Experts do not understand exactly why some people get ankylosing spondylitis, but they do know many factors play a role.
Genes and Family History
The clearest risk factors involve inherited genes and family history:
- A gene called HLA-B27 is found in 85% to 95% of people with ankylosing spondylitis, though some estimates are lower.1-4 Experts estimate only about 8% of people around the world have this gene.5
- Not everyone who has HLA-B27 will get AS. Experts estimate people who carry HLA-B27 have a 2% to 10% chance of developing spondyloarthritis.6
- A family history of ankylosing spondylitis is an additional risk factor for developing the disease. For example, a person who inherits HLA-B27 and has a parent with ankylosing spondylitis has about 20% chance of developing AS.7
- Black people who have ankylosing spondylitis are less likely to have the HLA-B27 gene than white people who have the disease.8
- In addition to HLA-B27, genetic variations in genes ERAP1, IL1A, and IL23R are also linked to ankylosing spondylitis.9
Exactly how genes and family history affect the risk of ankylosing spondylitis is not known.
Other Risk Factors
In addition to genes and family history, factors that can affect risk include:
Sex. Men are two to three times more likely to get ankylosing spondylitis than women.10 Men also tend to have more severe symptoms. Because AS is more common in men, diagnosis in women is often overlooked or missed, especially because women tend to have pain in the neck, hips, and/or peripheral joints instead of the low back.
Age. Ankylosing spondylitis is usually diagnosed in people in their teens, 20s and 30s. Ninety-five percent of patients are diagnosed before the age of 46.11,12 It is possible for children to be diagnosed: 15% of people with ankylosing spondylitis are diagnosed before age 15.13
Related diseases. Past or present history of verified Crohn’s disease, ulcerative colitis, or psoriasis increases the likelihood of developing axial spondyloarthitis.14
Bacteria. Everyone has bacteria that live in their intestines, and these bacteria—collectively called gut microbiota—are thought to be involved in the development and regulation of the immune system.5 Many scientists believe that gut microbiota influence the development of ankylosing spondylitis.16-20 While there is growing evidence for this connection, scientists are still learning about it.
Just because someone has risk factors for ankylosing spondylitis does not guarantee he or she will develop the disease.
These complications are quite rare and are generally only seen in the most severe cases of ankylosing spondylitis.