Tuesday, February 23, 2010

In Praise of...Parasites?

I've come across this idea several times now, first on NPR, then at this article, sent by my ceoliac friend, then in a detailed article in Mental Floss (sorry, article isn't available on-line) magazine.

All of this is in praise of parasites, specifically human intestinal worms, and there potential to dramatically reduce symptoms of many autoimmune disorders.

The idea is this: autoimmune disorders, in which the immune system, for varying reasons known and unknown, attacks a non-threatening substance or even body part, have risen in developed countries pretty much co-incident with the rise of widespread hygiene and the associated de-worming of our guts. According to Mental Floss, it took a dual-vocation gastrointologist and parasitologist and a long plane ride to notice this. As a gastrointologist, Dr. Joel Weinstock studied Chrohn's Disease, in which the immune system attacks the intestines, leading to constant surgery and a generally very painful life outlook. He and his team decided to experiment with parasitic worms, to an astounding success rate. One example: in 2005 he infected 29 Crohn's patients with Trichuris suis, a worm found in pigs and consequently, pig farmers, who tolerate them with little side effects. 23 patients showed improvements, 21 went into complete remission, according to a NY Times Article. Similar studies, by Dr. Weinstock and others who see some promise in the "hygene hypothesis" have been done with worms and type II diabetes, ceoliac disease, and asthma. All report very high success rates in terms of the reduction of symptoms. The theory is that for thousands of thousands of years, we co-evolved with the worms that lived in our guts. That co-evolution was not without it's benefits: apparently the worms kept some check on our otherwise excitable immune systems.

Now, I'm not particularly excited about the prospect of infecting myself with hookworm, even in light of my own extensive issues with allergies, and the success of an asthmatic man who walked across open toilets in the developing world. That man was also told by his doctor that he was lucky he didn't contract something deadly. Several friends have asked me, when I brought this up, and I agree that I want to know, isn't there a good reason we got rid of the worms in the first place?

According to the the Center for Disease Control's on-line archives, hookworm, the parasite most mentioned in the studies, is known to cause mild diarrhea and abdominal pain," while severe infections can cause "stunted growth and mental development." Anemia, protein deficiency, and weight loss may also result. Oh, and joy of joys, the life cycle of an adult hookworm involves attacking itself to the walls of your small intestine and sucking your blood. Perhaps we have such tough immune systems because we were living with these guys in our bodies. Now that we don't, what are a bunch of beefed up white blood cells to do?

In my case, they're gonna attack dust, mold, pollen, woodsmoke particles, with wild and willing abandon. Every single time they encounter a microscopic speck of that stuff.

So for people whose autoimmune disorders are debilitating, maybe the benefits do outweigh the costs. Aside from providing a new cause to hope for many of the worlds (unsqueamish) inhabitants of developed countries, there are several philosophical implications to this new look at parasites.

First is the notion that you can't completely overcome the forces of evolution. It could be argued that humans have already done this, in our outsmarting of the checks placed on us in Africa and subsequent colonizing of the entire world. Yet perhaps those ancestors never had asthma or diabetes. Of course, a lot of them didn't live long enough, because of other factors that may in fact include parasitic disease, to benefit from a wheeze-free, finger-prick free life.

Second is the value of the parasites themselves. We often think of parasites only as a disgusting plague on higher life forms, and to an extent that is a judgment based on our experience with the bad things they can do to us, and other species we care about. But they are actually quite interesting, even sophisticated. Certainly complex.

Take, for instance, toxoplasmosis. This is a parasite that begins in rats, is ingested by the cats who hunt the rats, and deposited with urine into litter boxes, where unsuspecting humans have the potential to contract it. This is why pregnant woman are urged not to clean litter boxes: toxoplasmosis has been linked to birth defects, and is also a suspected cause of schizophrenia. This worm ensures it's survival in a convoluted yet effective way: one of the symptoms of infected rats is their attraction to cat urine and lack of fear of their natural predator. An infected rat will present itself to a cat, in order for the parasite to be ingested.

Parasitic mind-control! (like in that hit series when I was in middle school, Animorphs). Something to be afraid of indeed, at least if you're a rat.

I learned of another one of these in sophomore ecology class. There is a species of parasite that only affects grasshoppers, yet begins its life-cycle as a larvae in the water. As an aquatic larvae, it is ingested by mosquitoes, who in turn are ingested by grasshoppers. This hapless grasshopper, normally ambivalent towards streams and the like, gets the notion in it's little brain that it wants to go swimming. This is not so good an idea for the grasshopper, because it can't swim. Yet it runs toward the water anyway, falls in and drowns, freeing the parasite to lay it's larvae and begin the process once again.

A rather specific, round-about way to do things, if you ask me, and not the kind of think they tell you about on those PBS Nature shows. Yet at the same time, pretty fascinating.

So am I going to infect myself with hookworm to save myself from this expensive and miserable hay fever I suffer?

Er...I'm probably too squeamish. But I'll keep an eye on the science.


  1. The Hygiene Hypothesis has a lot of intuitive appeal, and I believe it should be considered, but I have never seen it tested in any rigorous way. There should be some kind of correlation between these diseases and childhood sanitation. However, it is very hard to find good data on sanitation conditions 20 years ago. Surveys would not be reliable, because memories could be flawed or the questions could be leading. A well-designed survey asking specific questions might work, but ideally you would want some kind of a natural experiment that changed sanitation at random.

    There are very large benefits to eradicating parasites. Hookworms were so common and so devastating that they actually lowered the average IQ of southern states before they were eradicated. They killed and crippled thousands of people. Even if things like Chrohn's and allergies are the result of deworming, the costs of killing parasites are far smaller than the benefits, on average.

    Ideally, we would want to find some kind of organism that does not damage the host too much and cannot reproduce easily, but keeps the immune system busy. Then we can prescribe that as needed. That will take a lot of experimentation, which may or may not even be possible in our current legal and regulatory environment.

  2. I agree, we have benefited enormously from removing them and the host of other bacteria and disease associated with dirty living in general.

    It is good to have options: if I were suffering from Crohn's Disease, which sounds pretty terrible, or the severest level of asthma, I might be willing to put up with abdominal pain and anemia for relief. With the mostly-eradicated status of hookworm and others, only those people who really need the relief need be infected.