Embarrassing Illnesses

The exact cause (or causes) of Crohn’s disease are unknown. Most researches think that the condition could be caused by a combination of inter-related factors that are listed below.

• Genetics - genes that you inherit from your parents may increase your risk of developing Crohn’s disease (genetic susceptibility).
• The immune system - it appears that the immune system is responsible for the inflammation that occurs in Crohn’s disease.
• Previous infection - a previous bacterial, or viral, infection may trigger an abnormal response from the immune system.
• Environmental factors - the fact that Crohn’s disease is most common in ‘westernised’ countries, such as the UK, and least common in poorer parts of the world, such as Africa, may be due to environmental factors.

These four factors are discussed below in more detail.

Genetics

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

The direct evidence is that researchers have indentified 32 different specific genetic alterations (genetic mutations) that are more common in people with Crohn’s disease than in the population at large.

The indirect evidence is that Crohn’s disease can run in families. If you have a first-degree relative (mother, father, sister, or brother) with Crohn’s disease, you have a 5-10% risk of developing the condition yourself. And, if you have an identical twin with Crohn’s disease, you have 37% risk of developing the condition.

The fact that Crohn’s disease is more common in some ethnic groups compared with others also suggests that genetics is an important factor in the condition.

The immune system

The immune system plays a complex and demanding role in the digestive system. It provides protection against harmful bacteria that could potentially find their way into the digestive system.

However, the digestive system is also home to over 500 different types of so-called ‘friendly bacteria’ that help with the digestion of food. Therefore, the immune system has to recognise this type of bacteria and let them do their job without attacking them.

In Crohn’s disease, it appears that something disrupts the immune system which sends a special ‘killer 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 that is associated with Crohn’s disease

Previous infection

A related theory to the two that are outlined above is that in certain genetically susceptible individuals, a previous childhood infection may lead to an abnormal immune response, causing the symptoms of Crohn’s disease to occur.

One possible source of this infection is a bacterium called mycobacterium avium subspecies paratuberculosis (MAP). MAP is commonly found in cows, sheep, and goats and it causes a similar disease in animals that is known as Johne's disease.

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

MAP has been known to survive the pasteurisation process (where milk treated with heat to kill bacteria) so it could be possible that people have contracted 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 factors about Crohn’s disease that have led many researchers to believe that environmental factors may be involved. These factors are explained below.

• Crohn’s disease is a ‘disease of the rich’ - rates are highest in developed parts of the world, such as the UK and USA, and lowest in underdeveloped 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.

These factors seem to suggest that there is something that is associated with modern, western lifestyles which increases a person’s risk of developing Crohn’s disease.

One theory to explain this, known as the hygiene hypothesis, 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 evidence to support this theory.

An alternative theory is known as the cold-chain hypothesis. This theory suggests that the increase in cases of Crohn’s disease can 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. Therefore, as people eat more chilled foods, their exposure to these bacteria increases, and it is these bacteria that cause 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.

Risk factors

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 to non-smokers with the condition.