experts estimate that many allergies and immune-system diseases have doubled, tripled or even quadrupled in the last few decades, depending on the ailment and country.Immune Systems Increasingly on Attack trots out the "hygiene hypothesis" again. I never found the argument that overly zealous housekeeping and parenting is to blame for immunological disorders very convincing.
The theory has also gained support from a variety of animal studies. One, for example, found that rats bred in a sterile laboratory had far more sensitive immune systems than those reared in the wild, where they were exposed to infections, microorganisms and parasites.Were the two groups of rats genetically identical? Perhaps lab rats come from genetically susceptible stock? After all, many lines of lab rats and mice were bred to be genetically susceptible to cancer; some forms of cancer are caused or exacerbated by immunological dysfunction.
"It's sort of a smoking gun of the hygiene hypothesis," said William Parker of Duke University.
Which lifestyle is more stressful for rats? The laboratory environment or the wild?
Why are inner-city kids, who experience higher exposures to rat and cockroach dander, more rather than less likely to suffer from asthma than suburban kids?
What about the Japanese studies that showed that asthma and cedar pollen allergies increased the most in children that were exposed to both higher cedar pollen AND ozone counts? Kids exposed to either/or did not experience the huge increases in allergies and asthma. (I wonder if anyone followed up with whether there is something uniquely irritating about ozone or if it is a proxy for general air pollution?)
Unlike William Parker, I don't see a "smoking gun" cause and effect there. I have an alternate explanation which I hope people will set out to disprove with well-designed scientific studies. What if, in the past, fewer people with genes that cause immune dysfunction survived to reproduce?
Anyway, I am not a physician and this is not my area of research. I am merely a scientist who suffers from serious immunological dysfunctions. Blame my mother's housekeeping standards, and you will get me riled up. Criticize my housekeeping standards and I am not responsible for what happens to you.
BTW, I did have an infestation of worms as a child. In Taiwan, where I spent some of my youth, "night soil" was used to fertilize the vegetables. Worms were common. Cockroaches were common, and humongous. So don't blame the lack of worms or cockroaches for the state of my immune system.
I really can't believe that excessive housekeeping or cleanliness has anything to do with it. That is just another extension of this blind looking back at the "good old days" and forgetting about all the bad things that happened.
ReplyDelete"Why are inner-city kids, who experience higher exposures to rat and cockroach dander, more rather than less likely to suffer from asthma than suburban kids?"
ReplyDeleteThey're also exposed to more fumes from cars.
And in fact the hygeine hypothesis is somewhat misnamed - because it actually relates to how many bacterial infections you have as a child, not just to whether everything you put in your mouth was sterilised. And of course no-one is suggesting that *all* auto-immune diseases are caused by overly-clean environments, because clearly some happen in environments that are not overly clean.
There's a good article here about the whole thing. The study the article refers to is here, in abstract.
If it's all caused by genetic susceptibility, how do you explain why kids with siblings or pets are less likely to have allergies or asthma?
Seems to me that although the hygeine hypothesis is not 100% definitive, there is a fair bit of evidence stacked up, both in terms of population studies and explanations of the underlying biology, and the Ockham's razor thing would suggest that the hygeine hypothesis is a better explanation that a greater genetic predisposition developing in recent times.
I know that my kid is growing up without pets because she has a parent allergic to pets. If you control for that, the evidence on exposure to pets is much, much weaker.
ReplyDeleteMy understanding is asthma is way up in this country even in the last 50 years. Death-due-to-disease-before-child-rearing-age is down since 1958, but (and I say this not knowing the actual numbers) I guess I'd be surprised to learn that it's down enough to account for the big increase in asthma. I think your hypothesis is equivalent to saying that a large fraction of today's asthmatic children are being born to parents whose genetics are such that, had THEY been born in 1950, they would not have survived to bear children. The vast majority of children born live in 1950 lived to be 40 years old.
ReplyDeleteEric
I recently read an article on the heritability of traits that are NOT related to genetics, but rather to environmental influence.
ReplyDeleteThis flies in the face of widely accepted notions of genetics, but is apparently accepted now. The researcher explained that gene expression can be inhibited by environmental factors, and that this expression inhibition is heritable in some way.
This concept may have some applicability to the dirt exposure hypothesis.
Come to think of it, you might have seen this article too. I think it was in the Cal Alumni magazine.
I was thinking more along the lines of the genetic disease that I have. I was also thinking about my aunt who became ill as a young adult and died a few years later--with eerily similar symptoms to mine.
ReplyDeleteI spent many sleepless nights Googling the particular gene and related diseases. Several papers postulated that the gene appeared to have been dying out in most populations, until the onset of medical advances that made it possible for people with the gene to survive to reproductive age.
The same can be said for type I diabetes, another autoimmune disease. Until insulin shots became available, type I diabetes was dying out, too.
I also want to point out that genes aren't completely destiny. The gene has to be triggered. That environmental factors have a large role in gene expression, I have no doubt.
ReplyDelete