You are what you're infected with?

Rats infected with the parasite Toxoplasma gondii do crazy things.  They find the scent of cat urine sexy and attractive, they don't run from the actual beasts; they are more active in running wheels, which might indicate that the parasite induces increased activity which may more readily attract a cat's attention. When an infected rat is eaten by a cat, the T. gondii is passed on in the cat's feces to infect again.  T. gondii can only reproduce inside the cat.  Great survival strategy on the part of the parasite, this trick of making the rat no longer fear cats -- now that's really building a better mouse-trap! Did this strategy evolve by adaptive selection, or is it just something that happened?

Czech biologist, Jaroslav Flegr, thinks T. gondii infections do much the same to humans -- his story is told in the March 2012 Atlantic Monthly.  Toxoplasmosis, the infection caused by T. gondii, infects a significant segment of the world's population -- perhaps 20% of Americans, but 55% of French people are infected, probably because the French diet includes more rare or raw meat than the American diet.  The usual mode of transmission is from a member of the cat family to another warm-blooded animal via ingestion of feces from an infected cat, but raw or rare meat can be another source.  It can also be transmitted from mother to fetus, and can result in serious complications in an infected fetus, including stillbirth.  This is why pregnant women are told to avoid litter boxes.

Infection has long been supposed to cause mild flu-like symptoms in otherwise healthy individuals, but then it was assumed that the parasite lay dormant in cysts sequestered away inside brain cells.  People with weakened immunity were at greater risk, however; in the days before antiretroviral drugs for treating HIV, toxoplasmosis infections are thought to have caused much of the dementia in patients with end-stage AIDS.

But maybe the parasite actually does more damage than has been thought.

...if Flegr is right, the “latent” parasite may be quietly tweaking the connections between our neurons, changing our response to frightening situations, our trust in others, how outgoing we are, and even our preference for certain scents. And that’s not all. He also believes that the organism contributes to car crashes, suicides, and mental disorders such as schizophrenia. When you add up all the different ways it can harm us, says Flegr, “Toxoplasma might even kill as many people as malaria, or at least a million people a year.” 

Flegr's hypothesis comes directly from his own experience.  He wondered for years why he was willing to take risks that others wouldn't, like crossing a street in the middle of traffic, or speaking out against communism in Communist Czechoslovakia.  Entirely by fluke, he was tested for T. gondii by someone in his institution looking for infected people to study a diagnostic kit they were developing, and he was discovered to be positive. To him, this explained his bizarre risk-taking behavior.

He reasons that T. gondii is not the only parasite that affects behavior.  The rabies virus incites fury in infected animals, ensuring that they bite others, and thus pass on the infection.  Ants infected with parasitic Cordyceps fungi do all kinds of bizarre, self-destructive things, including climbing onto a blade of grass and then clamping on with their mandibles. Soon the fungus consumes the ant's brain, and fungal fruiting bodies sprout from the ant's head (as in the video) and burst, releasing spores into the air, to settle and find a home in another unsuspecting, soon to be robotic ant. Apparently the Cordyceps fungi release chemicals that change an ant's pheromone reception, which alters their sense of navigation. Is this coincidence, not specific enough to have evolved per se?  Or is it a specific adaptation?



Another example of zombie ants involves infection by the lancet liver fluke, Dicrocoelium dendriticum.  When infected, the ant again climbs onto a blade of grass where it clamps on, there to be eaten by a grazing sheep or cow.  The ant does this only in the evening, when the air cools, and if it survives the night uneaten, it climbs down and behaves normally again until the following evening, when the fluke regains control. Again, this is remarkable, but it is it specific enough and frequent enough to be a Darwinian adaptation?  And what's in it for the poor manipulated ant? 

Things that seem (to human observers) so bizarre probably would be expected to have a balance, or else the victim species would have evolved resistance.  So many questions are raised by these examples.  And there are many more like them. 

But in any case, parasite-induced behavior changes are not unprecedented.  Could T. gondii really do the same?

In the Soviet-stunted economy, animal studies were way beyond Flegr’s research budget. But fortunately for him, 30 to 40 percent of Czechs had the latent form of the disease, so plenty of students were available “to serve as very cheap experimental animals.” He began by giving them and their parasite-free peers standardized personality tests—an inexpensive, if somewhat crude, method of measuring differences between the groups. In addition, he used a computer-based test to assess the reaction times of participants, who were instructed to press a button as soon as a white square popped up anywhere against the dark background of the monitor.

The subjects who tested positive for the parasite had significantly delayed reaction times. Flegr was especially surprised to learn, though, that the protozoan appeared to cause many sex-specific changes in personality. Compared with uninfected men, males who had the parasite were more introverted, suspicious, oblivious to other people’s opinions of them, and inclined to disregard rules. Infected women, on the other hand, presented in exactly the opposite way: they were more outgoing, trusting, image-conscious, and rule-abiding than uninfected women.

