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Infections are mildly correlated to population, with little evidence supporting warming hypothesis from non-experts

Naegleria fowleri is one nasty microorganism.  A member of the amoeba family, it resides in inland bodies of water in North America.  It attacks humans -- particularly young men.  An alarmist report, backed by a non-expert, is proclaiming that global warming may drive infections.  Is this the case?  Let's examine the facts.

I. Background

The amoeba enters via the nose and works its way into the brain where it multiplies, slowly killing the victim.  The time between infection and onset of symptoms is nearly a week and the symptoms are rather nondescript at first -- headaches, vomiting, and confusion.

Once infected, the host's brain is literally eaten alive.  The disease is referred to as PAM (primary amebic meningoencephalitis).
PAM infection
[Image Source: Trevor Haggie]

Since 1962 only one patient (out of 128 confirmed infections) has survived and infection by the deadly parasite -- a 99 percent fatality rate.  In most cases, the infection isn't even caught.  Vanderbilt University medical school professor William Schaffner describes diagnosis as more luck than any other factor, remarking, "It’s not something that’s necessarily touched on in medical school.  You have to really probe what patients were doing in the last several days, you have to ask if they were swimming. Honestly, an accurate diagnosis is basically serendipity."

N. Fowleri
N. Fowleri is a deadly amoeba. [Image Source: CDC]

Given the low rate of infections -- and even lower rate of infections that are recognized before death -- treatment is more guesswork than science.  Patients are administered a cocktail of antibacterial and antifungal compounds.  Jonathan Yoder, an expert with the Centers for Disease Control’s division of parasitic diseases, describes in a recent interview with The Verge, "That’s very frustrating for us.  Did the treatment only work that one time? Does it work at all? Would other treatments work better? We just don’t know."

The potentially bad news is that PAM incidents -- at least correctly diagnosed ones -- are on the rise.  Here's a graph with the # of deaths and state of the death by year:

PAM incidents by year
PAM incidents by year [click to enlarge] [Image Source: CDC]

Notice that there were big jumps in confirmed infections in the 1970s and in the 2000s.  So what's to blame?

II. Hypothesis: Warming is To Blame

The Verge has run a rather alarming piece entitled "Brain-eating amoebas thrive in US lakes as global warming heats waterways".  To be fair they have a source for this -- Travis Heggie, a Bowling Green State University (Ohio) Sports and Leisure professor.  Now Professor Heggie has no professional degrees in epidemiology or microbiology, but he tracks N. fowleri infections as a hobby.

Yet, he speaks authoritatively, telling The Verge, "The climate is changing, and let me tell you, so is this.  If warm weather keeps up, I think we’ll see N. fowleri popping up farther and farther north."

Warming
Is warming to blame for the rise in infections? [Image Source: Tehran Times]

The Verge's Katie Drummond writes, "That speculation seems to be reinforced by recent cases of PAM, once a health woe confined to fresh water in southern states like Texas and Arizona. In Minnesota, public health officials were stunned to see two fatalities caused by N. fowleri — both young children — in 2010 and 2012. The cases are the only in state history, and occurred about 550 miles farther north than any previous reported PAM fatality in the US."

The author is correct, two infections have occurred in Minnesota which are the only infections reported in Northern states.  But note that nowhere in the piece do the real experts -- Dr. Schaffner and Professor Yoder -- suggest that the rise in infections is related to warming (a hypothesis fielded by Professor Heggie alone).  Let's dig into whether the rise in infections is truly correlated with warming.

III. The Boy Who Cried Warming (Hypothesis Invalidated)

Let us consider factors that could drive rates of confirmed infections up (along with a suggestion on how it might impact infection rates):
  1. Warming (most N. fowleri resides in water 86 degrees and up)
  2. National population (more people would mean more infections, you'd expect)
  3. Urban population (people use more crowded water resources)
  4. Papers on the disease (more interest means more awareness, more confirmations)
  5. Mutations (hardy strains survive in new habitats)
Let's acknowledge, but toss out #5, as we don't have the data to track that.

