Yummy or Yuck? Part 2: Let them eat dirt (or not?)

With discussions about increased allergies and hyper-sterile environments (in the (over)developed world at least), there is a body of parenting lore that supports the idea of building your child’s immune system by exposing them to germs through outside play, engagement with animals etc.

Now presumably this idea isn’t completely crazy. After all, we intentionally expose our children to Measles, TB, Polio etc through vaccinations specifically to build up their immunity to these potentially life threatening illnesses. But there is a significant difference between random exposure to bugs and intentionally exposing your child to a specific virus (which may even be dead virus) at a dose which has been carefully tested and under controlled conditions.

So my specific question harks back to part 1 of Yummy or Yuck?, in which I recalled my friend’s words “At least she is building a strong immune system” after eating something that had fallen on the floor…

Should I let my baby eat dirt?

Is there an increase in allergies? And is this linked to lack of exposure?

Let’s start at the beginning and examine the idea that our kids’ immune systems are weaker than in previous years. I’ve often heard that there is an increase in allergies and asthma in the developed world. A little reading seems to confirm that this is genuinely true, with research showing that migrants moving from the developing to the developed world are more likely to develop these issues the longer they remain in the developed country.

This trend has been explained in terms of the Hygiene Hypothesis which states that the increase of auto-immune diseases (such as asthma) and allergies in the developed world are linked to decreased level of infections.

Although there appears to be a fairly large body of data showing a correlation between increased hygiene and decreased immunity, the causal link has remained more elusive. There are however a number of new(ish) studies (such as the mouse study below) which are adding to the body evidence pointing to a causal link, lending credence to the idea that exposing your kids to germs could be a good thing.

Does exposure to microbes build our kids’ immunity?

A 2012 Harvard Medical School mouse study showed that early exposure to microbes plays an important role in building the immune system. Mice that were kept germ-free were much more vulnerable to asthma and a form of bowel disease (both autoimmune diseases).

A surprising 2013 Stanford study showed that adults’ immune systems are able to recognize and react to a large number of pathogens some of which they have never been infected by (like HIV). Reports on the research state that the researchers believe that early exposure to germs help the immune system develop a “memory” by the age of ten which activates when infections occur. (To be honest, this didn’t make a lot of sense to me, so anyone who can explain it, please help me out!)

In addition to these studies, I was seriously surprised to read that some researchers argue that particular intestinal worms play an important role in regulating the immune system. (So once again, bring on the cat food bowl!)

Some of the researchers even argue that a baby’s instinctive tendency to put everything in her mouth is an evolutionary advantage which has been retained to help strengthen our immune systems.

Considering the incredible impact that introduction of basic hygiene practices (hand-washing for example) has had in the past and continues to have in healthcare settings around the world, I am a little confused. Should I let my kids eat dirt and play with the animals and then insist they wash their hands before dinner?

Dirty dogs, contaminated cats and germy gerbils

The most surprising thing to emerge from my reading has been in relation to pets. Since we have three cats and are about to get a puppy, I am quite keen to know more.

So I explored this aspect of exposure to germs a little further: Do kids with pets have stronger immune systems?

Apparently, yes (though this is not uncontested). Some studies have shown that babies who are raised around pets from early babyhood have fewer allergies, asthma, colds and ear infections.

Some interesting elements of the research:

• Dogs have a greater protective effect than cats;
• Dogs who spend time outside have a greater protective effect than inside-only dogs (people have inside-only dogs?!?);
• The younger the child is exposed, the better;

Now there are some myths floating around about pet’s mouths being particularly clean. Specifically, some people believe that a dog’s mouth is cleaner than a human’s. This is apparently not true. Consider some of the things that dogs eat! It’s also apparently not true that dogs and cats have anti-bacterial properties in their saliva.

Nevertheless, feeding your child out of the pet bowl is not a great idea, as much because of what goes into the pet food as to the pet itself. There have been numerous reports of Salmonella in pet food, especially dry food, and as mentioned above, just think about where that cat’s tongue has been.

Ironically, all this reading, while intellectually allaying some of my concerns, has served to make me far more aware of germs than I ever was before. There has been much soaking of dish clothes in sterilizer and washing of hands in our family! But at the same time, my baby had her first doggy face lick.

Qualifier 1: If your child is immune compromised or you live or work in an environment that has a high risk level then clearly different considerations apply.

Qualifier 2: I am not a microbiologist, doctor or public health practitioner. I am just a plain old mom with questions. This post represents what I have gleaned from my research which has often included journalistic reports on the research, as I either don’t have access to the actual research or the scientific literacy to understand it. I am clearly open to additions, and corrections!

Sources

• T. Olszak et al., “Microbial exposure during early life has persistent effects on natural killer T cell function,” Science, doi:10.1126/science.1219328, 2012.
• Virus-specific CD4+ memory phenotype T cells are abundant in unexposed adults Laura F. Su , Brian A. Kidd, Arnold Han, Jonathan J. Kotzin, Mark M. Davis, Immunity February 2013
• Let Them Eat Dirt, by Megan Scudellari
• Kids need to eat dirt to be immune, www.thenakedscientists.com, by Chris Smith
• Babies Know: A Little Dirt Is Good for You
• Pets good for kids’ immune systems, researchers find
• 6 Ways Pets Can Improve Your Health
• The ‘hygiene hypothesis’ for autoimmune and allergic diseases: an update, H. Okada, C. Kuhn, H. Feillet and J.-F. Bach, Clinical & Experimental Immunology, Volume 160, Issue 1, pages 1–9, April 2010Article first published online: 11 MAR 2010, DOI: 10.1111/j.1365-2249.2010.04139.x