Flegr confirmed these surprising findings with further research, finding that infected men were suspicious, sloppy dressers, and introverted, while infected women were well-dressed and gregarious.  Reaction times of infected people were considerably slower than uninfected, and he found that they were 2 1/2 times more likely to be in traffic accidents -- this statistic has been replicated in other countries.  Flegr says that the personality changes are generally subtle, only detectable on a statistical basis.  But, it turns out that a fairly substantial percentage of people diagnosed with schizophrenia are T. gondii positive. 



What's the mechanism? 

Many schizophrenia patients show shrinkage in parts of their cerebral cortex, and Flegr thinks the protozoan may be to blame for that. He hands me a recently published paper on the topic that he co-authored with colleagues at Charles University, including a psychiatrist named Jiri Horacek. Twelve of 44 schizophrenia patients who underwent MRI scans, the team found, had reduced gray matter in the brain—and the decrease occurred almost exclusively in those who tested positive for T. gondii.

That's not clearly a mechanism, however, as the shrinkage could be entirely unrelated to schizophrenia.  Indeed, since only 1/4 of the patients tested showed reduced gray matter.  Anything more convincing?

Apparently, sequencing of the T. gondii genome suggests that it has 2 genes that can make the infected animal increase production of dopamine, and elevated dopamine levels are a mark of schizophrenia. Infection also, apparently, increases the infected animal's gregariousness, and in humans, increases sociability -- even infection with the influenza virus.  Infection can, apparently, even increase a person's (or a rat's) sex drive, and because many of these infections can be transmitted sexually, this improves their chances of being passed on.  This relates to any kind of infection that has been tested, not just T. gondii.

As it turns out, schizophrenia has been associated with a number of infections ("maternal rubella (German measles), influenza, Varicella zoster (chicken pox), Herpes (HSV-2), common cold infection with fever, or poliovirus infection while in childhood or adulthood, coxsackie virus infection (in neonates) or Lyme disease (vectored by the Ixodes tick and Borrelia Burgdorferri) or Toxoplasmosis" -- from a 2011 paper by C.J. Carter), and in fact, while genomewide association studies haven't found genes with major effects, or reliably replicated what they have found, for schizophrenia, itself, they have found 600 genes with small effect, many associated with inflammatory response, others implicated in the life cycle of the associated pathogens.  The same paper suggests that:

Schizophrenia may thus be a “pathogenetic” autoimmune disorder, caused by pathogens, genes, and the immune system acting together, and perhaps preventable by pathogen elimination, or curable by the removal of culpable antibodies and antigens.

That is, the authors suggest that the susceptibility genes code for proteins that are homologous to the pathogen's proteins, and that the latter might be intermingling or replacing endogenous proteins, and they are different enough to disrupt normal function, and lead to disease.

Pathogens' proteins may act as dummy ligands, decoy receptors, or via interactome interference. Many such proteins are immunogenic suggesting that antibody mediated knockdown of multiple schizophrenia gene products could contribute to the disease, explaining the immune activation in the brain and lymphocytes in schizophrenia, and the preponderance of immune-related gene variants in the schizophrenia genome. 

Further,

All of the pathogens implicated in schizophrenia express proteins with homology to multiple schizophrenia susceptibility gene products. The profile of each individual pathogen is again specific for different types of gene product, but all target key members of the schizophrenia network including dopamine, serotonin and glutamate receptors as well as neuregulin and growth-related or DISC1 related pathways.

So, the idea is that our genomes, our particular DNA variants, determine which human/viral matches we carry, and thus which pathogens we're susceptible to damage from.  So, in that sense, Carter, and others, suggest, schizophrenia and other behavioral disorders may be 'genetic', but environmental exposures, our vaccination history and so on determine the pathogens we might be infected with, and our immune system determines how we respond.

To be sure, these are statistical findings and there are so many genes associated with schizophrenia -- or perhaps more accurately so many genes not clearly but weakly, possibly, maybe, but not replicably associated, that it is possible one could almost always find some potential association with these pathways.  That makes it hard to evaluate the infectious scenario.

One clear point, though, is that even when what we are is genetic, the genes need not be those we were born with.  Bacteria, and hence their genes are vital to our survival and that appears just to be for starters.  When parasites affect our gene expression or function, their genomes become part of ours.  And from a biological point of view, our genetic battle for persistence -- for staying alive -- may have more to do with microbial challenges than with wearing out, which is basically what many GWAS targets are about (cancer, diabetes, etc.)

