Let Them Eat Dirt. B. Brett Finlay

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СКАЧАТЬ childhood, may be the cause of the rapid increase in allergy cases, since it prevents proper development of the immune system. This concept was later termed the “hygiene hypothesis,” and an increasing number of studies have explored whether the development of many diseases, not just allergies, can be explained by this hypothesis. There is now a large amount of very solid evidence, which we’ll examine in the following chapters, supporting Dr. Strachan’s proposal as generally correct. What remains less clear is what exact factors are responsible for this lack of microbial exposure. For his study on allergies, Dr. Strachan concluded that “declining family size, improvements in household amenities, and higher standards of personal cleanliness” contributed to this reduced contact with microbes. While this may be true, there are many other modern-life changes that have an even stronger impact on our exposure to microbes.

      One of these changes can be attributed to the use, overuse, and abuse of antibiotics—chemicals that are designed to indiscriminately kill bacterial microbes. Definitely one of, if not the greatest discovery of the twentieth century, the emergence of antibiotics marked a watershed before-and-after moment in modern medicine. Prior to the advent of antibiotics, 90 percent of children would die if they contracted bacterial meningitis; now most cases fully recover, if treated early. Back then, a simple ear infection could spread to the brain, causing extensive damage or even death, and most modern surgeries would not even be possible to contemplate. The use of antibiotics, however, has become far too commonplace. Between the years 2000 and 2010 alone there was a 36 percent increase in the use of antibiotics worldwide, a phenomenon that appears to follow the economic growth trajectory in countries such as Russia, Brazil, India, and China. One troubling thing about these numbers is that the use of antibiotics peaks during influenza virus infections, even though they are not effective against viral infections (they are designed to kill bacteria, not viruses).

      Antibiotics are also widely used as growth supplements in agriculture. Giving cattle, pigs, and other livestock low doses of antibiotics causes significant weight gain in the animals and, subsequently, an increase in the meat yield per animal. This practice is now banned in Europe, but is still legal in North America. It seems that antibiotic overuse in humans, especially in children, is inadvertently mimicking what occurs in farm animals: increased weight gain. A recent study of 65,000 children in the US showed that more than 70 percent of them had received antibiotics by age two, and that those children averaged eleven courses of antibiotics by age five. Disturbingly, children who received four or more courses of antibiotics in their first two years were at a 10 percent higher risk of becoming obese. In a separate study, epidemiologists from the Centers for Disease Control and Prevention (CDC) found that states in the US with higher rates of antibiotics use also have higher rates of obesity.

      While these studies didn’t prove that antibiotics directly cause obesity, the consistency in these correlations, as well as those observed in livestock, prompted scientists to have a closer look. What they found was astonishing. A simple transfer of intestinal bacteria from obese mice into sterile (“germ-free”) mice made these mice obese, too! We’ve heard before that many factors lead to obesity: genetics, high-fat diets, high-carb diets, lack of exercise, etc. But bacteria—really? This raised skepticism among even the biggest fanatics in microbiology, those of us who tend to think that bacteria are the center of our world. However, these types of experiments have been repeated in several different ways and the evidence is very convincing: the presence and absence of certain bacteria early in life helps determine your weight later in life. Even more troubling is the additional research that shows that altering the bacterial communities that inhabit our bodies affects not just weight gain and obesity, but many other chronic diseases in which we previously had no clue that microbes might play a role.

      Let’s take asthma and allergies as an example. We are all witnesses to the rapid increase in the number of children suffering from these two related diseases. Just a generation ago it was rather unusual to see children with asthma inhalers in schools. Nowadays, 13 percent of Canadian children, 10 percent of US children, and 21 percent of Australian children suffer from asthma. Peanut allergies? That used to be incredibly rare, but is now so frequent and so serious that it has led to peanut-free schools and airplanes. As with the obesity research, it is now evident that receiving antibiotics during childhood is associated with an increased risk of asthma and allergies.

      Our laboratory at the University of British Columbia became very interested in this concept and decided to do a simple experiment. As had been observed with humans, giving antibiotics to baby mice made them more susceptible to asthma, but what we observed next left us in awe. If the same antibiotics were given when the mice were weaned and no longer in the care of their mothers, there was no effect in susceptibility to asthma. There appeared to be a critical window of time, early in life, during which antibiotics had an effect on the development of asthma. When given orally, the antibiotic that we chose, vancomycin, kills only intestinal bacteria, and does not get absorbed into the blood, lungs, or other organs. This finding implied that the antibiotic-driven change in the intestinal bacteria caused the increase in the severity of asthma, a disease of the lungs! This experiment, as well as others from several different labs, came to the same conclusion: modifying the microbes that live within us at the beginning of our life can have drastic and detrimental health effects later in life. The discovery that this early period in life is so vulnerable and so important tells us that it’s crucial to identify the environmental factors that are disturbing the microbial communities that inhabit us during childhood.

      One of these factors has been observed by comparing children raised on rural farms to those raised in a city. Several studies have shown that exposure to a farming environment makes children less likely to develop asthma, even children from families with a history of asthma, and scientists are now beginning to learn why. Farm-raised children are exposed to more animals, more time outside, and a lot more dirt (and feces!), all things that are known to stimulate the immune system. A critical part of the training and development of the immune system occurs in the first years of life. Asthma, characterized by a hyperactive immune system, seems to have a higher chance of developing in a child with a limited exposure to these immune stimulants, because without them, the immune system does not have all the tools for proper development. By cleaning up our children’s environments, we prevent their immune systems from maturing in the way they have for millions of years before us: with lots and lots of microbes. Life for our ancestors involved massive exposure to microbes from the environment, food, water, feces, and many other diverse sources. Compare that to our current way of life, where meat comes on sterile Styrofoam pans wrapped in plastic wrap, and our water is treated and processed until it’s free of nearly all microbes.

Kids Will Be Kids

      A friend, Julia, moved to a small free-range pig and poultry farm when her first child was a preschooler. She observed firsthand how differently a kid grows up in a city and on a farm. She has always been outdoorsy, so even when she was living in the city she would let Jedd, her oldest child, play outside a lot. They would go to parks and playgrounds, where she would encourage Jedd to get dirty, play in sandboxes and mud puddles—she even allowed him to put (safe-sized) objects in his mouth, like big rocks or leaves. Her outdoorsy nature, she thought, would make their transition to rural life easier, and it did in many ways. But nothing prepared her for the things she’s seen her kids do on their farm. When her second baby was born, she would strap him on her back every morning so she could go to their chicken coop to pick up eggs. Jedd, timid with the animals at first, was now chasing and riding the chickens, tasting their feed and touching the fresh eggs. A couple of times she even caught him chewing on something he had picked up from the ground. Anyone who has stepped inside a chicken coop knows what’s on the floor, so she’s pretty sure Jedd has tasted chicken droppings at least a few times. Clearly, Julia freaked out at first, but it’s hard to prevent a five-year-old boy from getting dirty when you’re busy working and looking after a second child. After realizing that Jedd wasn’t getting sick from his newly acquired tastes of the farm, Julia relaxed a bit. Jedd, now eight years old, is responsible for gathering the eggs every morning. Newly laid eggs are often soiled and he doesn’t wear gloves. He washes his hands when he’s done, but it’s impossible that some of that stuff hasn’t made it into his mouth.

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