Monday, January 21, 2013

Disease driven poverty

In a few of my previous blog posts I’ve discussed the relationship between poverty and infectious disease.  Many of the most prevalent and severe infectious diseases in the world disproportionately affect the world’s poor.  Part of the reason is that the necessary resources aren’t available for tackling such diseases.  A lot of money is currently spent (wasted?) on designing biomedical ‘cures’ for diseases that persist in some places (usually economically poor places) while having already been eradicated in other places (usually economically rich).  It is my position that diseases such as malaria and tuberculosis remain major threats to some populations simply because of the way that financial resources are allocated in our extremely heterogeneous world.

But there is another angle to this story.

Not only does poverty lead to poor health, but poor health can also lead to poverty.  Quite frequently, that is, the arrows point both ways and the reality is a system in which there are “positive” feedback loops.  There is a growing literature on this type of system which is frequently referred to as a “poverty trap.”  Much of this literature has been in economics, where mathematical models have indicated that populations with infectious diseases are less able to ‘develop’ economically.

With economic development at the population level, e.g. with the growth of average income levels, we tend to see an increase in overall life expectancy at age 0.  Most likely this indicates a relationship between improved health and increased wealth.  However, most of the models that actually look at this relationship are either ecological (they are looking at the entire population and frequently assume homogeneity within the population) or at an individual level.  A few models have also looked at community or household levels.

One major problem with models of all types is that results can change when we change our unit of scale.  The effects of poverty on disease, for example, might be different if we look at a community level rather than a province/state level or even consider an entire nation to be a single population.  This is a problem known as the ecological fallacy (and is closely related to the modifiable areal unit problem) in which causal relationships at the population level don’t explain what is happening at, say, the individual level.  

Regardless of these problems and issues, there does appear to be a feedback loop between poverty and disease.

And this is an interesting thing from an anthropological view.  First off, poverty can mean different things to different people.  For example, to some, poverty means “not modern.”  Some indigenous groups actually choose to live in a traditional house rather than a more modern one.  In my opinion, “traditional” (or not modern) does not equal poverty, but it does get mistaken as poverty.  Poverty can also be a relative thing; something that becomes apparent when you don’t have as much stuff as the people with whom you are coming into contact.  Clearly this may lead to psychological and sociological issues, but it might also explain gradients in outcomes (relative health?)  Finally, there is a type of poverty that exists where people are simply unable to put food on the table.  While there can be some argument about the effects of modernization and relative poverty, I would suggest that this final type of poverty is unambiguous and its negative effects are less debatable.  

In poverty trap models we are frequently interested in investigating and understanding threshold levels under or above which equilibria are reached.  (There are quite a few relatively new papers out that are excellent references (see: Bonds, Keenan, Rohani, & Sachs, 2010; Plucinski, Ngonghala, Getz, & Bonds, 2013; Wood, n.d.)).  Perhaps it is easiest to consider at the unit of the household.

An already poor or marginally poor household in which the major breadwinner is afflicted by severe disease is plagued with multiple problems.  For example, aside from the risk of infection for other household members, if that person is afflicted by malaria or dengue fever, they may not be able to work for several weeks.  A house on the margin of poverty may then fall just enough behind in household money and/or food to fall into true poverty.  Households that are already poor may fall even further.

And an important aspect of this situation is that not only is the person who is actually infected met with further troubles; the entire house is also afflicted.  Furthermore, there tends to be heterogeneity in these effects even within households.  That is, poor households may see things such as greater infant mortality, and this effect may be exacerbated when there is a shortage of food or resources in the household.

In poverty trap models, there are usually equilibrium points in poverty levels that, once reached, are quite difficult to break.  From Bonds et al. (2010):

What may be most important in these debates is therefore not whether the effect of health on poverty is more significant than that of poverty on health, but whether the combined effect is powerful enough to generate self-perpetuating patterns of development or the persistence of poverty.  