Even more important, perhaps, and a hint that we need to pay more attention to, is that many GWA kinds of studies are finding genes in immune-related systems, or those related to 'inflammation' for what appeared to be totally non-infectious and non-behavioral diseases, even including diabetes, intestinal disorders, retinal disorders of the eye, and many others.  These would be genetic in the sense that genetic susceptibility is involved, but not in the sense of intrinsically harmful genetic variants.

Is this behavioral parasite work definitive?  Do we now know that schizophrenia, and other disorders, are infectious in origin?  No.  Many questions have yet to be answered.  Maternal or early childhood exposure seem to be associated with risk, but why does schizophrenia have such a relatively late age of onset, given early age of exposure?  And why so stereotypically in late adolescence?  And so on.

But, it's intriguing that many GWAS have found an albeit small proportion of risk of many diseases explained by immune genes.

Don't bug with evolution! The evolution of toxic E. coli

E. coli culture; Wikimedia

The sad and serious outbreak of E. coli toxicity in Europe illustrates many interesting, if in this case tragic, aspects of evolution. In this case, the evolution is fast, close to the gene level, and the theory works very well.  The explanation may or may not be (as of this posting) contaminated sprouts from an organic farm in the north of Germany.  If the source is eventually worked out convincingly, the reason will be that so much is known about the genes in E. coli and the process by which genes transfer among strains.

Normally (and again we're guessing here because the story in the media is not very specific), bacteria carry a variety of self-defense genes on small ring-DNAs called plasmids.  During their occasional version of mating, the two mating cells can exchange plasmids.  In this case somehow two different plasmids managed to get into the same E. coli cell which then proliferated in some animal's gut (that's where E. coli normally live) and then were spread somehow to  other animals and then became a contaminant of the food chain of commercial vegetables--or something like that.  It is possible the original gut source was human but again we don't know what the details are from what's been found so far. 

Two mutant genes have been found in these bacteria, suggesting a rare double event history.  The evidence suggests the new strain carries genes from two different groups of E. coli: enteroaggregative E. coli (EAEC) and enterohemorrhagic E. coli (EHEC).  The combination of these two mutants lead to E. coli with the ability to generate a deadly complication of haemolytic-uraemic syndrome.  This affects red blood cells and kidneys and has neurological complications as well.   It also contains several antibiotic resistance genes.  It is reported that the closest known strain was first isolated from the Central African Republic.  We guess from news stories that the bacteria produce two different toxic proteins as a result of these genes.

This is evolution in action because it changes the genome of the recipient bacterial cell, and is then inherited by its cellular descendants.  Because they reproduce rapidly, and with human agency somehow spreading them around, the new strain can expand quickly.  Its new phenotypic properties may or may not give it an evolutionary advantage (if it kills its host before being defecated out to spread further, that's not good for its future), but even if it has no selective advantage over other E. coli strains, it can persist if there's no selection against it.

It's because the toxic compounds are single-gene products with clear effects that the force of selection can be manifest, and that its evolution can be traced by comparison of its sequence to extensive E. coli strain sequence data bases.  Sequencing its small single-chromosome genome is very quick.

The evolution of infection, continued

We posted a few days ago about the proliferation of the way we're causing the evolution of genetically altered pathogens that are resistant to antibiotics. That's because we slam them with exposure to lethal agents, rapidly removing all but the few, or very, very few bacteria (or viral particles) that are resistant, leaving them an open field for competition-free access to resources and hence rapid reproduction.

We have an 'adaptive' immune system that generates clones of white blood cells or antibody molecules they produce. Because of a particular way in which the chromosome region that codes for these antibodies is scrambled to produce the antibody protein, we generate millions of random antibody structures. This is thought to be a way of avoiding the need to evolve pathogen-specific antibodies. If we make countless different antibody molecules, the odds are that at least one will be able to grab onto some part of any bacterial cell or virus that we may chance upon. We don't have to know what it will be ahead of time. Once we recognize it, the lineages of white cells that recognize it are induced to proliferate. They can destroy the pathogen, or recognize and kill cells that have been exposed to it.

Indeed, this molecular random-scrambling defense has found a match in some parasites who have many different versions of cell-surface proteins that they need for their life-cycle, and the idea (that we have about it) is that by the time our immune system has recognized the pathogen's characteristic, their cells switch to a different surface protein. We might kill those presenting the former protein but the new-presenters will have a chance to survive. This would lead to survival of these bugs, but at the same time our immune system will be able at least to contain the infection. Plants use somewhat similar switching strategies to fend off infection.

Such cat and mouse strategies should work. We should be able to detect and get rid of anything that may invade us that can be recognized and dealt with on a cellular level. So why then do we ever get sick? This is an interesting question.