To analyze this, let's first compile the rate of infections per decade which peaked from 2002-2011, thus far [Sources: 12].  For #1 let's take the average rise in surface air temperatures over the U.S. on a decade averaged basis [Source].  For #2 we can use widely available census data [Source].  For #3 we can use data from a world clearinghouse [Source].  For #4 we can use Thomson Reuters "Web of Knowledge" Product.

With this data in hand, we simply analyze the correlation of cases per decade versus influence and here is what we find:
PAM Correlation
Actually climate change has the lowest correlation of any examined factor.  The strongest correlations are with the population itself (around 0.86), and more specifically with the urban population (around 0.85).  

I'd be inclined not to discount the relatively high correlation to the amount of peer-reviewed research published per decade, though it's hard to say whether this is a cause or an effect.  Ironically The Verge report hints at this.  Mr. Yoder comments, "I'll swear my life to that — this is being wrongly diagnosed [for bacterial meningitis].  If someone died of meningitis, I’d suggest looking into that case a little more closely ... and looking at where the brain was eaten away at."

A 1970 study shows that for every 16,000 cases of bacterial meningitis, one was actually a misdiagnosed case of PAM.

But even these correlations are highly speculative when one looks at the graph of change in the various variables, scaled to change in PAM infections per decade:

PAM Correlation

Is there an outside possibility that warming is having some affect on PAM?  Perhaps, but it seems unlikely, given that most cases remain in southern states, some of which have actually cooled (like Georgia):
NOAA warming by state
[Image Source: EPA]

Furthermore, while Minnesota is among the states to have warmed more over the past century, Michigan and the Eastern states have warmed at least as much -- yet had no cases.  Michigan and Minnesota have roughly the same number of lakes [Source: 12] over 100 acres -- yet there were no infections in Michigan.

In other words, while N. fowleri is certainly a poorly understood disease and frightening public health threat.  But to blame warming for this disease (or publicize such claims) is highly dubious.  Quite simply put, the correlation is weak, as is the supporting evidence for such a claim.  It appears this is yet another case of crying wolf.

Sources: CDC via Epidemiology and Infection [abstract], The Verge



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This article is over a month old, voting and posting comments is disabled

True trend line
By JPForums on 6/28/2013 9:39:46 AM , Rating: 2
I'm having a hard time seeing the alarmingly rising trend that is suggested. Rather than a steady rise, I'm seeing what looks more like a cyclical pattern with a slightly rising trend.

If look at the worse 5yr period post-1990 (picked arbitrarily) and compare it to the worse 5yr period pre-1990 you find that both the periods 1998-2002 and 1977-1981 net you 20 cases (see bar graph). Expanding to find the worse 10yr period you have both the periods 1998-2007 and 1978-1987 with 32 cases. I suppose you could accuse me of cherry picking, but I did so for both periods.

Putting that aside, without adding them all up, there does seem to be more cases on the more recent half of the chart than the earlier time period. One could say there is an slight increasing trend. However, it is logical to assume that many cases in the period after its discovery and before widespread awareness were simply missed. That said, while awareness as most certainly improved, there is no way of knowing how many cases go misdiagnosed today. It could simply be that we catch more cases now than we did before and the true trend line is flat. The correlation with the papers released suggest this is at least partly true.

In my opinion, we have both better awareness as suggested by the paper correlation, and a very slightly increasing trend line that probably has a lot to do with population growth. It also makes sense that it would be more highly correlated to urban population growth as Naegleria fowleri proliferates in warm bodies of water, including warm water discharge from industrial plants (http://en.wikipedia.org/wiki/Naegleria_fowleri).

I commend your skepticism Jason. Travis Heggie, a Sports and Leisure professor has no more business spreading such alarmist theories about microorganisms he has no expertise in than your typical DailyTech poster (like me). Granted, it was The Verge that ran the story, but who gave it to them it the first place.




RE: True trend line
By maugrimtr on 6/28/2013 11:22:48 AM , Rating: 2
Is it alarmist theory? Yes. Absolutely. The data is almost flat post 1970 and you can cherry pick similar counts between different decades. That all reeks of the rate of detected infections being highly variable and quite possibly down to dumb luck in diagnosis.

So the reality is that we have no reliable data on which to base any hypothesis.


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