Children who grow up in households with frequent food shortages may not have the same physical or cognitive abilities as others.  Their immune systems, already taxed by years of exposure to pathogens, may not be able to fight off diseases as well as their healthy counterparts.  Therefore, when they begin their own households, they are already behind in the nutrition, health, and economic game.  And once again, when adults in the new household fall ill and cannot put food on the table, the children will be disproportionately affected.  

This cyclical pattern, where disease leads to poverty and poverty can lead to disproportionate disease, provides a perfect storm in which there aren’t enough resources to keep from getting sick, where once sick you are likely to fall further into poverty, and once you fall further into poverty you are even further away from pulling yourself and your family out.  This leads to the maintenance of poverty and sickness across the generations.

And this story could perhaps get even more complicated when we consider some evolutionary implications.  For example, populations that have historically been afflicted with malaria also tend to have high proportions of blood and blood-related disorders that seem to protect against malaria.  Almost all of these disorders are harmful in some cases (for example, in homozygotes).  Therefore the evolutionary history of disease can lead to a situation where some individuals are actually plagued with sickness from the very beginning of life.  Paradoxically, under situations of heavy malaria burden, some people with these disorders will apparently be healthier than their non-affected counterparts.  I don’t know whether the side effects of these disorders are enough to lead to poverty traps on their own.

Finally, in an age when many scientists appear to be looking “for the gene for (fill in your favorite thing to study)”, poverty traps and households are an interesting thing to ponder.  Poverty and the apparent predisposition of household members toward succumbing to disease can look like a genetic effect.  If it runs in families, and certainly both poverty and sickness do, then it can look a whole lot like there is a genetic reason for it.  I think that poverty trap models are therefore a nice illustration of how we could arrive at the same phenotype (poverty and sickness) from purely socio-economic and ecological factors.  


Bonds, M. H., Keenan, D. C., Rohani, P., & Sachs, J. D. (2010). Poverty trap formed by the ecology of infectious diseases. Proceedings of the Royal Society B: Biological Sciences, 277(1685), 1185–92. doi:10.1098/rspb.2009.1778

Plucinski, M. M., Ngonghala, C. N., Getz, W. M., & Bonds, M. H. (2013). Clusters of poverty and disease emerge from feedbacks on an epidemiological network. Journal of The Royal Society Interface, 10(80), doi: 10.1098/rsif.2012.0656.

Wood, J. (in press). The Biodemography of Subsistence Farming: Population, Food and Family. Cambridge University Press.


Anonymous said...

It makes me wonder that if we really wanted to improve health in this country, rather than large scale sequencing projects, we should do more to eliminate poverty.

Ken Weiss said...

I personally would say that we should first eliminate those causes of misery and shortened or worsened life, before we spend so much resources in the way we're doing.

Or, another way to put it, is that we know very well a number of diseases that truly are genetic. Let's put the resources into showing that we can use genetic knowledge to alleviate that suffering, instead of pouring resources into trying to turn primarily environmental problems into genetic ones.

Not all of the former are due to poverty. Many common diseases of aging are due to dysbehavior by the privileged, not just the poor.

But both poverty and indulgent, lazy lifestyles obscure what are really genetic disorders. Of course, as far as the serious challenge of eliminating poverty or changing indulgent life-styles goes, lotsa luck!

Daniel M Parker said...

I would say that if we could actually eliminate poverty, then we'd probably get more bang for our buck. Eliminating poverty is no easy thing to do though! But perhaps it is easier to raise the floor to a minimum acceptable level rather than to think there will ever be real equality. As an example, I'm not saying everyone should be wealthy or even middle class, but in a region as wealthy as North America, there shouldn't be anyone going hungry.

Also, as Ken mentioned, I'm not sure what good it does to spend time and money looking for genetic factors in diseases that are clearly the result of socio-political and economic factors. We know of quite a few diseases that are genetic, maybe our genetics-based efforts should spend more time working on alleviating them.