind May/June 2011 Infected rodents lose their fear of cats. By (some measures, they even appear to be mildly attracted.)

cause cats hunt live prey and do not eat eases is tantalizing but remains murky. carrion, T. gondii must not immediately Recent claims go so far as to argue for kill its temporary host. a role of T. gondii in shaping distinct cul- tural habits, depending on the rate of in- Rodents Aren’t Superheroes fection in the population. A prospective How does T. gondii effect its insidi- study tracking the road safety in Czech ous changes in the host? Experiments by recruits during their 18 months of com- Joanne P. Webster of Imperial College pulsory military draft found a rate of ac- London, Robert Sapolsky of Stanford cidents six times higher in affected driv- University and others have shown that A cyst of Toxoplasma gondii resides in the ers. Are the young men with toxoplas- infected rats or mice do not turn into the brain. Can these silent invaders, which re- mosis infection simply slowed down? Or main for life in the brain of the infected indi- murine equivalent of Siegfried, the hero vidual, control us without our awareness? do they drive more aggressively? of Wagner’s Ring who knew no fear. No, In my November 2009 column, I de- they still avoid open spaces, remain noc- fection rate is seven to eight times higher, scribed the discovery by cognitive neu- turnal creatures, retain their aversion to possibly because of the widespread con- roscientists that the feeling of freely will- the urine of other predators and learn to sumption of uncooked and undercooked ing an action (called authorship or agen- fear a tone associated with a foot shock. meat). Human toxoplasmosis is usually cy) is a subjective, conscious sensation Might the protozoa have stunted their considered to be symptom-free (what doc- no different, in principle, from the con- smell? After all, if they cannot smell tors refer to as asymptomatic). Exceptions scious awareness of seeing the azure blue anything anymore, they would not are patients with a weakened immune sys- sky or feeling the sharp pain of a tooth- know how to avoid places smelling of tem and the unborn (hence the need for ache. When I engage in a dangerous pur- cat urine. But infected mice still avoid pregnant women to avoid cleaning cat- suit, such as taking the end of the rope food if it smells different—an aversion litter boxes). on a steep section of a granite wall in that arose partly because for centuries Science has known for a long time Yosemite Valley while climbing, I feel as humans have been trying to control ro- that schizophrenic patients are two to if “I freely decided” to do so, whatever dents by poison. The infected mice also three times more likely to carry antibod- this might mean in a metaphysical sense. respond appropriately to the smell of ies to T. gondii than are controls who are Yet my action is most likely caused by an their littermates. not schizophrenic. Furthermore, anti inexhaustible multiplicity of factors not Clues about how the parasites affect psychotic drugs that block the action of accessible to my conscious introspec- the animal come from several observa- dopamine, such as haloperidol, com- tion, including, yes, possibly some tiny tions. First, the density of cysts in the monly used in the treatment of schizo- single-celled parasites lodging in my amygdala is almost double that in other phrenia, are also effective in combating brain and making me act out their silent brain structures involved in odor percep- toxoplasmosis in both rats and people. commands. The wonder of it all. M tion. Parts of the amygdala have been And some infected adults go on to de- linked to anxiety and the sensation of fear. velop psychotic symptoms similar to CHRISTOF KOCH is Lois and Victor Troendle Second, the genome of T. gondii contains schizophrenia. Little is known about Professor of Cognitive and Behavioral Biology two genes related to mammalian genes in- the mode or site of action of this patho- at the California Institute of Technology. He nc. I volved in the regulation of dopamine, the gen in the human brain. The exact link serves on Scientific American Mind’s board molecule associated with reward and plea- between T. gondii and psychiatric dis- of advisers. sure signals in the brain, including in ours. So perhaps the creepy protozoa makes sui- cidal activities, such as hanging around (Further Reading) places frequented by cats, feel more plea- surable for the infected rodent? ◆◆Bugs in the Brain. Robert Sapolsky in Scientific American,Vol. 288, No. 3, pages 94–97; March 2003. What elevates this vignette about evo- rce/photo researchers, researchers, rce/photo ◆◆The Effect of Toxoplasma gondii on Animal Behavior: Playing Cat and Mouse. Joanne P. lution and life in the wild to epic propor- Webster in Schizophrenia Bulletin, Vol. 33, No. 3, pages 752–756; 2007. tions for humanity is that about a tenth of ◆◆Manipulation of Host Behavior by Toxoplasma gondii: What Is the Minimum a Proposed the U.S. population is infected by T. gondii Proximate Mechanism Should Explain? Ajai Vyas and Robert Sapolsky in Folia Parasito-

u so science (in some countries, such as France, the in- logica, Vol. 57, No. 2, pages 88–94; 2010.