Episode 18: Nodding Disease and Traveling Life

While I’m here though, I’ll pose some questions on the Nodding Disease segment since it seemed less clear to me than most segments. It seems accepted that O. Volvulus has something to do with Nodding because of the 93% of Nodding sufferers have it, but what about the last 7%? If we expect the number to be 100%, is the uncertainty in the statistic just that large? Or if we don’t expect 100%, what does the high incidence of O.Volvulus actually tell us? Other than the two diseases are endemic to the same areas. Maybe it’s whatever causes Nodding Disease that makes a person more susceptible to catching O. Volvulus.

The concomitant immunity hypothesis seems to beg the same question. How can you suppose that O. Volvulus provides concomitant immunity when most Nodding sufferers have it and obviously aren’t immune? You’d expect most Nodding children to not have Volvulus. Unless you assume the Nodding parasite is being detected as Volvulus, in which case, wouldn’t you expect 100% incidence? (Again, I wish we had error bars.) In any case, it seems far fetched to me that *all* adults who could otherwise have caught Nodding Disease have all got River Blindness instead.

Anyway, I’m no parasitologist, and there are probably coefficients of susceptibility and relative aggressiveness and progression rates of the parasites that make it all work out. Just doesn’t seem like the most obvious conclusion to me.

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KWeinersmith Post authorApril 6, 2012 at 12:19 AM

Hey!

Thanks for asking for clarification. It’s useful for me to learn when I’m doing a good job of explaining something and when I’m not.

I also don’t think that the O. volvulus explanation is particularly convincing. There are, however, a few reasons why you can have 7% of infected people not have the disease while still considering that parasite to be a reasonable explanation for the disease. The reasons are mainly error-related. So it’s possible that those 7% are people who have some other disorder that resembles Nodding Disease, but is not actually nodding disease. Perhaps they are having neurological symptoms because they encountered some environmental contaminant, and the symptoms looked enough like Nodding Disease that they were incorrectly diagnosed as having it. Alternatively, it’s possible that these 7% have Nodding Disease, but whatever test they use to diagnose Nodding Disease yielded a false negative (i.e., the test said they didn’t have the disease when they actually do).

Quick summary: Those 7% may not have Nodding Disease in the first place, or they do have it and a mistake was made in the test to confirm that they have it.

You make a very good point. It is possible that whatever it is that does cause Nodding Disease could simply make people more susceptible to O. volvulus. Or, Nodding Disease and River Blindness may share a similar risk factor (e.g., both can be caught near rivers), so children who are likely to have one are likely to have both.

Zach mentioned after the show that I may not have explained concomitant immunity very well. Sorry about that. I meant to say that concomitant immunity can happen when someone is infected by a parasite, and the immune system of the host uses this parasite as a template so it knows to infect things that look similar to that parasite in the future. This means that, in the future, the immun system will be much more likely to destroy parasites of the same species, and sometimes evene closely related parasite species.

So, imagine you have 2 types of O. volvulus. O. volvulus Type 1 causes River Blindess, and never moves to the nervous system. O. volvulus Type 2 is the same parasite species, but instead of staying in the skin it moves directly to the nervous tissue. So in both cases it is one species, but each type is doing something slightly different. I’m imagining that Type 2 is a mutated version of Type 1. If all adults became infected by Type 1 when they were kids, then they would be protected from Type 2 as adults. So they’ll have River Blindness, but not Nodding Disease. The reason I brought Nodding Disease up is because I was trying to work my way through the argument for O. volvulus as the cause of Nodding Disease if none of the adults were getting infected. For concomitant immunity to be a reasonable explanation then either all adults would have to be infected with Type 1 O. volvulus, or the only adults who encounter Type 2 O. volvulus are adults who are only infected with Type 1. I don’t actually know what percentage of the adult population is infected with O. volvulus.

I agree that the hypothesis that Nodding Disease is caused by the same parasite that causes River Blindness isn’t a particularly satisfying one. From my readings it seems to be the most popular hypothesis at the moment, so I was trying to walk through various ways in which this hypothesis could be supported. Apparently I wasn’t very effective! Hopefully I’ll do better next time.

Thanks for the comment and thanks for listening! Hope this explanation helped a bit!

Best,
Kelly

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1. Morris Keesan April 22, 2012 at 1:18 AM
  
  “the immune system of the host uses this parasite as a template so it knows to infect things that look similar to that parasite in the future.”
  I hope that was just a typo, and that you meant to say “so it knows to ATTACK things that look similar”. Otherwise, I’m totally confused, and don’t understand the human immune system as well as I thought I did. Our immune system works by our white blood cells detecting and destroying intruders (such as bacteria), right? But the WBCs don’t “infect” those things, do they?
  
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  1. KWeinersmith Post authorApril 23, 2012 at 2:08 PM
    
    You are correct. I meant attack, not infect. Thanks!
    
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