Causes of Crohn’s disease

The exact cause of Crohn’s disease is unknown. Most researchers think that it is caused by a combination of factors.

The main contributory factors are thought to be:

• genetics
• the immune system
• previous infection
• environmental factors
• smoking

These are discussed in more detail below.

Genetics

There is evidence to suggest that genetics plays a role in the development of Crohn’s disease.

Researchers have identified 32 different specific genetic mutations (alterations) that are more common in people with Crohn’s disease than in the general population.

There is also evidence that Crohn’s disease can run in families. About 3 in 20 people with the condition have a close relative (mother, father, sister or brother) who also has Crohn’s disease. If you have an identical twin with Crohn’s disease, you have a 70% chance of also developing it.

The fact that Crohn’s disease is more common in some ethnic groups than in others also suggests that genetics plays an important role.

The immune system

The immune system provides protection against harmful bacteria that could potentially find their way into the digestive system.

The digestive system is also home to more than 500 different types of so-called ‘friendly bacteria’ that help to digest food. The immune system has to recognise these bacteria and let them do their job without attacking them.

It seems that, in Crohn’s disease, something disrupts the immune system, which sends a special antibody, known as tumour necrosis factor (TNF), to kill all bacteria, regardless of whether they are friendly or not. It is the TNF antibodies that cause most of the inflammation associated with Crohn’s disease.

Previous infection

In certain genetically susceptible individuals, a previous childhood infection may lead to an abnormal immune response, causing the symptoms of Crohn’s disease.

One possible source of this infection is a bacterium called Mycobacterium avium subspecies paratuberculosis (MAP). MAP is commonly found in cows, sheep and goats.

Research has found that people with Crohn’s disease are seven times more likely to have traces of MAP in their blood compared with the general population.

MAP has been known to survive the pasteurisation process (where milk is treated with heat to kill bacteria), so it is possible that people have become infected with MAP by drinking milk from contaminated animals.

However, the exact role that MAP may play in the development of Crohn’s disease is uncertain.

Environmental factors

There are two unusual aspects of Crohn’s disease that have led many researchers to believe that environmental factors may play a part. These are explained below.

• Crohn’s disease is a ‘disease of the rich’. The highest number of cases occurs in developed parts of the world, such as the UK and US, and the lowest number in developing parts of the world, such as Africa and Asia.
• Crohn’s disease became much more widespread from the 1950s onwards. Before the Second World War the condition was rare.

This suggests that there is something associated with modern, western lifestyles that increases a person’s risk of developing Crohn’s disease.

One theory to explain this is known as the hygiene hypothesis. It suggests that as children grow up in increasingly germ-free environments, their immune system does not fully develop due to a lack of exposure to childhood infections. However, there is little in the way of hard, scientific evidence to support this theory.

An alternative theory is the cold-chain hypothesis, which suggests that the increase in the number of cases of Crohn’s disease might be linked to the increased use of refrigerators after the Second World War.

Psychrotrophic bacteria are bacteria that can survive, and sometimes grow, in cold environments. The cold-chain hypothesis theorises that, as people eat more chilled foods, their exposure to these bacteria increases and it is this that causes the abnormal immune response.

Traces of psychrotrophic bacteria have been found in people with Crohn’s disease, although, as the researchers behind the cold-chain hypothesis admit, this could be pure coincidence.

Smoking

Aside from family history and ethnic background, smoking is the most important risk factor for Crohn’s disease. Smokers are twice as likely to develop Crohn’s disease compared with non-smokers.

Furthermore, people with Crohn’s disease who smoke usually experience more severe symptoms compared with those with the condition who do not smoke.

Read more about how to get help to stop smoking.