Bacterial link to Crohn's disease?

Written by Simon Crompton

Much controversy surrounds a link between germs that can live in milk and Crohn's disease - what's the science behind it all?

Can drinking milk cause Crohn's disease? It's a controversial idea that, at the very start of the 21st century, was regarded as a mere theory from a lone British researcher. But as the years have passed, it has gained credibility as other scientists have looked into the link, and been unable to discount it.

In science, just because a theory is credible, it doesn't mean it's true. A link between two things doesn't mean that one causes another

It's not the milk itself that's causing the debate, but the bacteriaA group of organisms too small to be seen with the naked eye, which are usually made up of just a single cell. that may live in it.

In 2000, Professor John Hermon-Taylor, then the professor of surgery at London UK's St George's Hospital Medical School, announced to the Royal Society of Medicine that there was widespread contamination of milk with a bacterium called Mycobacterium avium paratuberculosis (MAP). This was, he said, a 'public health disaster of tragic proportions', because the bug was responsible for causing a substantial proportion of cases of Crohn's disease. Dramatic stuff.

Today, Professor Hermon-Taylor, now at the Division of Nutritional Sciences at Kings College London, maintains that the link is all but proven. He puts three pieces of evidence together:

  • First, MAP infectionInvasion by organisms that may be harmful, for example bacteria or parasites. is widespread in livestock
  • Second, the bowels of most people with Crohn's disease have been found to be infected with MAP - significantly more than the population in general 
  • Third, it has been established that MAP can cause inflammationThe body’s response to injury. of the intestineThe section of gut, or gastrointestinal tract, from the stomach to the anus. in animal species.

Add that up, he says, and you have proof enough.

In 2003, Hermon-Taylor's research using DNAThe building blocks of the genes in almost all living organisms - spelt out in full as deoxyribonucleic acid. testing provided further evidence that the majority of people with Crohn's disease were infected with MAP.

His ideas are credible because they build on the now widely accepted belief that Crohn's disease is caused by a combination of geneticRelating to the genes, the basic units of genetic material. predisposition, an abnormal immune response and some sort of environmental trigger - a response to food, or to microorganismsOrganisms that are too small to be seen with the naked eye, such as bacteria and viruses. in the gut. Hermon-Taylor believes that MAP is the trigger in susceptible people.

But in science, just because a theory is credible, it doesn't mean it's true. A link between two things doesn't mean that one causes another.

To get the full picture, you can read below about:

What recent research tells us

The link between Mycobacterium avium paratuberculosis (MAP) and Crohn's disease has been looked into seriously since Hermon-Taylor spoke up. Although scientists argue that it is still far from proven, Hermon-Taylor is not now alone in saying that the MAP factor needs thorough investigation.

A 2005 review of the evidence by a panel of experts for the British voluntary body the National Association for Colitis and Crohn's Disease in the UK concluded:

  • MAP is present in milk and possibly in water supplies
  • Crohn's disease is likely to be a collection of different conditions with differing causes, and if MAP has a role, it may affect only one type of the disease
  • There is no proof at present that MAP causes Crohn's disease.

Another comprehensive review of all the evidence published in the authoritative medical journal The Lancet Infectious Diseases in 2007 took a similar line.

It said that milk was widely infected with MAP, and that pasteurisation didn't always destroy it. Equally, it confirmed that infectionInvasion by organisms that may be harmful, for example bacteria or parasites. with MAP is substantially more common in people with Crohn's disease than in people without the condition.

But could this study either confirm or exclude MAP from having a causal role? No, not with any certainty, concluded the authors. One of the problems was that having Crohn's disease might make people more susceptible to having a thriving colony of the bacteriaA group of organisms too small to be seen with the naked eye, which are usually made up of just a single cell. in the gut, not the other way round.

What this means for governments

Though not widely publicised, the Crohn's disease/MAP debate is the hottest of hot topics, because it potentially affects not just public health, but milk industries, governments and the livelihoods of millions around the world.

In Britain, government food and agriculture bodies have taken the link seriously enough to conduct studies showing that milk pasteurisation doesn't always get rid of the bug, with 2 per cent of pasteurised milk on the shelves found to contain Mycobacterium avium paratuberculosis (MAP). They have also adopted a strategy to try to control MAP in milk production; recent studies have indicated that 75 per cent of British milk herds may be infected.

In other countries, the extent of dairy MAP infectionInvasion by organisms that may be harmful, for example bacteria or parasites. will vary, as will the strategies taken by governments to tackle it.

In the United States, for example, where a 2007 study showed that 68.1 per cent of dairies were infected with MAP, producers are implementing management practices to reduce MAP transmission - if nothing else, to reduce the risk of their cattle contracting a livestock disease called Johne's disease, also linked to MAP.

What this means for us

Even in the light of this debate, no one is suggesting that we should stop drinking milk, which can be a very valuable nutritional source, particularly of calciumAn element that forms the structure of bones and teeth and is essential to many of the body's functions.. The consensus is that there should continue to be more research, and continuing efforts to control Mycobacterium avium paratuberculosis (MAP) in milk.

The National Association for Colitis and Crohn's Disease in the UK says that for the vast majority of people, the presence of MAP in milk is unlikely to be a problem. It advises that if you are concerned about MAP in milk, it is extremely unlikely to be present in UHT milk (which has been sterilised at very high temperatures).

For people who already have Crohn's disease, the association advises that it is not yet known how continuing exposure to MAP might affect their condition - whether, for example, it might trigger new bouts of inflammationThe body’s response to injury. during periods of remission.