The explosive growth of peanut allergies: The Hygiene Hypothesis

The rate of childhood peanut allergies has tripled from 1997 to 2008. During this 11-year period it has gone from 0.4% of kids younger than 18 in 1997 to 1.4% in 2008 [link].

What is the explanation of this explosive growth in peanut allergies?

First, peanut allergies are not an isolated example. As described in an article in Vox by Susannah Locke, childhood skin and food allergies in general are on the rise (see Figure 1). Indeed skin allergies seemed to have increased from ~7% in 1998 to over 12% in 2010.

So then what is the cause of this general increase in childhood allergies?

An explanation I favor is the Hygiene Hypothesis:

"The overall increase in allergies is still a big mystery in epidemiology. One of the leading explanations is the hygiene hypothesis, which posits that society has become too clean and hygienic. Essentially, children aren't exposed to enough bacteria, viruses, and possible allergens early on, which in turn inhibits the development of their immune system. That leads to more problems later on, including allergies and asthma. Researchers are still trying to figure out if this hypothesis is true."

It seems counterintuitive that the fact our environment is becoming too clean can cause our immune system to become "hyperactive". Another Vox article by Joseph Stromberg provides the current rationale for this hypothesis:

"A child's immune system needs education, just like any other growing organ in the human body," says Erika von Mutius, a pediatric allergist at the University of Munich and one of the first doctors to research the idea. "The hygiene hypothesis suggests that early life exposure to microbes helps in the education of an infant's developing immune system." Without this education, your immune system may be more prone to attacking the wrong target — in the case of autoimmune diseases, yourself.'

In other words, without proper exposure to real-world antigens from actual pathogens, the immune system may instead target antigens from innocuous sources such as food (e.g. peanuts) or one's own body (resulting in autoimmune disorders like asthma, multiple sclerosis, lupus, or rheumatoid arthritis).

The evidence for the "hygiene hypothesis" comes from studies in which people who grow up in "dirtier" environments tend to have fewer immune disorders than people who grow up in "cleaner" (typically wealthier) environments:

"Other doctors have found that, on the whole, people in wealthy, more heavily sanitized nations have much higher rates of asthma and allergies than those in the developing world. This could be a function of natural variations among the populations, but more recently, doctors have found that people who move from a developing country to a wealthier one have a higher chance of developing these diseases than people who stay in their country of origin."

Returning back to peanut allergies, what are the possible treatment options? According to experts, "peanut allergies tend to be lifelong, although studies indicate that approximately 20 percent of children with peanut allergy do eventually outgrow their allergy." One strategy is to slowly build up tolerance for peanuts:

"But scientists are also developing treatments that might reduce how allergic children are. In the past decade or so, researchers started compiling good evidence that by very carefully exposing children to tiny bits of peanut, they could very slowly work up children's tolerance. Patients eat tiny doses of peanut or use a peanut patch worn on the skin, and medical professionals stand ready to jump in with an adrenaline shot in case of a severe reaction. Over time, as the dose increases, the body learns that the peanut is not the enemy.

One small 2014 study in The Lancet showed encouraging results. In a randomized, controlled trial of 39 patients, the researchers got 54 percent to tolerate the equivalent of roughly 10 peanuts. If you want to read more about this technique, Carlyn Kolker wrote an essay in The New York Times about her son participating in a similar study."

Thus, a hyperactive immune system that hasn't been properly trained to attack actual pathogenic antigens, must be domesticated in their response to "normal" antigens such as peanuts or "self" antigens. Further studies should shed light on whether this type of treatment can be successful.

Figure 1. Allergies are on the rise in kids (CDC); one possible explanation is the Hygiene Hypothesis.