Solving the asthma riddle

The Bavarian hills look like they could be a set from The Sound of Music. The air is crystal clear, cowbells are tinkling and the countryside is dotted with picture-perfect wooden farm houses.

Over the last 100 years, farming has not changed much here. Cows, pigs and chickens live in barns beneath the houses. Mothers tend to the barn animals. Children grow up doing farm chores and drinking fresh, unpasteurised milk.

It’s an idyllic, healthy way to grow up. And it shows. These farm children have around half the rate of asthma of children in nearby towns.

This natural human experiment has intrigued researchers. Around the world asthma rates rise with modern living. But why? Is it city life, with its greater exposure to smog and other pollutants? Or does an ingredient in the rural lifestyle hold the secret to healthy lungs?

Asthma researchers agree there is no simple answer. Multiple factors influence who becomes asthmatic: genetic make-up, viral infections that damage the airways, microbes in the gut, exposure to household chemicals and air pollution all play a part. Another complication is that asthma diagnosis varies with the latest medical fads, making it difficult to compare asthma rates over time.

But the Bavarian experiment has reduced some of the variables. The people in the towns and farms, for instance, are of the same genetic stock, and are being diagnosed with the same tests. And researchers are taking advantage to drill down into what is protecting children on the farm.

In a paper published in Science in September, an international team led by Ghent University researchers in Belgium exposed mice to dust collected from a Bavarian cow barn – and it protected them from asthma. Intriguingly, though, the dust only protected mice that carry an intact copy of the gene A20 that plays a key role in training the immune system. People lacking a functional form of A20 are also more susceptible to asthma, the team showed.

“What’s striking about this study is how complete it is; it seems to add up and make sense,” says Manuel Ferreira, who studies genetic predisposition to asthma at the Queensland Institute of Medical Research.

The results offer new leads for preventing and treating asthma in the form of vaccines or drugs that mimic the effects of farm dust. “It’s a big step forward,” says Erika von Mutius, a co-author of the present study who heads the Asthma and Allergy Department at the Children’s Hospital of the University of  Munich.

Asthma rates began to rise in the 1950s, especially in Western countries. In Australia the rise was particularly dramatic. In 1964, the percentage of seven year olds with asthma or wheeze was reported as 19% based on parents’ responses to a survey. By 1990, the same type of survey reported that 46% of children were affected. In Scotland the rate rose from 28% in 1964 to 64% in 1999.

Such spikes could be attributed, at least in part, to a medical fad. What had once passed as wheezy bronchitis was relabelled asthma. But there’s no doubt the overall trend was real. “However you define it, many studies showed a rise,” says epidemiologist Guy Marks at the University of NSW. Yet since its peak in the mid-1990s, the rate of asthma has fallen and now seems to have plateaued. In Australia, the latest (2011) report by the Australian Institute of Health and Welfare is that the percentage of school-aged boys with asthma is 11.4%; for girls the figure is 7%.

It’s difficult to compare asthma rates over the last six decades, but it’s clear something about modern living raises the risk – a finding supported by rising asthma rates in developing countries. One theory is that modernity makes us too clean. Known as the hygiene hypothesis, the term was coined by British epidemiologist David Strachan in 1989. He observed that the more elder siblings a child had, the lower their risk of allergies.

Strachan’s hypothesis has generally come to mean that the cleaner the environment, the higher the risk of asthma. Children in affluent countries grow up in small families with little exposure to animals, and antibiotic use reduces their exposure to microbes. This might leave their immune system with not enough to do, making it more likely to over-react to “innocent” environmental factors – most often house dust mite poo, mould, cat hair (actually salivary proteins left after licking) or grass pollen.

Farm studies that compare asthma rates between town and country folk in a particular region have been one of the best ways to put the hygiene hypothesis to the test. More than 30 of these studies, carried out over the last 20 years, have found farm living protects against asthma, as well as against the related allergic conditions eczema and hay fever. Large effects are seen in rural Germany, Switzerland and among the Amish of Pennsylvania, where farm-living almost halves a child’s risk.

Australian studies have found a link too – and shown that not just any farm will do. A 2001 study by Sara Downs at the University of Sydney, showed children growing up with livestock in Wagga Wagga were more protected from allergies than those from Moree, a cropping region.

As von Mutius points out, the protective effects are strongest when children are raised in traditional farms with close exposure to animals. The critical period of exposure seems to be the first two years, and before – the time a mother spends in the barn while pregnant also offers protection. And the more types of animals she is exposed to the better.

Many factors have been linked to the protective effect of farm living: hay, exposure to a variety of microbes, drinking unpasteurised milk. The Belgian researchers wanted to drill down further. They starting by testing farm dust from Bavarian cow barns. For two weeks, mice were exposed to barn dust every second day; the dust was daubed on their nostrils. Then they were exposed to a notorious trigger of asthma: house dust mite poo. The untreated mice duly had an asthma attack. The barn dust snorters did not.

