Nobody knows what causes Ankylosing Spondylitis but genetics do play an important role. Over 90% of people who are diagnosed with AS have a gene called HLA-B27.
The latest research would seem to suggest that bacterial infections and the environment have roles in the triggering of AS.
It is felt that there may be up to 5 or 6 other genes involved in susceptibility to AS but none of these have yet been identified.
It is thought by some that the initial inflammation may be caused by the body's autoimmune system failing to switch itself off after an infection or a combination of infectious microbes.
There is also considerable regional variation in the incidence of AS- in the US 1% of the population has AS whilst 7% of the population has the HLA-B27 gene whereas in Northern Scandanvia the figures are 1.8% and 24% respectively. There are also variations between different ethnic and racial groups, in Caucasians over 95% of people who have AS also have HLA-B27 whilst the figure is 50% for Afro-Americans and almost 80% for people from Mediterranean countries.
Sometimes bowel infections appear to trigger AS whilst symptoms sometimes start following a period of enforced bed rest.
People with Reiter's Syndrome may also develop AS. Reiter's syndrome is characterised by inflammation of the iris and of the urethra.
One theory says that AS starts when "the defences of the intestine start breaking down and bacteria from the intestines pass into the bloodstream directly into the region where the sacroiliac joints are located."
