Tuesday, March 8, 2016

Murmurations and you

I have a doctorate in Public Health which means that, unlike a 'real doctor', I was trained to think in terms of the health of populations, not of specific individuals.  Public Health of course, when done appropriately, can have an enormous impact on the health of individuals, but in a very real way that's a side effect of gathering group information and instituting measures meant to affect a group.  Clean water, fluoridated water, vaccinations, window screens, anti-smoking campaigns, and so much more are all public health measures targeting whole populations, without regard for the specific cavities or cases of cholera or lung cancer that the measure will actually prevent.  This is because, of course, smoking doesn't make every smoker sick, just enough of them that aiming to convince whole populations not to smoke can have a large enough difference on population health that it's worth the cost and effort.

You've probably seen those murmuration videos showing enormous flocks of birds flying as if they were one; undulating, turning, responding as though they have a collective mind.  Here's one is of a flock of starlings being hunted by a peregrine falcon one evening in Rome. The starlings fly so unpredictably that, at least this time, the falcon is unable to catch a meal.

Source: BBC One

According to the Cornell Lab of Ornithology, murmurations almost always arise in response to the detection of a predator; a falcon or a hawk that has come for its dinner, as the starlings in Rome.  So, a bird or birds detect the predator and sound the alarm, which triggers the whole flock to take off. But, how do they stay together?  Who decides where they're going next, and how does the rest of the flock get the message?

Young et al. report, in a 2013 paper in PLOS Computational Biology, that once in flight each bird is noticing and responding to the behavior only of its seven nearest neighbors.  The murmuration, the movement of the group, then, is due to local responses that create the waves of motion that can be seen in the evening sky.  There is no single leader, just many, many local responses happening almost simultaneously.

The same kinds of dynamics explain the movements of schools of fish as well.  They work to some extent, but fish are routinely attacked by sharks, which can scoop up multiple individuals at a time, and surely sometimes birds of prey manage to snap up a luckless bird among the thousands or millions in a flock.  But, most of the fish or the birds do get away, so it's a winning strategy for the group.  Public Health in action.

Well-known, very prolific British epidemiologist George Davey Smith was interviewed on the BBC Radio 4 program The Life Scientific not long ago.  He's a medical doctor with a degree in Public Health as well, so he's been trained to think in terms of both the population and the individual.  He is currently interested in what genes can tell us about environmental influences on health.  One of his contributions to this question is the analytical tool called Mendelian Randomization, which aims to tease out environmental triggers of a trait given a particular genetic risk factor.  That is, the idea is to divide a study sample into individuals with and without a particular genetic variant, to determine whether their history of exposure to an apparent risk factor might be responsible for the disease.  In this instance, the gene isn't modifiable, but exposure might be.

In the interview, Davey Smith said that his primary interest is in population health, and that if a Public Health measure can reduce incidence of disease, he's happy.  So, if everyone in a population is on statins, say, and that reduces heart disease and stroke without major side effects, he would consider that a successful Public Health measure.  Even if it's impossible to know just who's stroke or heart attack was prevented.  Success of Public Health can only be evaluated on the population, not the individual level.

So much for personalized, predictive medicine.  That's fine, my training is in Public Health, too, so I'm ok with that.  Except that Davey Smith is also a fan of large, longitudinal studies maintained in perpetuity because, as he said, they have yielded more results at lower cost than most any other kind of epidemiological study.

But there are problems with such studies, and if the idea is to identify modifiable environmental risk factors, a major problem is that these studies are always retrospective.  And, as we've written here so often, future environments are not predictable in principle.  Presumably the aim of these large studies is to use Big Data to determine which Public Health measures are required to reduce risk of which diseases, and if that is done -- so that large segments of the population are put on statins or change from saturated to unsaturated fats or start to exercise or quit smoking -- this changes environmental exposures, and thus the suite of diseases that people are then at risk of.

So, Public Health has to always be playing catch up.  Controlling infectious diseases can be said to have been a cause of the increase in cancer and obesity and heart disease and stroke, by increasing the number of people who avoided infectious disease to live to be at risk of these later diseases.  So, in that sense, putting whole populations on statins is going to cause the next wave of diseases that will kill most of us, even if we don't yet know what these diseases will be.  Maybe even infectious diseases we currently know nothing about.

Even though, after putting their favored Public Health measure into effect, all the starlings outwitted the falcon that particular night in Rome, they're all eventually going to die of something.

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