Some pathogens are larger than a single cell or hide themselves in various ways. The malarial parasite gets into our red blood cells, and apparently is not vulnerable to our adaptive immune system. So we have evolved other kinds of defenses, such as red cells that resist invasion by the parasite. Of course, it doesn't always work.

In the end we need to play an extinction game: develop a means of assault at something so fundamental to a pathogen's lifestyle that it can't out evolve us. This is clearly possible in principle: most species that have lived on earth have become extinct. In each case the cause is different. But it should be possible, and some diseases like polio and small pox have been pushed close to the edge of extinction.

On the other hand, the more we present a ready target for rapidly evolving parasites, the faster we'll have to develop strategies to combat them.

Still, it would seem that many kinds of bacteria would never be able to develop characteristics we can't recognize. They may develop resistance to antibiotic attacks on some aspect of their biology, but why can't our immune system always eventually produce an effective antibody? Different diseases will provide different answers, and in most cases the answer is not known. Certainly we don't know the answers. But the evolutionary story is that the rapidly evolving simpler pathogens continually challenge our rapidly changing array of immune attacks.

Evolution for real, and for keeps

Of all the things we should be spending research funds furiously on, infectious disease should be at or near the top of the list. Yet another story has appeared on the proliferation of antibiotic resistant species that are causing death and disease, mainly in hospital patients at present. But there's no reason to expect this to stay contained. And here we face real instances of genetic and evolutionary determinism. Since proper nutrition and exercise would also be a way to avoid massive numbers of otherwise early deaths, because hale people have at least better chances of surviving infection, we should refocus effort, away from luxury and made-up 'genetic' disease to these major societal problems.

There is, in fact, evidence that an increasing number of supposedly old-age, lifestyle-related diseases are going to turn out to be related to infection after all--either directly, or indirectly. Here we are learning from many avenues of research, including, yes we're free to acknowledge it, from GWAS approaches (case-control studies that searched for regions of the genome whose variation was associated with risk). Some cancers like stomach and cervical, among others, turn out to be due largely to bacterial or viral infection. Some heart disease is like that, too. But the genes identified by some of the more successful GWAS turn out to have connection with inflammatory gene networks. These include problems with intestinal, eye, and even psychiatric functions.

Direct infection may cause the disorder (the GWAS would be identifying genes whose variation somewhat protects, or makes one somewhat more susceptbible to the disorder). Or in a substantial number of diseases, the effect may be indirect: autoimmune diseases in which the immune system, after a real infection, mistakes your own cells for attacking bacteria; or perhaps infection in which the successful immune response then leads to chronic inflammation that itself may be damaging (these are called autoimmune disorders).

In genetic studies susceptibility variants may be brought detectably to the fore by the strong point-cause effects of infection, and that could explain why a disproportionate fraction of detected genes--of GWAS genomic 'hits'--seem to be involve the immune system. Since the susceptibility variants are not nearly fixed in the population--it's the fact of their variation that makes them detectable by association studies--they are probably not the result of a history of widespread or very serious infection with strong natural selection effects. And the signals usually only account for a small fraction (usually very small) of what are complex traits involving many contributing genes and environmental exposures.

But antibiotic resistance is a kind of classic Darwinism. Pathogens have simple genomes, huge population sizes, and rapid generational turnover. Antibiotics slam them with a sledgehammer kind of very strong selective effect. This is a cocktail of factors ready-made for the rapid evolution of the favored few--the lucky bacteria or viruses whose makeup makes them resistant. The wipe-out of their susceptible peers leaves an open field for the resistant ones to proliferate, and the rapid turnover and great population growth rate favors quick fixation of the new genetic variant in the population. Further, the dense population of very similar food items (humans, particularly hospitalized humans, at least so far) favors rapid growth and spread of the pathogen population, too.

Here is where evolutionary and genetic theory meet in classically relevant ways. The standard deterministic models work well, because the signal can be so much stronger than the statistical 'noise' of chance events in the lives and times of individual bacteria.

Whereas genetic susceptibility to disease in the host (us) is fairly similar in most individuals, we represent in a sense a standing target for the bugs to adapt to. We do have an immune system designed to evolve as fast as microbes do (it's called our 'adaptive' immune system, and is too intricate to go into here--but it is covered in our book and of course in many sites on the web), even this system may be unable to cope with the hardy pathogens that evolve in survivors of our antibiotic assault.

For whatever reason even the adaptive immune system in vertebrates, including humans, may not have been exposed to environments so changeable as those we're likely to subject ourselves to in the near future if we're not careful. Whatever the reason, if we're slow to respond we're going to teach ourselves a clear lesson in evolutionary genetics.