Archive | February, 2012

Dogs, Dust, and the Hygiene Hypothesis

7 Feb Since I’m still only getting about 3 or 4 hours of sleep at night, extracurricular reading for this blog has to be uber-interesting to maintain my attention long enough to keep me from falling asleep halfway through the abstract.  Not to say that anything microbial isn’t particularly interesting, but the bar has been set a little higher now that I’m a SciMom, and baby girl likes to eat… a lot.

So, in looking for something exceptionally interesting to blog about this week, I stumbled on the perfect storm for today’s paper, which piqued my interest for several reasons that may or may not be obvious you folks out in readerland.   First, I just had a baby a few months ago and find myself continuously thinking I need to do more research on this or that which will affect the baby’s health and wellbeing.  Second, I have “indoor pets”, namely several German Shepherds (see the shamelessly cute photo below) and two cats.  Third, I was diagnosed with allergy-induced asthma when I was about 9 years old and have struggled with it ever since (you may be wondering at this point, why on earth I would have so many indoor pets if I have allergic asthma… just call me a glutton for punishment, but I love my dogs).

The unifying theme of the paper, however, and overarching interest for me is, of course, due to the very cool and omnipresent impacts of microbial ecology.

Exactly how microbial ecology plays a role in a study on prenatal and early childhood exposure to indoor pets and how that affects immunoglobulin E may not be overtly obvious, so let me explain.

What does hygiene have to do with it?

First, you need a cursory understanding of something called the “hygiene hypothesis”.  It’s pretty huge in the health news these days so I wouldn’t be surprised if you’re at least vaguely familiar with the concept.  Essentially, it’s the idea expressed beautifully by Rob Dunn in Eating off the floor: How clean living is bad for you.  The basic idea is that the more of our little microscopic friends that we are exposed to early in life, the healthier we are overall due to stimulation of our immune systems in a way that allows natural development and response to true pathogens.  Early exposure to a range of microbes is thought to essentially “teach” our immune systems what is good, safe, or at least not a threat, versus those organisms that can really make us sick and should be eliminated.

The slightly longer than 5-second rule...

If we live in an environment where everything is sterile (at least as much as possible), our air is filtered, antimicrobials are everywhere, antibiotics are over-prescribed, and we aren’t exposed to microbes and parasites, then our immune systems may begin to overreact to anything and everything regardless of the potential harm it might cause us. This would explain the rise in autoimmune disorders and asthma, some even include eczema and hay fever.

As cool as all that is, it is only a hypothesis at the moment, which means we need (lots) more evidence (a.k.a. data) to be able to assert these ideas as a theory, much less a fact (of course we all know that in Science, reaching the status of “theory” is as good as it gets… only the media and snopes assert “facts”).  And it’s even a fairly vague hypothesis at that.  We’re not sure which types of microbes we need to be exposed to, how many, at what times, and precisely which aspects of our immune system will or won’t be affected.

The word for the day: Pets

There is currently a lot of work ongoing around the globe to get at some of these different aspects of the hygiene hypothesis and hopefully begin to provide us with the data we need to assert that, in fact, microbial exposure is a good thing for the development of the human immune system.  One such study came out in 2010  in the Journal of Allergy and Clinical Immunology which compared the microbial communities in dust of homes with and without pets.  They proved what all us pet-owners already know, that the dust of homes with pets supported a much more diverse microbial population than the homes without pets.

Dust mite

In this most recent paper, Suzanne Havstad and her colleagues took this idea one step further.  They set out set out to provide more details of the dynamic interplay between these pet-based dust microbial populations and the human immune system.  This particular puzzle piece involved testing immunoglobulin E (IgE) levels in the blood of young children in response to their exposure, or lack thereof, to indoor pets.

What is IgE, anyway?

Admittedly, the various components of human immune system (and vertebrate biology, for that matter) are well outside the scope of my own training and expertise.  I will therefore refer you to the following article on Immunoglobulin E and this medical article on pediatric asthma (you’ll find the bits on the hygiene hypothesis in under “Pathophysiology”).

In a  nutshell, IgE is part of our immune system which is triggered by allergens and parasites in your environment, including worms.  It causes inflammation that helps your body defend against intruders and keep you healthy.  However, it’s also thought to play a very significant role in pediatric asthma since blood IgE levels in asthma sufferers are typically much higher than non-asthmatics.  Essentially, some people’s immune systems overreact to things like dust, pollen, dander, etc, which leads to excessive production of IgE which in turn causes inflammation of tissues in lungs and bronchial tubes and asthma (not being able to breathe) is the result.

What Suzanne Havstad and her colleagues were able to conclusively prove was that children who were exposed to indoor pets in utero or in their early years had significantly lower IgE levels than those who weren’t exposed to pets.  The more diverse microbial community of the homes with pets must be interacting with the children’s immune systems and fostering the lower IgE levels, right?  It’s certainly possible and makes all the sense in the world to somebody like me.

However, just because [A (pets) = B (more diverse dust  microbial communities)] and [A (pets) = C (lower IgE levels)], doesn’t necessarily in this case mean that B =C as well.  There could be any number of other factors for children in the homes with indoor pets that may or may not have been measured, such as lifestyle, diet, exercise, and exposure to other allergens or toxins in their environment.  These other factors could also be influencing IgE levels.

However, what makes this study more compelling is that they also found an effect of the child’s mode of delivery:  vaginally versus Cesarean section.   The reason I say that this makes the study more compelling is because it’s been found that infants delivered vaginally have an entirely different microbiome (indigenous microbial community) than those delivered by C-section.  This lends some weight to the idea that microbial populations influence IgE levels.

What does it all mean?

This study seems to doubly lend support to the idea than early microbial exposure impacts human immune

A shamelessly cute German Shepherd puppy.

response, particularly via IgE levels.   Indoor pets can clearly increase the diversity and abundance of the microbial populations of your household dust (not a fact, that in and of itself, I really want to spend a lot of time thinking about) and exposure to that dust may actually be a good thing, in moderation (i.e. we probably shouldn’t spoon-feed the dust to anybody), for children’s developing immune systems.  Take home message: every kid needs a dog, or a cat, or both, or to at least get a little dirty every now and then… which means my little girl has got this covered!
Havstad, S., Wegienka, G., Zoratti, E., Lynch, S., Boushey, H., Nicholas, C., Ownby, D., & Johnson, C. (2011). Effect of prenatal indoor pet exposure on the trajectory of total IgE levels in early childhood Journal of Allergy and Clinical Immunology, 128 (4), 880-8850000 DOI: 10.1016/j.jaci.2011.06.039