What was the magic ingredient in the cow barn dust? At least in part, the protective agent was a substance called endotoxin – the broken up bits of the cell wall of  “gram negative” bacteria. Dosing the mice with endotoxin alone partly replicated the dust’s protective effect, the team showed.

But how, exactly, was exposure to endotoxin preventing the mouse immune system over-reacting to the presence of dust mite poo?

The immune system operates like an army. And like all armies it relies on sentries – a collection of cells known as the innate immune system. They are the first responders and they brief the command centres on the nature of the assault.

Do the invaders like to hide inside cells – like viruses and some bacteria? Or are they like tiny helminth worms – a type of parasite – that proliferate openly in the bloodstream? Different invaders require a different response. If it’s the intracellular variety, the immune system mounts a TH1 response. Like sending troops to fight within a town, the immune forces must carefully go from house to house to check for hidden combatants. If it’s worms in the bloodstream, the immune system mounts a TH2 response; that’s more like dropping bombs on an enemy out in the field. It’s less precise and runs the risk of more collateral damage.

At birth, the human immune system seems to be set to a TH2 default state. During the first years of life, as it is exposed to the environment, it learns to rebalance towards TH1. In asthmatics, the immune system seems to have adapted badly: they remain more inclined towards the carpet bombing of a  TH2 response.

Endotoxin may be one of the environmental triggers that helps usher the immune system from TH2 to TH1. How does endotoxin achieve this? Researchers knew it must be connected to the way the immune system’s sentry cells signal the state of the battlefield to command headquarters.

Endotoxin is known to activate receptors on sentry cells. But what was the next signal? The researchers suspected it might be the product of a gene called A20.

To prove A20 was the crucial go-between, the Ghent team eliminated it from mice. They found the protective effect of endotoxin and farm dust was greatly diminished, confirming the importance of A20. It was not only the sentry cells that relayed the A20 signal to the immune system. The cells that line the airways and provide a barrier against microbes – so-called epithelial cells – also played this role.

So that’s the story in mice. What about humans? To find out, the researchers took swabs of epithelial cells from the airways of normal and asthmatic patients and grew these cells in dishes. They were exposed to endotoxin every second day for one week, and then sprinkled with house dust mites.

Cells in dishes can’t have an asthma attack, but they did produce chemicals typical of a TH2 response. However, just as in the mice, the cells pre-treated with endotoxin showed far lower levels of these chemicals. Interestingly, the cells from asthmatic people produced less A20 than those of non-asthmatics – a finding consistent with the notion that their immune armies had been poorly trained because they carry a defective form of this gene.

These findings tally well with studies of children living on farms. For instance the GABRIELA study of 1,707 rural children aged six to 12 from four European countries (including Germany), found that those children most susceptible to asthma carried a defective form of the A20 gene.

“The new study adds to decades of research that all point in the same direction,” says Peter Sly, director of the Children’s Health and Environment Program at the University of Queensland.

Starting from the womb, infants are exposed to farm dust which carries endotoxin among other things. It triggers the release of A20, which signals the assembly of a better functioning immune army.

So should pregnant mothers start sniffing farm dust and giving it to their infants as well?

“Although tempting, I wouldn’t advise it,” says Martijn Schuijs, the lead author of the Belgian study. “We are still not sure what it is that is protective in the cocktail of farm dust.” It’s not likely to be endotoxin alone, since that is plentiful in cities too – it exists wherever there are bacteria. He suspects the protective element is connected to cows and hay.

But he does not rule out a type of vaccine in the future that, like endotoxin, could be used to train the human system. Alternatively, knowing the crucial signalling role of A20 points to a new target for developing drugs that raise its activity.

In the meantime, should children attend farm-based crèches? “Yes, I like that idea,” says von Mutius, who has studied these farming environments for more than a decade. Farm crèches are already popular in the Netherlands.

But childhood exposure may not be enough to recruit a healthy immune army for life. A 2014 study in Silesia, Poland showed that once people in traditional villages stopped keeping animals they lost much of their protection against allergies.

In 2003, their rates of allergy were about a third that of people in the nearby towns. But in 2004, Poland joined the European Union and many villagers, rather than conforming to costly new husbandry standards and the requirement to pasteurise their milk, jettisoned their animals. The result was that their allergy levels rose to match the level of the people in the towns.

Asthma rates have not followed suit, but Paul Cullinan, a respiratory physician at London’s Imperial College and senior author of the study, suspects this will come. He cites the “allergic march” often seen in apprentice bakers. First they develop allergies to wheat flour; after several months their allergic response is followed by asthma. For Cullinan, the lesson of Poland is: “If you want to stop being allergic, you need to live like an 18th century villager.” But there are costs. Drinking unpasteurised milk carries the risk of being infected with dangerous bacteria including listeria, salmonella and enterohaemorrhagic E. coli that can damage the kidneys.

Since living like 18th century villagers is not an option for most of us, let’s hope the new findings on how barn dust trains the immune system will provide another answer.

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